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In this article we will discuss about Phylum Mollusca:- 1. Definition of Phylum Mollusca 2. Habit and Habitat of Phylum Mollusca 3. Taxonomic Retrospect 4. Characteristic Features 5. Classification 6. Number and Size 7. Mantle and Mantle Cavity 8. Excretory System 9. Reproductive System 10. Development.
Contents:
- Definition of Molluscs
- Habit and Habitat of Molluscs
- Taxonomic Retrospect of Molluscs
- Characteristic Features of Molluscs
- Classification of Molluscs
- Number and Size of Molluscs
- Mantle and Mantle Cavity of Molluscs
- Excretory System of Molluscs
- Reproductive System of Molluscs
- Development of Molluscs
1. Definition of Molluscs:
Phylum Mollusca is a group of invertebrates which has fascinated man from the dawn of civilization. It is a major group in the animal kingdom as regards the number which occupies next to arthropods. The members of this group have adapted either on land or in water.
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Bilaterally symmetrical, coelomate protostomia with spiral cleavage and soft un-segmented body, covered by a thick muscular fold, the mantle which secrets a single or many shells, and a radula in the oesophagus except Bivalvia, and ventral muscular foot for locomotion.
2. Habit and Habitat of Molluscs:
Molluscs are noted for their adaptive modifications to different modes of life. They are distributed in almost all the parts of the earth and exhibit variety of forms. Besides their abundance in space, they have left behind a continuous palaeontological records since Cambrian period.
Majority of the Molluscs are aquatic and a few are adapted to terrestrial environment. Usually the snails and slugs lead a land life. The aquatic Molluscs are mostly marine. Few Bivalves and snails inhabit freshwater or brackish water.
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They usually live in the sea shores or in shallow water. Some Molluscs are pelagic and a few are recorded to sink down to the depth of about 35,000 feet. Most of the Molluscs are nocturnal.
Greatest number of Molluscs are free- living forms. As regards locomotory power, they range from very slow moving to fast swimming forms. They exhibit variety of modifications. By the modification of foot, they can creep, leap, burrow, float or swim. Some Bivalves fix their bodies on the substratum by the byssus apparatus.
Molluscs are mostly herbivorous and live on available vegetable by scraping with the radular apparatus. Most of the Bivalves live on micro-organisms, but the larval forms for some time lead ectoparasitic life in the gills of fishes. All Cephalopods of predaceous forms live on small fishes and Crustaceans. The Gastropods have different food habits.
They are mostly vegetable feeders and some are predaceous. Amongst the Gastropods, the members of the order—Pyramidellacea lead semiparasitic life, whereas that of endoconchidae includes all endoparasitic forms living in the body of Holothurians.
3. Taxonomic Retrospect of Molluscs:
1. Aristotle, the father of Malacology described many molluscs, specially the cephalopods and divided molluscs into two groups depending on the presence or absence of shell.
2. Johnstonus (1650) coined the term Mollusca and his Mollusca included barnacles.
3. Linnaeus (1758) also retained the term Mollusca and included heterogenous soft-bodied forms like tunicates, anemones, cephalopods and polychaeties.
4. Cuvier (1795) threw the modern light on molluscan taxonomy. He divided the Mollusca into Cephalopoda, Gastropoda, Pteropoda and Acephala. The Acephala includes Tunicata, Brachiopoda and Cirripedia.
5. Lamarck (1815-1819) excluded bivalves from the group Mollusca.
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6. De Blainville (1832) adopted a new name, Malacozoa for the molluscs. Although his name is no longer retained in modern textbooks.
7. The term ‘Malacology’ signifying the science of mollucs is still in use. The barnacles had long been included under Mollusca, but Thompson (1830) and Burmeister (1834) established their crustacean affinities and excluded them from molluscs.
8. The class Amphineura was formed by Loven (1841) and Graff (1875, 1877) and later established by Von Ihering (1877) and Spengel (1881).
9. The group solenogastres (Aplacophora) established by Gegenbaur (1878) had long been taken as holothurians, but Graff (1875-1877) studied their molluscan nature and included them under Mollusca.
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10. The inclusion of scaphopods under Mollusca had long been a controversial issue. Cuvier and Lamarck placed them with annelids, but their molluscan organisation was established by Deshayes (1825), Clark (1851) and Keferstein (1862-1886). The class Scaphopoda was finally established by Bronn (1862).
11. The class Gastropoda was introduced by Cuvier (1795). Cuvier’s Gastropoda underwent profound alterations during nineteenth century but the stability of the classification of the Gastropoda started since Milne Edwards (1848) and Spengel (1881) and finally it was established by Lankester (1883).
12. The Bivalvia the name first employed by Linnaeus, were named Lamellibranchiata by De Blainville (1816) and it was also changed Pelecypoda by Goldfuss (1821).
13. Cuvier (1795) first named Cephalopoda for the cuttle fish.
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14. Lankester (1901-1909), Pelseneer (1906) first divided the phylum into 5 classes:
(i) Amphineura,
(ii) Scaphopoda,
(iii) Gastropoda,
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(iv) Pelecypoda and
(v) Cephalopoda.
15. Parker and Haswell (1940) classified into 6 classes such as:
(i) Solenogastres,
(ii) Placophora (Loricata),
(iii) Gastropoda,
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(iv) Scaphopoda,
(v) Bivalvia and
(vi) Cephalopoda (Siphonopoda).
16. Morton and Yonge (1964) classified Mollusca into 6 classes:
(i) Monoplacophora,
(ii) Amphineura,
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(iii) Scaphopoda,
(iv) Gastropoda,
(v) Bivalvia and
(vi) Cephalopoda.
This scheme has presented in Parker and Haswell’s, Vol. I (7th ed.).
17. Hyman (1967), Abott (1973), Peter Dance (1977), Barnes (1980), Ruppert and Barnes (1994), Anderson (1998), Pechenik (2000), Brusca and Brusca (2003) divided Mollusca into 7 classes in which the subclass rank of Aplacophora and Polyplacophora under Amphineura have upgraded into classes.
Morton (1979) divided Phylum Mollusca into two Subphyla:
(i) Aculifera including the classes:
(a) Aplacophora and
(b) Polyplacophora and
(ii) Conchifera which includes the rest 5 classes such as:
(a) Mono- placophora,
(b) Gastropoda,
(c) Bivalvia,
(d) Scaphopoda and
(e) Cephalopoda.
4. Characteristic Features of Molluscs:
1. Majority of the members aquatic and a few terrestrial.
2. Bilaterally symmetrical body except some gastropods and cephalopods become secondarily asymmetrical.
3. Body soft and un-segmented in the adult stage.
4. Exoskeleton, in the form of calcareous shell, present in most of them except aplacophorans, nudibranchs and most octopuses. The shell is mostly located externally, although a few forms with internal shell.
5. Cephalization well-marked in gastropods and cephalopods but in other classes the head is small or poorly differentiated.
6. Visceral mass is enclosed by a thick muscular fold of the body wall, called mantle or pallium, which secretes spicules or shell. It is the unique feature of all molluscs.
7. A ventral muscular foot is present which helps in locomotion and may be secondarily modified in some forms.
8. A radula is present in the oesophagus which is a ribbon-like structure with recurved chitinous teeth stretched over a cartilaginous base helps as a scraper in feeding; absent in bivalves.
9. Respiratory organs are of comb-like structures known as molluscan gills or ctenidia or branchiae lodged in the mantle cavity, found in aquatic forms. In most pulmonate gastropods the mantle cavity is transformed into a pulmonary sac, helps in aerial respiration.
10. A chemoreceptor or tactile receptor organ, called the osphradium, generally located adjacent to the ctenidium. Other sense organs are eyes, tentacles and statocysts in most of the groups.
11. Blood-vascular system is a haemocoel, usually in the form of several large connected sinuses and serves as a hydrostatic skeleton in the locomotion of some species. The two-chambered heart is situated in the haemocoelomic cavity. The type of circulation is open type. In open type the capillaries are absent and the blood bathes tissues directly.
12. Respiratory pigment is usually haemocyanin, a oxygen-carrying blood pigment. Haemoglobin is found in a number of species (e.g., Solen, Arca, Planorbis).
13. Excretory organs in most cases include kidneys or called metanephridia.
14. A well-defined nervous system with a primitive brain in most cases, consists of a circumoesophageal nerve ring with a pair of pedal nerve cords and a pair of visceral cords.
15. Sexes usually separate (gonochoristic or dioecious) but hermaphrodite (monoecious) in few groups. Mostly non-marine gastropods, some bivalves and cephalopods are hermaphrodites.
16. Fertilization may be external or internal. External fertilization takes place in many marine gastropods and most bivalves, but internal fertilization in non- marine gastropods.
17. Development through spiral cleavage.
18. In marine and freshwater forms the development involves by two larval stages—the multiciliated trochophore is the first larval stage and the veliger is the second larval stage.
19. Direct development mainly in Gastropoda and Cephalopoda.
5. Classification of Molluscs:
Classification in Outline (Up to Orders):
The classificatory scheme followed in the present text is based largely on the plan laid down by E.E. Ruppert and R.D. Barnes (1994) in their book “Invertebrate Zoology”.
Phylum Mollusca:
Class 1. Aplacophora
Subclass Chaetodermomorpha (= Caudofoveata)
e.g., Chaetoderma.
Subclass Neomeniomorpha (= Solenogastres)
e.g., Neomenia, Proneomenia.
Class 2. Polyplacophora
Order Palaeoloricata
Order Neoloricata
e.g., Lepidopleurus, Cryptoplax.
Class 3. Monoplacophora (Gastroverms)
e.g., Neopilina.
Class 4. Gastropoda
Subclass Prosobranchia
Order Archaeogastropoda (Diotocardia)
e.g., Haliotis, Fissurella, Patella, Trochus, Nerita.
Order Mesogastropoda
e.g., Pila, .Turritella, janthina, Strombus, Lambis, Cypraea, Cymatium, Tonna.
Order Neogastropoda
e.g., Murex, Thais, Oliva, Mitra, Conus, Terebra.
Subclass Opisthobranchia
Order Cephalaspidea
e.g., Bulla.
Order Pyramidellacea
e.g., Pyramidella.
Order Acochlidioidea
e.g., Acochlidium.
Order Anaspidea or Aplysiacea
e.g., Aplysia.
Order Notaspidea
e.g., Pleurobranchus.
Order Sacoglossa
e.g., Oxynoe, Elysia.
Order Thecosomata
e.g., Cavolina.
Order Gymnosomata
e.g., Cliapsis.
Order Nudibranchia
e.g., Doris, Kalinga.
Subclass Pulmonata
Order Systellommatophora
e.g., Onchidium
Order Basommatophora
e.g., Planorbis, Limnaea.
Order Stylommatophora
e.g., Helix, Achatina.
Class 5. Bivalvia
Subclass Protobranchia
Order Nuculoida
e.g., Nucula.
Order Solemyoida
e.g., Solemya.
Subclass Pteriomorphia
Order Arcoida
e.g., Anadara, Arca
Order Mytiloida
e.g., Mytilus, Modiolus.
Order Pterioida
e.g., Pteria, Pinna.
Order Ostreoida
e.g., Ostrea, Pecten, Chlamys, Placuna.
Order Limoida
e.g., Lima.
Subclass Palaeoheterodonta
Order Unionoida
e.g., Unio, Anodonta.
Order Trigonioida
e.g., Trigonium.
Subclass Heterodonta
Order Veneroida
e.g., Cardium, Mactrea, Tellina, Solen, Ensis.
Order Myoida
e.g., Mya, Pholas, Teredo, Bankia.
Order Hippuritoida
e.g., Chama.
Subclass Anomalodesmata
e.g., Pandora, Poromya
Class 6. Scaphopoda
e.g., Dentalium.
Class 7. Cephalopoda (= Siphonopoda)
Subclass Nautiloidea (= Tetrabranchiata)
e.g., Nautilus.
Subclass Ammonoidea
e.g., Ammonites.
Subclass Coleoidea (= Dibranchiata)
Order Belemnoidea
e.g., Belemnites.
Order Sepioidea
e.g., Sepia, Sepiola, Spirula, Rossia.
Order Teuthoidea
e.g., Loligo, Sepiolenthis, Architeuthis, Onychoteuthis, Chiroteuthis.
Order Vampyromorpha
e.g., Vampyroteuthis.
Order Octopoda
e.g., Opisthoteuthis, Octopus.
Classification with Characters:
Class 1. Aplacophora:
[Gk. a = without + plakos = a sheet of wood + phoros = a bearing]
Features:
1. It includes primitive molluscs. Elongated bilaterally symmetrical wormlike bodies covered by cuticle.
2. Cuticle bears calcareous spicules.
3. Body lacks a distinct head, and shell complexly absent.
4. Foot absent or reduced to a ventral ridge.
5. Intestine uncoiled.
6. Vascular system rudimentary.
7. Nervous system is primitively built and consists of two pedal and two pleural nerve cords which are connected anteriorly with an oesophageal nerve ring. The cerebral ganglion is either single or double.
8. Sexes united or separate.
9. Sex cells discharged into the pericardial cavity.
10. Fossil forms are not known.
Habitat:
Marine, found in Australian oceans, from shallow water (200 m) to mostly in deep water (3000 m).
It includes two subclasses:
Subclass 1. Chaetodermomorpha (= Caudofoveata):
1. Mid-ventral ridge is lacking.
2. A pair of bipectinate gills in the mantle cavity.
3. Sexes separate.
4. Burrowing footless aplacophorans
It includes 3 families and approximatley 120 species.
Examples:
Chaetoderma (Fig. 16.54), Falcidens, Limifossor.
Subclass 2.
Neomeniomorpha (= Solenogastres) [Gk. solenos = a channel + gaster = stomach]
1. Mid-ventral longitudinal groove with ciliated ridge helps in locomotion.
2. Gills absent.
3. Sexes united.
4. Animals are associated with cnidarians.
It includes 21 families with approximately 200 species.
Examples:
Neomenia, Proneomenia, Strophomenia, Pruvotina, Epimenia.
Class 2. Polyplacophora (Chitons):
[Gk. poly = many + plakos = a sheet of wood]
Features:
1. Bilaterally symmetrical untorted bodies.
2. Body more or less oval, dorsoventrally flattened.
3. Dorsal side convex and flat ventral side.
4. Dorsal side is covered by transversely placed 7 to 8 shell plates, held together by a surrounding fleshy thick girdle. The shell plates are arranged in an imbricate fashion.
5. Ventral side occupies a broad sucker-like creeping foot.
6. Head incopspicuous, but eyes and tentacles absent.
7. Multiple gills (6-80 pairs) located in the mantle grove.
8. A pair of kidneys present.
9. Free-swimming trochophore larva present but veliger larva absent in the life cycle.
They are called the armadillos of the sea and also called chitons.
Origin:
Ordovician.
Habitat:
Exclusively marine, found in the intertidal zone, remain attached to the rocks and crevices.
It includes 2 orders:
Order 1. Palaeoloricata:
Palaeozoic and Mesozoic chitons in which the middle calcareous layer absent in shell plates. Priscochiton (Ordovician), Helminthochiton (Silurian), Gryphochiton (Carboniferous).
Order 2. Neoloricata:
Fossil and living polyplacophorans, and the middle calcareous layer present in shell plates.
13 families and about 600 living species.
Examples:
Lepidopleurus, Mopalia (Hairy Chitons), Hanleya, Hemiarthrum, Choriplax, Chitonellus, Chaetopleura, Tonicella, Loricata, Acanthochiton, Chiton (Common Chiton), Ischnochiton (Giant chitons), Lepidochiton, Cryptoplax, Katharina [Fig. 16.54(i)]
Class 3. Monoplacophora (Gastroverms):
[Gk. mono = single; plakos = a sheet of wood; phoros = a bearing. Gk. gastros = stomach, verms = worm.]
Features:
1. Body bilaterally symmetrical and remains covered by a mantle.
2. Dorsal side covered by a shield-like shell.
3. Ventral side occupied by a flat creeping foot.
4. Multiple pairs of monopectinate ctenidia (3-6 pairs), nephridia (6-7 pairs) and foot retractor muscles (usually 8 pairs) repeat along the length of the body.
5. Well-formed coelom.
6. Head reduced and eyes absent.
7. Radula and velar lobes present.
8. Sexes separate except Micropilina arntzi which is hermaphrodite.
9. Exclusively marine
Geological record:
Mostly extinct (Lower Cambrian to Devonian), a few living species.
Geographical distribution:
Deep waters of Pacific and the North and South Atlantic and the Indian Ocean.
Examples:
19 species have already described and included in a single family. Neopilinidae (Pechenik, 2000). Neopilina, Vema, Micropilina, Rokopella, Laevipilina.
Class 4. Gastropoda (Gk. gastros = stomach, podos = foot) [Limpets, Snails, Slugs, Whelks]. Approx. 30,000 species.
Features:
1. Asymmetrical bodies due to torsion and detortion.
2. A single piece spirally twisted shell. Shell sometimes reduced or absent.
3. Head well-differentiated bearing eyes and tentacles in most cases.
4. Foot usually flat on the ventral surface, situated behind the head usually used for creeping.
5. Mantle and mantle cavity containing typically two plume-like ctenidia or a modified lung. Ctenidia may be single or replaced by the secondary gills.
6. Buccal cavity with an odontophore and a radula.
7. Intestine much coiled.
8. Siphons often present.
9. An operculum on the upper surface of the foot which closes the shell aperture may be present.
The Gastropoda is the largest living class among molluscs and includes about 30,000 to 70,000 living species.
Origin:
Early Cambrian Period.
Habitat:
Marine, freshwater and terrestrial. Among the members of 7 classes some members of this class have ventured onto land and a few are parasitic.
It contains 3 subclasses:
Subclass 1. Prosobranchia (= Strepto- neura) [Gk. prosos = forward + branchia = gills; Streptoneura, Gk. streptos = twisted + neuron = nerve]
Features:
1. Gills when present are placed anterior to the heart, so the name of the subclass is Prosobranchia.
2. Visceral hump is twisted due to torsion. So the pleuro-visceral connectives are crossed like figure ‘8’, hence the other name is Streptoneura.
3. Shell and operculum present in most cases.
4. Mantle cavity with organs generally anterior, due to torsion.
5. Aquatic species with one or two gills within mantle cavity.
6. Sexes separate.
140 families with approximately 20,000 species.
Habitat:
Mainly marine, some freshwater, a few terrestrial.
The subclass prosobranchia is divided into 3 orders:
Order 1. Archaeogastropoda (= Diotocardia, Aspidobranchia) [Archaeogastropoda. Gk. archaios – primitive, podos = foot]
(i) Primitive gastropods with two auricles, two usually bipectinate gills, two nephridia and two osphradia generally present.
(ii) Right gill may be reduced or absent, even if only one gill present, it is bipectinate.
(iii) Siphon and proboscis absent.
(iv) Operculum present in a few cases.
(v) Nervous system exhibits less concentration.
They are sometimes called Aspidobranchia (Gk. aspid = shield, branchia = gills). It is a taxonomic order of abalones, limpets, top shells, etc. used in conventional classification.
Habitat:
Mostly marine, a few freshwater and terrestrial.
Examples:
Slit snails (Pleurotomaria, Scissurella); Abalones (Haliotis); key hole limpets (Scutus, Diodora, Fissurella, Macrochisma); True limpets (Acmaea, Patella, Cellana, Lepeta); Deep sea limpets (Cocculina); Top shells (Trochus, Monodonta, Bankivia, Tegula, Calliostoma, Astele, Gibbulla, Lischkela, Diloma, Clanculus, Tectus); Star shell (Astraea); Turbans (Turbo, Phasianella); Nerit shells (Nerita, Neritina)-all marine; Septaria (freshwater and brackish water); Hydrocena, Alcadia (all terrestrial forms).
Order 2. Mesogastropoda (= Taenio- glossa, Monotocardia or Pectinibranchia) [Mesogastropoda, Gk. mesos = middle]:
(i) Single monopectinate gill (only the lamellae occur on one side of the taenidial axis), single auricle, single osphradium and single kidney present as the consequence of torsion.
(ii) Siphon and operculum and penis present.
(iii) Radula taenioglossate, i.e., radula consists of seven teeth in each transverse row.
(iv) Nervous system lacks pedal nerve cords.
(v) Fertilization internal.
Habitat:
Mainly marine, many freshwater and terrestrial forms but the members of Entochonchidae (e.g., Entoconcha, Entocolax, Enteroxenos) are endoparasites of sea cucumbers).
Examples:
Cyclophorus, Viviparus, Pomacea, Pila—all freshwater genera; Lacuna— cold water marine snail; Periwinkles (Littorina—shore between tide marks and also in mangrove swamps, Tectarius—warm and tropical seas); Pomatias—terrestrial; Rissoina—warm seas; Turret snails (Turritella—almost all seas); Sundial shells (Architectonica, Heliacus—warm and tropical seas); Planaxis—tropical seas; Siliquaria, Vermicularia—warm seas; worm shells (Vermetus); Telescopium, Terebralia— muddy shores near high tide level); Horn snails (Cerithium—marine); Violet snails (Janthina—pelagic, marine); Eulima, Stylifer (marine, mostly ectoparasite on echinoderms); Thyone, Entoconcha, Entocolax, Enteroxenos (endoparasites of sea cucumbers (holothurians); Carrier snails (Xenophora, Stellaria—shallow or moderately deep water in tropical seas); Wentletraps (Epitonium—all seas); Capulus, Calyptraea, Crucibulum—permanently attach themselves to shells or other hard objects, marine); Slipper snails (Crepidula—warm and temperate seas); Conchs (Strombus—marine); Terebellum, Tibia—tropical seas; Spider conch (Lambis—tropical seas); Cowries (Trivia, Erato, Pedicularia, Jenneria, Cypraea—tropical seas); Volva, Ovula— Indo-Pacific; Moon snails (Natica, Polinices, Sinum, Lunatum—all seas); Helmet snails (Morum, Phalium, Casmaria, Cassis, Cypraecassis); Triton snails (Apollon, Charonia, Cymatium— all marine); Frog shells (Bursa—generally found on sand or coral rubble); Tun snails (Tonna, Malea—tropical and temperate seas); Fig shell (Ficus—Indo- Pacific); pelagic snails with finlike foot, marine (Atlanta, Carinaria and Pterotrachea).
Order 3. Neogastropoda (= Stenoglossa) [Neogastropoda = Gk. neos = new, young]
Most of the characters are similar with mesogastropods except:
(i) Radula usually rachioglossate (= Stenoglossate) type, i.e., 3 teeth in each row of radula (radular row with a median tooth [rachidian], and lateral and marginal teeth) except the superfamily conacea.
(ii) A single monopectinate ctenidium, a single kidney, and a heart with an auricle.
(iii) A complex osphradium present.
(iv) Siphonal canal much elongated.
(v) Mostly carnivores.
Habitat:
The members of all genera are marine.
Examples:
Rock shells (Murex, Haustellum, Chicoreus, Hexaplex, Muricanthus, Ceratostoma, Pteropurpura, Homalocantha, Drupa, Thais, Rapana); Axis shells (Latiaxis); Mud snails (Nassarius); Fulgar whelks (Busycon, Buccinum); Tulip snails (Fasciolaria, Fusinus); Dove shells (Columbella); Olives (Olivet, Ancilla, Olivancillaria); Mitre snails (Pusia, Vexillum Mitra, Strigatella); Vase shells (Vasum, Tudicla, Tudicula); Chank (Xancus); Harp snails (Harpa); Volute snails (Voluta, Lyria, Cymbium, Cymbiola); Cone shells (Conus); Turrial snails (Turris, Nihonia, Turricula, Perrona); Japanese wander shells (Thatcheria); Augar shells (Terebra).
Subclass 2. Opisthobranchia (Euthy- neura) [Gk. opisthen = behind + branchia = gills]
Features:
1. There are single plicate gill, single auricle and single nephridium but display detorsion.
2. Gills, if present, placed behind the heart generally, hence called Opisthobranchia.
3. Auricle usually placed behind the ventricle.
4. Ctenidia often replaced by secondary gills (= branchiae).
5. Reduced mantle cavity shows a tendency to occupy in the posterior side due to detorsion of the visceral hump.
6. Shell and operculum usually reduced or absent. Shell absent in Doris or present in mantle cavity in Aplysia.
7. Head commonly with two pairs of tentacles.
8. Nervous system shows anterior concentration of ganglia and posterior part shows ethyneurous (Gk. euthus = straight, neuron = nerve) condition (untwisted loop).
9. All hermaphroditic.
120 families include about 2000 species.
Habitat:
Mostly marine, and many such like pyramidellids are parasitic.
It includes 9 orders:
Order 1. Cephalaspidea:
(i) Presence of head shield is the most peculiar feature.
(ii) Shell usually present and remains partly or wholly within mantle.
(iii) Parapodial lobes present.
(iv) Single plicate gill (folded gill) on right side.
Examples:
Bubble shells (Acteon, Cylichna, Bulla, Hydatina, Scaphander).
Order 2. Pyramidellacea:
(i) Ecoparasites of bivalves and polychaetes.
(ii) A spirally coiled shell and an operculum present.
(iii) Gills and radula are lacking.
(iv) A long retractile proboscis with a stylet present.
Examples:
Pyramidella, Odostomia, Brachystomia.
Order 3. Acochlidioidea:
(i) Shell and gills are lacking.
(ii) Visceral mass separated from the foot.
(iii) Most species small (2 mm to 5 mm).
(iv) Mostly marine, a few are freshwater, found in West Indies, Palau and Indonesia.
Examples:
Acochlidium, Hedylopsis, Unela, Ganitus.
Order 4. Anaspidea or Aplysiacea:
(i) Shell reduced to a flattened plate and remains concealed in the mantle except Akera where shell is external.
(ii) Gill and mantle cavity present.
(iii) Lateral parapodial lobes well-formed.
(iv) Head bears a pair of cephalic tentacles and a pair of rhinophores (2nd pair). First pair tentacle is called oral tentacle.
(v) The male gonopore is situated at the base of the right rhinophore.
Examples:
Sea hares, Aplysia, Petalifera, Notarchus, Bursatella, Akera.
Order 5. Notaspidea:
(i) Shell may or may not be present.
(ii) Mantle present but without mantle cavity.
(iii) Gill bipectinate type.
(iv) Gill and osphradium on the right side of the body.
Examples:
Tylodina, Umbraculum, Pleurobranchus (Fig.16.56A).
Order 6. Sacoglossa (L. saccus = sac; Gk. glossa = tongue):
(i) Shelled [e.g., Berthelinia (Fig. 16.56B), Julia] or shell-less (e.g. Elysia) opisthobranchs. Berthelinia and julia have bivalved shells.
(ii) Mantle cavity, gill and osphradium may be present (e.g., Berthelinia) or absent (e.g., Elysia).
(iii) Radula bears a single row of teeth adapted for suctorial feeding.
(iv) Parapodia and cerata may be present.
Examples:
Oxynoe, Lobiger, Berthelinia (Fig. 16.56B), Julia, Elysia (Fig. 16.56C), Caliphylla.
Order 7. Thecosomata (= Pteropoda):
[Thecosmata = Gk. theke = case + Gk. soma = body]
(i) Pelagic opisthobranchs with large muscular wing-like parapodia.
(ii) Mostly shelled forms.
(iii) Shell placed externally and of different types.
Examples:
Sea butterflies or shelled pteropods (Spiratella, Limacina, Cavolina, Clio, Cymbulia, Corolla, Desmopterus).
Order 8. Gymnosomata:
[Gk. gymnos = naked + Gk. soma = body]:
(i) Species are pelagic, and shell and mantle cavity are absent.
(ii) Parapodial fins are present.
Examples:
Naked pteropods (Cliopsis, Pneumoderma).
Order 9. Nudibranchia:
[L. nudus = naked + branchiae = gills]
(i) Shell, mantle cavity, gill and osphradium absent.
(ii) Body bears numerous cerata or respiratory outgrowths. Some have simple cerata (e.g., Aeolida, Glaucus), or simple to branched cerata (e.g., Dendronotus, Tritonia).
(iii)Some species have secondary gills around anus (e.g., Doris, Glossodoris) and some have plate-like gills beneath mantle edge or cerata (e.g., Armina).
(iv) Visceral loop greatly reduced.
Examples:
Nudibranchs or Sea slugs [Doris, Glossodoris (Fig. 16.56D), Chromodoris, Cadlina, Jorunna, Kalinga, Tritonia, Dendronotus (Fig. 16.56E), Armina, Aeolidia, Glaucus, Pleurophyllidia, Eolis, Aeolis, Tethys).
Subclass 3. Pulmonata:
[L. pulmonatus = having lungs]
Features:
1. True gill is absent and respiration is performed by pulmonary sac or lung. Mantle cavity is transformed into a pulmonary sac.
2. There are a single auricle and a single kidney.
3. Shell may or may not be present, but operculum is absent.
4. Nervous system is secondarily symmetrical due to detorsion.
5. They are hermaphroditic.
Origin:
First appear in the Devonian Period.
About 16000 extant species.
Habitat:
Mostly terrestrial or freshwater, a few are marine.
It includes 3 orders:
Order 1. Systellommatophora:
(i) Pulmonates possess anus at the posterior end of the body.
(ii) Single pulmonary sac present but the lung absent in Veronicellidae.
Examples:
Marine slugs (Onchidium, Rhodope).
Order 2. Basommatophora:[Gk. basis = base, omma = eye, pherein = to bear]
(i) Eyes located at the base of the tentacles.
(ii) One pair of tentacles present.
Habitat:
Mainly freshwater forms; a few are marine and brackish water forms.
Examples:
Planorbis (Indoplanorbis), Lymnaea, Anisus, Bulinus, Physa (freshwater forms); Siphonaria—marine; Amphibola (the estuarine form and the most unique having operculum in this genus).
Order 3. Stylommatophora: [Gk. stylos = pillar, omma = eye, pherein = to bear]
(i) Two pairs of retractile tentacles present.
(ii) Eyes located at the tip of the posterior pair of tentacles.
(iii) Mostly terrestrial and sometimes found in the gardens and parks on the trees. Many members lead to amphibious life.
Examples:
Achatina, Achatinella, Partula, Pupilla, Clausita, Helix, Glessula, Ancylus, Macrochlamys, Ariophanta, Planispira Succinea, Subulina, Rachis, Limax, Milax, Zebrina.
Class 5. Bivalvia (= Pelecypoda)—Cockles, Mussels, Oysters, Clams. [L. bi = two + valvae – folding doors; Gk. pelekys = a hatchet + podos, pons = a foot]—Approx. 20,000 species.
Features:
1. Body bilaterally symmetrical and compressed laterally, which is enclosed by two lateral shells (valves), secreted by the mantle, and are hinged dorsally.
2. Foot is a laterally compressed muscular organ, usually hatchet or plough-share shaped and is placed ventrally, commonly used for crawling or for burrowing purposes.
3. There are two leaf-like lobes, one on each side of the body, forming the mantle which secretes the shell.
4. Indistinct head and a radula, jaws, eyes and tentacles absent.
5. Mouth is provided with two pairs of labial palps, and most bivalves are ciliary feeders.
6. The posterior margin of the mantle forms the paired tube-like, protrusible siphons for passing of inhalant or exhalant current.
7. Respiratory organs are a pair of plate -like ctenidia and are used in respiration and often for food collection.
8. Mostly unisexual.
9. Fertilization external.
10. Development indirect and completed by trochophore and veliger larvae.
Bivalvia means possession of two pieces of enclosing valves and Pelecypoda means hatchet-footed ones. Sometimes Lamellibranchiata (means plate gill) is applied to this class.
Origin:
Lower Ordovician.
Habitat:
Mostly marine and some are freshwater, and no terrestrial forms.
The class is divisible into 5 subclasses:
Subclass 1. Protobranchia [Gk. proto = first, primitive + branchiae = gills]
Features:
1. Most primitive bivalves.
2. There are two bipectinate gills, each with two rows of gill filaments (protobranchiate).
3. Usually isomyarian (more or less equal adductor muscles).
4. There are a pair of palpal tentacles.
5. Siphons usually absent.
6. Foot is not compressed but has a flattened ventral surface upon which the animal creeps.
Habitat:
Marine.
The subclass is divided in 2 orders:
Order 1. Nuculoida:
(i) Shell small to triangular and equivalve.
(ii) Numerous teeth arranged in a row on each side of ligament.
Examples:
Nucula (distributed in most seas), Yoldia (cooler water).
Order 2. Solemyoida:
(i) Thin, fragile, equal, somewhat elongated shells.
(ii) Hinge has no teeth.
Distribution:
The animal buries itself in sand or mud, found in the east coast of N. America and Mediterranean coasts.
Example:
Solemya.
Subclass 2. Pteriomorphia
Features:
1. Specimens are attached to the solid substratum by byssus threads or by cementation but some become secondarily free.
2. Mantle margins are not fused.
3. Foot small or poorly developed.
4. Filibranchiate (i.e., Bivalve gills consisting of individual filaments that linked together by tufts of specialized cilia).
Habitat:
Marine and freshwater.
The subclass in divided into 5 orders:
Order 1: Arcoida:
(i) Hinge straight and with numerous small, similar V-shaped or straight teeth.
(ii) Adductor muscles equal or subequal.
(iii) Shell equivalve, sub-quandrangular.
Distribution:
Warm and tropical seas.
Examples:
Ark shells (Area, Barbatia, Trisidos, Anadara, Senilia, Noetia, Glycymeris).
Order 2. Mytiloida:
(i) Bivalves attached in the hard substratum by byssus threads.
(ii) No byssal notch but a gap for the byssus.
(iii) Hinge with few teeth or absent.
(iv) Anterior adductor muscle small.
Distribution:
In shallow water of most seas.
Examples:
Sea mussels (Mytilus, Brachidontes, Ischadium, Perna, Septifer, Musculus, Modiolus, Lithophaga).
Order 3. Pterioida:
(i) Shells usually thin, brittle with triangular, fan-shaped, wing-like and more or less equal.
(ii) Abductor muscles unequal or monomyarian.
(iii) Mostly attached by byssus threads.
Distribution:
Temperate, warm and tropical seas.
Examples:
Pen shells (Pinna, Atrina), winged oysters (Pteria, Pinctada, Malleus).
Order 4. Ostreoida:
(i) Shells thick or thin with unequal or equal valves.
(ii) Left valve usually attached to other objects by a byssus or cemented by right valve.
(iii) Adductor muscles are aniso or monomyarian.
(iv) Ornamented with irregular, concentric shell layers.
Distribution:
Widely distributed in most of the seas.
Examples:
Scallops [Amusium, Chlamys, Decatopecten, Pecten, Fig. 16.55(ii) A], Thorny oyster (Spondylus), Jingles (Anomia), Window pane clams (Placuna), Oysters (Ostrea, Saccostrea, Crassostrea).
Order 5. Limoida:
(i) Equivalve shells.
(ii) They are attached by byssus threads or some are free.
(iii) Hinge teeth weak or absent.
(iv) Adductor muscles monomyarian.
Distribution:
Occur in most seas.
Examples:
File clams (Lima, Ctenoides).
Subclass 3: Palaeoheterodonta [Gk. palaeo = ancient, old; heteros = other + odous = tooth]
Features:
1. Equal two valves with a few hinge teeth.
2. Two equal sized adductor muscles.
3. Gills are Filibranch or eulamelli- branchiate.
4. Siphons usually absent.
5. An inner nacreous layer present.
Habitat:
Marine and freshwater.
The subclass is divided into 2 orders.
Order 1. Unionoida:
(i) Shells oblong, ovate, elliptical or rhomboidal.
(ii) Hinge always complete with well- developed teeth.
(iii) Single cardinal tooth may be present.
Habitat:
The genera of the order inhabit the muddy or sandy bottoms.
Examples:
Freshwater mussels (Unio, Margaritifera, Anodonta, Lamellidens, Indonoia, Nodularia).
Order 2. Trigonioida:
1. Shells triangular in outline.
2. Median tooth of left valve concave or deeply emarginated below.
3. Two teeth of right valve have transverse ridges.
Example:
Only one genus Neotrigonia is living in Australian (South Australia and Tasmania) waters.
Subclass 4. Heterodonta [Gk. heteros = other, different + odous = tooth]
Features:
1. Shells are equal-sized and with a few large cardinal and lateral hinge teeth.
2. Siphons are usually present.
3. Shell without nacreous layer.
4. Eulamellibranch gills (the gill filaments of a bivalve gill are connected by tissue junctions).
5. Absence of byssu gland in the adult foot.
Habitat:
Marine and freshwater.
This subclass includes 3 orders:
Order 1. Veneroida:
(i) Shells equivalve, circular, ovate or trapezoidal and moderately compressed and isomyarian.
(ii) Two cardinal teeth.
(iii) Pallial sinus absent.
Habitat:
All seas.
Examples:
Lucina, Codakia, Divaricella, Chama; Entovalva (only endoparasitic bivalve, lives in the sea cucumber); Cockles (Cardium, Cardita, Acanthocardia, Trachycardium, Fragum, Corculum), Giant clam (Tridacna, Hippopus), Surf clam (Mactra, Scissodesma, Lutraria), Rajor clams or Jacknife (Solen, Ensis, Phaxas, Siliqua), Tellin clams (Tellina, Strigilla, Tellidora, Gastrana), Wedge clams (Donax, Iphigenia, Gari), Venus clams (Venus, Circe, Gafrarium, Meretrix, Trivela, Callista); Rock borer (Petricola, Mysia. They can bore into mud, chalk and coral).
Order 2. Myoida:
(i) Inequivalve, the right valve is more convex than the left.
(ii) No true teeth on hinge plate.
(iii) Pallial sinus usually large.
(iv) Siphons very long and broad.
Habitat:
Most species are burrowers in sand and mud in warm seas.
Examples:
Soft shell clams (Mya), Boring clams (Pholas, Barnea, Martesia, Xylophaga), Wood borer in shipworms (Teredo, Bankia).
Order 3. Hippuritoida:
(i) Irregularly rounded thick shells.
(ii) One valve cemented to substrate permanently or temporally.
(iii) At least one large cardinal tooth in either valve.
(iv) No pallial sinus.
Habitat:
Tropical seas.
Examples:
Jewel boxes (Pseudochama, Chama).
Subclass 5. Anomalodesmata [Gk. an = not + homalos = even + desmos = bond]
Features:
1. Shell varied in shape but thin.
2. No hinged teeth.
3. Mantle edges extensively fused and siphonate.
4. Byssus threads rudimentary or absent.
5. Two equal adductor muscles.
6. Eulamellibranchiate or septibranchiate (i.e. Bivalves in which the gills are modified to form a muscular septum).
7. Hermaphrodite.
Habitat:
Marine, found in most seas.
Examples:
Watering-pot clams (Lyonsia, Pandora, Clavagella), Poromya, Cuspidaria.
Class 6. Scaphopoda [Tusk shells] [Gk. skaphe = a boat + podos – a foot] Approx. 400 species.
Features:
1. Tusk-shaped conical shell, open at both ends (Fig. 16.58).
2. Anterior part of the shell much wider than the posterior end.
3. Foot narrow, trilobed (wedge-shaped) and capable of being protruded through the anterior opening of the shell.
4. Small thread-like with ciliated, adhesive knobbed tentacles, called captacula (Fig. 16.58), that surround the mouth being used both as sensory and for feeding.
5. Mouth on the oral proboscis.
6. Radula present but eyes absent.
7. Gills, heart and circulatory system lacking.
8. Gonad unpaired and kidneys parried.
9. Fertilization external.
10. Eggs planktonic.
11. Both trochophore and veliger larva in the life cycle.
Geological period:
Cambrian—Present.
Habitat:
Exclusively marine; widely distributed in all seas. The animals remain buried on sandy or muddy sea bed with posterior end.
It includes two families:
(1) Siphonodentalidae and
(2) Dentalidae.
Family 1. Siphonodentalidae:
(i) Small animals with vermiform elongated foot.
(ii) Shell translucent and fragile.
(iii) Shell swollen in the middle.
Examples:
Siphonodentalium, Cadulus, Pulsellum, Entalina.
Family 2. Dentalidae:
(i) Comparatively larger and stronger species.
(ii) Trilobed foot.
(iii)Shell tubular, tapering and colourless.
(iv) Posterior opening of the shell often notched.
Examples:
Dentalium (Fig. 16.57), Antalis.
Class 7. Cephalopoda (= Siphonopoda) [Cuttle fish, Squids, Nautiluses, Octopuses] = head + Gk. podos = foot] living species and 40,000 fossil species.
Features:
1. Body bilaterally symmetrical with a well-developed head that bears a crown of mobile muscular appendages surrounding the mouth.
2. Foot modified into oral appendages.
3. Appendages bear suckers or hooks except Nautilus.
4. Shell usually internal either reduced or absent (e.g., Idiosepius and some octopuses) and enveloped by the mantle in most species. An external shell occurs only in Nautilus.
5. Mouth with horny or calcareous beaklike jaws.
6. A funnel or siphon present which expels water from the mantle cavity, helps in jet propulsion.
7. Gills (ctenidia) are a single pair (e.g. cuttle fishes, squids and octopuses) or two pairs (e.g., Nautilus).
8. Blood vascular system with branchial heart, arteries, veins and capillaries present. Haemocyanin in the blood present.
9. Most of the cephalopods possess an ink gland except Nautilus with a duct opening into the rectum that produces a brown or black ink (sepia) containing melanin pigments and serves as a protective smoke screen.
10. Nervous system complex and highly developed.
11. Highly developed eyes.
12. Sexes separate; external sexual dimorphism in some species.
13. Males in many cases possess 1 or 2 modified spoon-like arms, called hectocotylus, for transferring spermatophores to the female.
14. Cleavage meroblastic; spiral cleavage absent.
15. Development direct.
Origin:
Lower Cambrian.
Habitat:
Exclusively marine, found from the surface to the 5000 m depths. They are found as epibenthic and pelagic in the seas or in the open oceans.
It is divided into 3 subclasses:
(i) Nautiloidea
(ii) Ammonoidea
(iii) Coleoidea.
Subclass 1. Nautiloidea (= Tetrabranchiata)—Chambered nautiluses [L. Nautilus = nautilus + Gk. eidos = form]:
Features:
1. Shell external which may be chambered and coiled, Suture simple.
2. Circumoral appendages vary 63-94.
3. Tentacles suckerless.
4. Hood, the enlarged outer lobe of the foot serves to close the aperture when the animal withdraws tentacles into the shell.
5. Funnel or infundibulum incomplete and consists of two separate parts.
6. All the chambers except the last or body chamber are filled with gas, giving buoyancy to the shell.
7. Siphuncle (= Siphonal tube) present and helps in swimming.
8. Eyes simple and without lenses or cornea.
9. Four ctenidia, four auricles and four kidneys present. The other name of Nautiloidea is also called Tetrabranchiata for the presence of 4 ctenidia.
10. Inc sac and chromatophores lacking.
11. Osphradia are present in Nautilus among cephalopods.
Geological record:
They have recorded since the Lower Cambrian Period and all the members have become extinct during Jurassic Period except Nautilus (Fig. 16.60A). The earliest nautiloids were entirely uncoiled and later the coiling forms are seen. Some fossil genera are Volborthella, Actinoceras, Gomphoceras, Orthoceras, Phragmoceras, Lituites and Aturia, etc.
Geographical distribution:
They are found in the Indo-West Pacific region at the depth of 500 m. In India the chambered nautiluses, Nautilus pompilius are found mainly in and around the coasts of Andaman and Nicobar Islands. Very rarely they are found at the coasts of Digha (W. Bengal), Talsari (Odisha) and South Indian coasts. This species is characterized by filled umbilicus with a concretion and colour pattern extending nearly entire shell.
The subclass contains a single family Nautilidae and is represented by four extant species:
(i) Nautilus macromphalus,
(ii) Nautilus pompilius,
(iii) Nautilus belauensis and
(iv) Nautilus scrobiculatus.
N. macromphalus is found in Western Pacific, New Caledonia and Loyalty Islands. N. belauensis found in Palau and Western Caroline Islands, and N. scrobiculatus in Solomon Island and New Guinea.
Subclass 2. Ammonoidea (Ammonites):
Features:
1. All extinct with coiled external shell.
2. Coil is piano-spiral type.
3. Complex suture lines and sculptured external surface.
4. Siphuncle generally at the outer.
Geological record:
They have recorded in Upper Silurian to Upper Cretaceous.
Examples:
Agoniatites (Lower Devonian), Trachyceras (Triassic), Pinacoceras (Triassic), Phylloceras (Fig. 16.59A) (Jurassic-Cretaceous), Macroscanoceras (Fig. 16.59B) (Jurassic-Cretaceous), Stephanoceras (Fig. 16.59B) (Jurassic- Cretaceous), Macroscaphites (Fig. 16.59C) (Lower Cretaceous), Turrilites (Fig. 16.59D) (Upper Cretaceous).
Subclass 3. Coleoidea (= Dibranchiata) [Gk. Koleos = sheath]:
Features:
1. Oral appendages bear suckers.
2. Shell internal which may be reduced or absent, and when present completely surrounded by mantle tissue.
3. Two ctenidia, two auricles and kidneys present.
4. Funnel tube-like.
Dibranchiata is also called for the presence of two gills.
Geological record:
They range from Mississippian to the present.
The subclass Coleoidea is divided into 5 orders and are approximately 1000 extant species distributed in 43 families.
Order 1. Belemnoidea:
(i) They are all extincts.
(ii) Shell internal, unbranched and conical.
(iii) The posterior rostrum solid and varies considerably in shape and size.
(iv) Phragmacone (i.e., the conical posterior terminal part of the cuttle bone or gladius, homologous to the phragmacone of fossil teuthoids) is a hollow cone and divided into chambers by septa.
Geological record:
They have recorded from Upper Mississippian to Cretaceous.
Examples:
Belemnites, Belemnetella, Dactyloteuthis, Pachyteuthis.
2. Sepioidea [Cuttle fish, Sepiolids]:
(i) Cuttle fishes are broad, sac-like or flattened dorsoventrally with lateral fins that either are narrow and extend the whole length of the body (e.g., Sepiidae) or are short, become round and flap-like (e.g., Sepiolidae).
(ii) Posterior fin lobes (lateral fins) not connected at the midline.
(iii) Members of the order possess 8 arms and 2 long tentacles.
(iv) Tentacles retractile into pockets between the arms III and IV.
(v) Internal shell (cuttle bone = sepion) may be straight, coiled laminate or rudimentary or lost (e.g., Sepiadariidae).
(vi) Internal shell may be calcareous (e.g., Sepia, Spirula, Fig. 16.60B) or chitinous (Sepiolids).
(vii) Suckers stalked (pedicel) with chitinous rings.
The order includes 5 families:
(i) Sepiidae, e.g., Sepia (Fig. 16.60C), Sepiella, etc.
(ii) Sepiadariidae, e.g., Sepiadarum,
(iii) Sepiolidae, e.g., Sepiola, Euphrymna, Sepietta, Rossia (Fig. 16.60D), Neorossia,
(iv) Idiosepiidae, e.g., Indiosepius and
(v) Spirulidae, e.g., Spirula.
Geographical Distribution:
They are inhabitants of warm and temperate seas.
Habitat:
They are generally bottom dwellers of sandy, rocky and muddy areas, even found in coral reef areas.
Order 3. Teuthoidea (Squids):
(i) Mostly torpedo-shaped body.
(ii) Members possess 8 arms and 2 tentacles (4th pair).
(iii) Fin lobes (lateral fins) connected at the midline of the posterior.
(iv) Tentacles not retractile but contractile, and do not possess pockets.
(v) Internal shell or gladius (= pen) is broad, rod-shaped or feather-shaped and chitinous.
(vi) Radula well-developed.
(vii) Suckers stalked with chitinous rings.
Generally they are called “squids”.
The order Teuthoidea is divided into 2 suborders:
Suborder 1. Myopsida:
(i) Arms and tentacular clubs are present with suckers.
(ii) Eyes are covered by transparent membrane (cornea).
The suborder Myopsida contains 2 families:
(i) Loliginidae and
(ii) Pickfordiateuthidae.
Habitat and Biology:
The members of Loliginidae are inhabitants of shelf and shore areas of all oceans except polar region. They show diel movements and enter into the brackish water. The family is represented by 8 genera. These are Loligo, Lolliguncula, Doryteuthis, Sepioteuthis, Alloteuthis, Uroteuthis, Loliolus and Loliolopsis. The pickfordiateuthidae is represented by a single genus—Pickfordiateuthis.
Suborder 2. Oegopsida:
(i) The squids do not contain corneal membrane over the eye.
(ii) Suckers absent.
It consists of 23 families of which the representatives of Architeuthidae (Architeuthis sp.) are the largest animal not only in cephalopodes but also among invertebrates. The animals are 20 m in total length and most records are from standings or whale stomach.
Though it is worldwide in distribution but the members are abundant in the shore of Newfoundland, Norway (North Atlantic) and in the North Pacific and also in the Southern Ocean. It is believed that the giant squid occurs between 200 m to 400 m depths and sometimes some specimens are seen at the surface of the sea.
Some other genera are Onychoteuthis, Onykia, Moroteuthis, Octopoteuthis, Watasenia, Ancistrocheirus, Gonatus (Fig. 16.61A), Gonatopsis, Abralia, Bathyteuthia (Fig. 16.61B), Ctenopteryx, Brachioteutnis, Batoteuthis, Todaropsis, Histioteuthis (Fig. 16.61C), Martialia, Dosidicus, Thysanoteuthis, Valbyteuthis, Mastigoteuthis (Fig. 16.61D), Leachia, Taonius and Egea.
Order 4. Vampyromorpha (Vampire squids):
(i) 8 oral appendages present only which are united by a web.
(ii) Two small tendril-like appendages between the bases of 1st and 2nd arms.
(iii) Internal shell chitinous, broad plate-like structure.
(iv) Colour of the body black.
(v) Stalked suckers.
The order is represented by a single family Vampyroteuthidae and contains a monotypic genus Vampyroteuthis with a single species V. infernalis (Fig. 16.62A) squid, occurs at depths between 300 m – 3000 m.
Order 5. Octopoda (Octopuses) [Gk. Oktipous, okto = eight + pous = foot]:
(i) Generally called “devil fish”.
(ii) Globose-shaped body in most cases.
(iii) 8 oral appendages, no tentacles and the bases of the arms are connected by a membranous web.
(iv) Appendages bear sessile suckers and without chitinous rings along the length of the arms.
(v) Internal shell absent or represents a reduced, vestigial, cartilaginous or a U-shaped support (e.g., Cirrothauma, Cirroteuthis).
(vi) Lateral fins either absent or separate paddle-like fins in some deep sea forms (e.g., Cirrothauma).
(vii) Body colour dark brownish but never black.
The order Octopoda is divided into two suborders:
Suborder 1. Cirrata:
(i) Arms are connected by a Secondary web.
(ii) Cirri present on arms.
(iii) Internal shell present.
(iv) Paddle-shaped lateral fins present.
(v) Mostly deep sea pelagic and epibenthic forms.
The suborder is represented by three families of which:
(i) Cirroteuthidae (e.g., Cirrothauma, Fig. 16.62B, and Cirroteuthis) is characterized by the web, saddle-like shell, and paddle-like fins at the postero-lateral part of the body.
(ii) Opisthoteuthidae (e.g., Opisthoteuthis) is characterized by the flattened body and straight or slightly bent shell. The web is absent.
The members of the Cirrata are mostly deep sea pelagic and epibenthic forms.
Suborder 2. Incirrata:
(i) 8 arms with suckers present.
(ii) Cirri and fins absent.
(iii) Shell absent.
The suborder Incirrata represents 7 families, such as:
(i) Alloposidae—Biserial suckers on arms (e.g., Alloposus),
(ii) Bolitaenidae— Uniserial suckers with stomach posterior to liver (e.g., Japetella),
(iii) Amphitretidae— Uniserial suckers with reduced mantle opening (e.g., Amphitretus),
(iv) Vitreledonellidae —Uniserial suckers with wide mantle opening (e.g., Vitreledonella, Fig. 16.62D),
(v) Tremoctopodidae—Deep web, smooth mantle and water pores (e.g., Tremoctopus, Fig. 16.62C),
(vi) Ocythoidae—Reduced web and water pores (small apertures in the web of some octopuses) (e.g., Ocythoe),
(vii) Argonautidae— A thin shell-like egg case and reduced web (e.g., Argonauta) and
(viii) Octopodidae—Mantle globose, saccular, elongate, ovoid and males with left or right third arm hectocotylized, and no fins with reduced web (e.g., Octopus (Fig. 16.62E), Scaeurgus, Cistopus, Pteroctopus, Eledone, Benthoctopus).
The representatives of this suborder Incirrata are the inhabitants of shallow depth to moderately deep depth in seas. They are of epipelagic and benthic forms. Among the genera, the genus Octopus represents a major number of species and occurs from the coast to a depth of 1000 m and even enters into the estuarine region during night.
The family Argonautidae represents by a single species, Argonauta argo, commonly called “paper nautilus” and is peculiar for the size of the adult females which are 10 to 15 times larger than the adult males.
The web of the females secretes and holds a thin, calcareous shell-like egg case in which they reside. The ‘argonaute’ specimens are found occasionally in different fish landing centres along the coasts of Gulf of Mannar, Tamil Nadu, and India.
6. Number and Size of Molluscs:
The phylum Mollusca is noted for its numerical abundance. In this respect Molluscs occupy a position next to Arthropods in the animal kingdom. It is extremely difficult to give an accurate data of the total number of species under the phylum.
Molluscs are remarkable for their range of size and forms. The size varies from 1 mm to 18 m long.
This phylum includes the largest invertebrate form known in the world, the following account will give an idea about the different size of molluscs:
Chitons:
The size varies from 1.3 cm to 30 cm. The largest genus is Amicula of Pacific coast.
Scaphopods:
The elongated shell is usually 6.5 cm in length but the length may extend up to 15 cm.
Gastropods:
They have usually globose body. The average diameter is less than 5 cm. Hemifusus proboscifera has the shell of 60 cm long. Aplysia California has the largest body of 1 m length.
Bivalves:
Usually the size ranges between 1.3 cm to 11.7 cm. The body of Tridacna gigas reaches a length of 1.35 m and weighs about 200 kg.
Cephalopods:
Some squids and Octopuses have 2.6 cm long body. The giant squid, Architeuthis of north Atlantic has a 20 m long body with the arms.
7. Mantle and Mantle Cavity of Molluscs:
Presence of mantle is a very distinctive feature of Mollusca. The mantle cavity usually communicates to the exterior and encloses the respiratory organs. The mantle secretes the shelly matter and the edge of the mantle is the active productive centre.
The mantle cavity is modified in different groups of Molluscs. In Prosbranchia, the mantle cavity is situated on the left side of the body and its anterior portion is prolonged into a tubular siphon.
In Pulmonata the mantle fold encloses the pulmonary sac. In Bivalvia mantle cavity is equally developed on either side of the body and the mantle forms a sort of lining to the valves of the shell. Progressive fusion of the edges of the mantle is observed in different bivalves (Fig. 16.63).
In Nucula, Area, Trigonia the mantle edge is perfectly free and the siphons are absent. In Mytilus, Cardita, Astarte the mantle edge is fused at one point in the middle region of the posterior side and forms the rudiment of the ‘Branchial siphon’. In Venus, Cardium, Lucina the branchial aperture is divided and the mantle is fused at two places to form three apertures. Through the ventral aperture protrudes the foot.
In Tridacna, Chama the fused area become extended and in Teredo, Pandora fusion of the mantle edge forms a closed cavity excepting at three openings, one for foot and the other two for the siphons. In some forms, a fourth aperture is present which bears a definite relation to the byssus apparatus. In Lyonsia a thick byssus protrudes through this particular opening.
In some Molluscs the mantle may be reflected over the external surface of the shell and subsequently the mantle edge may fuse to make the shell internal. Transitional stages are observed in Cypraea, Marginella, Scutus, where a considerable portion of the shell remains enclosed by mantle.
But in Aplysia, Lamellaria the shell is completely enclosed by mantle. Amongst Pulmonata subsequent transformation from a completely external shell to internal shell is also observed. Indication of such changes is also observed in some Bivalves.
Shell of Phylum Mollusca:
The shell is one of the most important diagnostic structures in Molluscs. It has previously been employed as a basis for classification. Like all other structures, the shell also exhibits wide variations in different molluscs. It is present in almost all the members of Mollusca, but varies greatly in disposition.
The shell may be present outside the body as in most Gastropods, Bivalves and Chitons. Internal shell occurs in most Cephalopods and in some Gastropods. Transitional stage, where the shell is partly internal and partly external is observed in Hemphillia and Naticidae. In some Cephalopods and Nudibranchia, the shell is absent in adult stage.
Types of Shell:
The majority of molluscs possess univalve shell and the r4st may have bivalved shell or may even be composed of eight plates as in Chiton.
Univalve shell:
Normally the univalve shell is an elongated cone which becomes twisted into a spiral round an axis. The shell was originally a simple cone, but due to the increase in size of the visceral hump the shell has become twisted to accommodate whole of the visceral organs.
In some cases the shell instead of forming an elongated spire, forms either a flattened spire (Polygyrantia) or a globular spire (Pila). Amongst cephalopods the shell varies in form. It may be simple as seen in the Squids.
In other forms, it is usually spiral and chambered. The shell of Spirula and the shell of Nautilus are typical representatives. In Spirula, the shell is spirally twisted but the spirals do not touch, and internally it is chambered like that of Nautilus. But the relation of the soft parts with reference to the shell differs in Spirula and Nautilus. The shell in Spirula shows endogastric curvature but in Nautilus it is exogastric.
In Spirula the shell is internally placed and remains covered completely by mantle. The shell of extinct Ammonoidea is more or less similar to that of Nautilus. Octopus lacks shell excepting a pair of small vestiges. In female Argonauta, an external thin shell is present (see Fig. 16.53A). It is delicately coiled and is not chambered.
Types of spiral:
In different molluscs considerable modifications of the spiral occur. Usually the spire is more of less obliquely coiled round the axis. Two types of spirals are usually encountered in molluscs.
They are:
Dextral:
In most of the univalve shells, the spiral is dextral meaning thereby that the spiral is right-handed round the axis.
Sinistral:
This type of spiral does not occur commonly. This condition is just reverse to the dextral form, i.e., the spiral is left-handed. Occurrence of sinistral type of coiling may be regarded to be normal in certain species and in most cases sinistral type of shell represents abnormal forms.
In cases where sinistral monostrosity or in which a sinistral and dextral forms are interchangeable, the internal organs become affected in relation to such coiling.
The direction of spiral does not show uniformity in all cases. In some cases, the sinistral type of shell in the embryonic stage is converted into dextral type in adults. Abnormal growth of shell occurs in nature which varies from simple to complex. In some cases the abnormality reaches the peak and may change the entire normal organisation of the creature.
Bivalve shell:
This type of shell is the characteristic of the class Bivalvia. Based on the relation of two valves, the shell may be equivalve or inequivalve. The dorsal margin of the two valves are usually united by ligament. The two valves can be opened or closed by adductor muscles. The protractor muscle serves to thrust forward the foot and retractor muscle retracts the extended foot of the bivalves.
The impressions of the adductor muscles are visible on the inner surface of the valves. The inner surface of the valves shows pallial lines which are actually the impressions produced by the muscular edge of the mantle. The ligament which unites the two valves consists of two parts, the externally located ligament proper and the internal cartilage.
In Pecten the external ligament is ill-developed and thin. The internal ligament is well-developed and is situated in a shallow pit. The ligament proper is non- elastic and is insoluble in caustic potash, whereas the cartilage is elastic and is soluble in caustic potash.
The valves are articulated by a hinge, which in most cases, is furnished with interlocking hinge-teeth. In some Bivalves like Anodonta and Mytilus, hinge- teeth are absent. The hinge-teeth are derived from the crenulations of the shell surface. The absence of hinge-teeth in Anodonta may be the result of secondary degeneration.
Composition of Shell:
The main material constituent of the shell is carbonate of lime. Traces of phosphate of lime and an organic base, called Conchiolin (chemically allied to chitin), are present. In addition, carbonate of magnesium and trace of silica are also detected. Conchiolin constitutes a sort of membranous framework for the shell. It is generally regarded that the shell has two parts, the cellular and the membranous parts.
Cellular structure is very rare and the membranous part constitutes the sole material basis. The membranous part was once a part of the mantle. Majority of the Gastropods possess porcellaneous shell consisting of three layers, each of which is composed of plates.
The orientation of the plates in the three layers has definite regularity. If in the middle layer the plates are transversely arranged then the outer and the inner layers have longitudinally arranged plates. In Bivalves the nature of formation and disposition of layers are similar to that of Gastropods.
Formation of Shell:
Opinions differ as regards the formation of shell. Bower-bank and Carpenter advocate that the formation of shell is an organic process. It grows in the same manner as the teeth and bones of higher organisms.
But the modern workers like Reaumur and Eisig hold that the shell is the by-product of excretion and is deposited like the cuticle of Arthropods. Shell is formed by a large number of calcareous particles which are held together by some kind of glue.
The shell is formed from the margin of the mantle. The margin of the mantle is the main source of deposition of shell matter and the rest of the mantle helps to thicken the innermost layer. The shell-depositing cells are present in all parts of the mantle. The carbonate of lime from the circulating blood is separated by the epithelial cells of the mantle edge.
The carbonate of lime assumes granular or crystalline forms and becomes hard on exposure. The shell matter in some cases may be deposited by foot also. Deposition of shell matters is not a continuous process but it shows periodicity which is manifested by the lines of growth.
Operculum:
The operculum is present in almost all marine Prosobranchia. It is a cuticular development on the foot. The operculum exactly fits into the mouth of the shell. In all Opisthobranchia excepting Acteon and in all Pulmonata excepting Amphibola operculum is absent. The shape of the operculum differs in different forms.
8. Excretory System of Molluscs:
In Molluscs the excretory system comprise of the kidneys and the pericardial gland. Each kidney is a special portion of the coelom and remains in communication with other parts of the coelom. The glandular tissue of the kidneys is arranged in various ways in different forms.
The kidneys are usually paired, symmetrical and coiled structures. The kidneys are dorsally placed near the pericardium. One end of the kidney opens externally into the mantle cavity while the other end opens internally into the pericardium by the reno-pericardial aperture.
Besides kidneys, there is special glandular tissue in the pericardial or viscero-pericardial division of the coelom. This tissue constitutes the pericardial gland.
Kidneys:
The kidneys in molluscs represent the special portion of the coelom. In all the Molluscs excepting Nautilus, the kidneys communicate with the coelom. The most primitive and typical condition of the excretory system of Molluscs are observed in Chiton. The kidneys consist of two symmetrical glandular structures on the dorsal part of the body and are located near the pericardium.
Each kidney communicates to the mantle cavity by one end near the anus and the other end opens into the pericardial cavity by reno-pericardial aperture. The arrangements of the glandular tissue in the excretory organs vary greatly in different forms of Molluscs. The number of the kidney in different Molluscs depends upon the number of auricles and ctenidia.
The epithelial lining of the pericardium usually contains glandular tissue and constitutes the pericardial gland. In some cases the glandular tissue may also be present in the auricular wall.
Kidneys in different Molluscs vary greatly. In Bivalves the kidneys are two in number. They may assume the form of twisted tubes which may be dilated at certain regions. The reno-pericardial aperture is usually lined by an yellowish glandular tissue which can also extract the excretory products directly from the blood. In a few Bivalves the two kidneys are in communication with one another.
The glandular portion of the kidney in Ostrea is branched and ramified encircling the visceral mass. In Scaphopoda the kidneys are also paired. As regards the existence of renal organs in Solenogastres there exist two opposite and contradictory views.
According to some, renal organs are unknown, while others hold that the kidney in Solenogastres is tube-like and bent on themselves opening into the cloaca by a common duct.
Like all other organ systems, the excretory system in Gastropods exhibits great diversities. In majority of the forms two kidneys are present, but they are unequal in size. In some Gastropods only the left kidney (originally the right kidney) is persistent in the adult. Paludina possesses two kidneys in embryonic stage, but in course of development, the kidney of the right side disappears.
In Aspidobranchia, excepting Neritidae, the left kidney is smaller and the right kidney is larger. The right kidney, in addition to its renal function, sub-serves as the passage for the genital products. In Haliotis the gonad opens into the right kidney by a large aperture. Both the kidneys open into the mantle cavity, one on each side of the anus. But with regard to their pericardial opening there are some variations.
In Trochus, Turbo and Haliotis, the left kidney is small and opens into the pericardium, while the right one is without the pericardial opening. In Fissurella (Fig. 16.73) the right kidney has a pericardial opening.
In other Gastropods, one of the two kidneys is either greatly reduced or totally absent. The absence of the left kidney appears to be the primitive condition. Generally, the kidneys open externally near the anus into the mantle cavity. The kidneys may, sometimes, open into the anus with or without a long ureter.
The reno-pericardinal aperture varies in different Molluscs. In Haliotis, Trochus and Turbo, the right kidney is devoid of reno-pericardial aperture, while the left kidney possesses it.
Though not universally accepted, some authors have claimed the absence of reno-pericardial aperture in Patella. In some Nudibranchs, like that of Chiton, the non-glandular renal sac gives off glandular tubular branches which perform excretory function.
In dibranchiate Cephalopods there are two kidneys which may be separate as seen in Octopoda or may be connected anteriorly. Typical condition in Cephalopods is already discussed in the biology of Sepia.
In Nautilus there are four kidneys and the four afferent branchial vessels are covered by glandular tissue, which project into the pericardium and thus constitute the pericardial gland.
The nitrogenous wastes are eliminated in the form of guanin in Cephalopods, as uric acid in Opisthobranchs, and as urea in Lamellibranchs.
Relationship of Kidney with the genital organs:
In Molluscs, the excretory system and the genital system are closely related. In majority of the Gastropods the renal organ, in addition to its own function, may subserve as the passage for the genital products. In Chaetoderma and Neomenia, gametes are discharged into the pericardium and they ultimately pass out through kidney.
In Haliotis, Fissurella, Nerita, Patella, the right kidney acts as an outlet of the genital products. In some Bivalves such as Avicula, Modiola, Pecten, the genital gland communicates directly to the kidney. In Ostrea cyclas, the genital gland joins the renal duct. But in most Bivalves the genital and the renal glands are closely placed but they open separately.
9. Reproductive System of Molluscs:
In Molluscs, the sexes may be united or separate (gonochoristic). The Solenogastres, excepting the Chaetoderma, are all hermaphroditic. The generative organs are usually paired. In Chaetoderma the gonads are fused together to form an unpaired one. There is no separate gonoduct and the sexual products are discharged into the pericardial cavity. From there these are transported to the exterior by a pair of coelomoducts.
The sexes are separate in Polyplacophora. Gonads are unpaired, sac-like structures and are similar in appearance. The gonoducts are paired. Sexes are separate in Scaphopoda. An elongated unpaired gonad is present anteriorly which narrows to form the gonoduct. The gonoduct opens near the anus.
In Bivalves, excepting some forms like Ostrea, Cardium, Pisidium, Poromya, the sexes are separate. The genital glands usually lie in the visceral mass and may extend into the mantle lobes as in Mytilus. The ovary and testis look alike and are distinguishable by colour only. In hermaphrodite forms, the whole of the generative gland is hermaphrodite.
In Ostrea edulis and Plicata the hermaphroditic gland produces ova and spermatozoa alternately. The male and female follicles in the hermaphroditic gland may be separate as seen in Pecten and Cyclas. They usually open by a common duct. In fresh-water mussels, Unio and Anodonta, hermaphroditism is sometimes observed.
Some Oysters are protandrous, i.e., the gonad produces first spermatozoa and then ova. In some of these forms a part of the gonad acts as an ovary and the rest as testis. Accessory glands are totally lacking.
Most of the Gastropods are dioecious (sexes separate). In dioecious Streptoneura the generative organ is a racemose glandular structure which usually opens into the kidney. But in Neritidae and Pectinibranchs, the gonad has its own independent duct.
Accessory glands are usually absent in dioecious forms. In hermaphroditic Streptoneura, single hermaphroditic gland (ovotestis) is present. Accessory glands are quite well-formed in hermaphroditic Streptoneura. Euthyneura is also hermaphrodite and the hermaphroditic organ is more complex in nature. The male and female germinal follicles are usually separate.
In Pleurobranchs and majority of the Nudibranchs, several female follicles are arranged around a central male follicle and open into it. The hermaphroditic gland leads into hermaphroditic duct which exhibits wide variations amongst the Gastropods. The hermaphroditic duct is of three types.
They are as described below:
Monoaulic Forms:
In most of the cases as in Bullids and Aplysids, the hermaphroditic duct is undivided.
Dialuic Forms:
In Valvata and majority of the Pulmonata, the hermaphroditic duct is divided into a male part (vas deferens) and into a female part (oviduct). The male and female genital apertures may be quite widely separated as in Valvata, many Basommatophora, Onchidium, but may be placed very closely as in most Nudibranchs.
In Stylommatophora and Siphonophora the two ducts unite to form a common duct which opens into cloaca.
Triaulic Forms:
In hermaphrodite forms seminal receptacle is present. In monoaulic forms the seminal receptacle opens into the hermaphroditic duct into the oviduct in the dialulic forms but in Doris the seminal receptacle not only opens into the oviduct but also communicates with vagina. Such disposition is usually designated as the triaulic arrangement.
Cephalopods are unisexual and in most cases sexual dimorphism is present. Generative gland is single and the gonoduct in one side is usually reduced or vestigial. In Nautilus, both male and female, the left gonoduct is vestigial and the right one persists and sub-serves the genital function.
In dibranchiate Cephalopods the left gonoduct is present only in males. The same condition prevails in females excepting Oegopsida and Octopoda (except Cirroteuthis) where both the ducts are present and functional.
10. Development of Molluscs:
The development of Molluscs excepting the Cephalopods is usually indirect. In most of the forms, a typical larval form, called Trochophore, is present. The structure of the Trochophore larva resembles that of annelids. This larval form was first discovered in 1840 by a Swedish Naturalist, Loven.
For this reason it was called Loven’s larva for many years since its discovery. Ray Lankester (1877) named the Loven’s larva as Trochophera. But in 1878, this larval form was thoroughly studied by Hatschek and he named it Trochophore.
The body of the Trochophore larva is more or less oval in shape (see Fig. 16.12). The apical pole has a tuft of long cilia at the centre of the lobe and a circlet of cilia, called prototroch. Another circlet of cilia is present at the meridian of the body.
At the basal part of the body two lobes are present which become transformed into the foot. Close to this, another ciliated band, called telotroch, is present. The region dorsal to the prototroch becomes the shell which later on assumes a cup-like form. This stage is called the veliger larva.
The prototroch of the Trochophore expands in the veliger larva to form a ciliated disc, called velum (Fig. 16.74). In some Gastropods the velum is produced into ciliated lobes.
In Solenogastres development is not fully known. The trochophore larva is present and is provided with calcareous plates. In Polyplacophora typical trochophore larva is present. The apical plate is absent in early phase. Primitive kidneys are lacking. In Scaphopoda also the spical plate and the primitive kidneys are absent.
Typical trochophore larva is present in Bivalves and Gastropods. In land pulmonata the young’s assume the adult form and a slight trace of formation of velum is encountered. In other Gastropods typical molluscan larval stages are observed and the larvae may possess certain special organs such as the contractile sinuses (blister-like extension of the integument assisting larval circulation) and the larval kidneys.
In Cephalopods the eggs are large and heavily yolked. There is no larval stage and the young’s are hatched in the form of adult. The two types of larval forms, viz., trochophore larva and veliger larva, are seen in different molluscs. They are built more or less on the same structural plan with certain variations and modifications.
In freshwater bivalves, trochophore stage is suppressed and the veliger stage is represented by either Glochidium larva or Lasidium larva or Haustoria larva. The structure of Glochidium larva has already been described in the biology of Unio (see Fig. 16.37). Fig. 16.75 shows the structure of a Lasidium larva in South American fresh-water mussels and Haustoria larva in an African mussel, Mutela.
