Phylum Ctenophora: Features, Characters and Other Details

Phylum Ctenophora: Features, Characters and Other Details! 

Habit and Habitat of Phylum Ctenophora:

Ctenophores are very common marine animals found in diverse habitat. They are widely distributed being specially abundant in the warmer seas, though some occur in temperate or arctic regions.

They are of planktonic habit floating in the surface waters, mostly near shores but a few live to depth of even 3000 metres. Ctenophores feed on small marine animals including the eggs and larvae of molluscs, crustaceans and fish.

External Features of Phylum Ctenophora:

Pleurobrachia has a pear-shaped body about 5-20 mm in diameter, and of glass transparency. The mouth is situated at the centre of the oral pole and the opposite or aboral pole is occupied by a complicated and characteristic sense organ. This sense organ is a modified statocyst and acts as an organ of equilibrium.

From the opposite sides of the broad end hang two long tentacles provided with numerous little tag-like processes. Each tentacle springs from a deep cavity of sheath into which it can be retracted completely.

The surface bears eight equidistant meridional bands or swimming plates or costae starting from near the aboral pole and extending about two-thirds of the distance towards the oral pole.

Each band is constituted by a row of transversely arranged comb-like structures, consisting of narrow plates frayed at their outer ends. During life the frayed ends are in constant movement lashing to and fro, and so propelling the animal through the water.

Histology of Phylum Ctenophora:

The body is covered externally by a delicate ectodermal epithelium.

The epithelium of stomodaeum is ectodermal, that of infundibulum and its canals endodermal. Both ectodermal and endodermal epithelia are ciliated. The space between the external epithelium and canal system is filled by a soft, jelly-like mesogloea. The tentacle sheath is an invagination of the ectoderm, and the tentacle itself is covered by a layer of ectoderm.

Delicate muscle fibres lie beneath the external epithelium and beneath the epithelium of the canal system, and also traverse the mesogloea in various directions. Ectodermal cells are cuboidal or columnar in shape and are ciliated in certain regions. These cells are interspersed with numerous gland cells and sometimes certain pigment granules or branched pigment cells.

The sensory cells are of two types:

(i) One type is having several stiff bristles and

(ii) The other type is with a single stout bristle.

Digestive System:

The mouth, lying in the centre of the lower end, is an elongated slit leading into a deep flattened tube called the stomodaeum. Towards its upper or aboral end the stomodaeum gradually narrows and opens into a cavity called the infundibulum, which is probably equivalent to the stomach of an anthozoan or a medusa, and is flattened in a direction at right angles to the stomodaeum, i.e., in the transverse plane.

From the infundibulum three tubes are given off one, the infundibular canal, passes directly upwards; and immediately beneath the aboral pole divided into four short branches, two of which open on the exterior by minute apertures, the excretory pores. The two other canals given off from the infundibulum are the pre-radial canals.

The per-radial canals pass directly outwards, in the transverse plane, and each divided into two inter-radial canals, which in their turn divide each into two ad-radial canals. Each per-radial canal gives off a stomodael canal which passes downwards parallel to and in close contact with the stomodaeum and a tentacular canal which extends outwards and downwards into the base of the corresponding tentacle.

Most of the digestion is completed extracellular in the stomodaeum. Undigested food passes out through either the mouth or anal pores.

Sense Organ:

The chief sense organ (Fig. 37. 4) is a peculiar apparatus situated at the aboral pole. In this region is a shallow depression lined by ciliated epithelium and produced in the transverse plane into two narrow ciliated areas, the polar plates.

From the depression, arise four equidistant groups of very large S-shaped cilia, united to form as many springs, which support a mass of calcareous particles. From each spring a pair of ciliated grooves proceeds outwards, and passes to the two swimming plates of the corresponding quadrant.

The calcareous mass, with its springs, is enclosed in a transparent case or bell, formed of coalesced cilia. It appears that the whole apparatus is a kind of steering gear or apparatus for the maintenance of equilibrium.

If the body is inclined, the statolith presses more heavily the ciliary tuft of the inclined side and the stimulus appears to be transmitted by the corresponding ciliated groove to a swimming plate and results in vigorous movement of the combs. A sub-epithelial plexus of nerve fibres with nerve cells extends all over the surface of the body, and nerve elements can also be traced in the mesogloea.

Reproductive System:

The ctenophores are hermaphrodite, i.e., both the sexes are present in the same individual. The gonads are developed in the meridional canals. The ova are formed on one side and the spermatozoa on the other side of each meridional canal, from its endodermal lining. The matured ova and sperms are discharged into the canals and finally escape by the mouth. Fertilisation is external.

Development of Phylum Ctenophora:

Development of Pleurobrachia is not well-known but the following description of development in other forms of Ctenophora will- give an idea of the development in Ctenophora. After fertilisation, the zygote undergoes cleavage and gives rise to four blastomeres by two meridional cleavages.

The third cleavage is nearly vertical resulting in a curved plate of eight cells arranged in two rows. The eight blastomeres divide twice along the horizontal plane and give rise in each time eight small cells, the micromeres and eight large cells, the macromeres. The micromeres are the source of ectoderm and the macromeres give rise to endoderm in due course.

Micromeres undergo rapid division and proliferate as a wreath of small cells over the macromeres which ultimately grow down as a one-layered sheet. Invagination starts and gastrulation goes on by combined processes of emboly and epiboly. Micromeres cover the embryo to become the ectoderm.

Four inter-radial bands of small rapidly dividing cells become noticeable which differentiate extensively to form stomodaeum from which coelenteron arises by endodermal outgrowth. Biradial symmetry persists throughout the development.

Affinities and Systematic Position of Ctenophora:

The ctenophores possess many characters of Coelenterata, but they differ considerably from the other members of the phylum Coelenterata. This group has also some similar features with different animals of diverse phyla, which will be discussed here to judge its systematic position.

1. Resemblance with Coelenterata:

(1) Possession of radial symmetry and tentacles.

(2) Arrangement of parts along an oral-aboral axis.

(3) Lack of coelom.

(4) Two cell-layers with a gelatinous mesogloea in between.

(5) Branching gastro vascular canals.

(6) Presence of mesogloea.

(7) Presence of diffused nerve network.

(8) Presence of statocyst as sense organ.

(9) General lack of organ system.

(10) Endodermal origin of gonads.

(11) Absence of nephridia. Due to the above similarities, the ctenophores are considered by many zoologists to be a class of Coelenterata.

2. Resemblance with Hydrozoa:

An anthomedusan form Ctenaria shows remarkable similarities with a cydippid such as Hormiphora in the following characters:

(1) Presence of two tentacles, situated at opposite per-radii, in sheath.

(2) Presence of eight radial canals formed by the bifurcation of four inter-radial pouches of the stomach.

(3) The subumbrellar cavity of the Ctenaria can be homologised with the stomodaeum of Hormiphora.

But these similarities are superficial and the claim that Ctenaria represents a form directly intermediate between the Hydrozoa and Ctenophora seems to be untenable due to the following objections:

(1) The tentacles of Ctenaria have no muscular base.

(2) Eight rows of nematocysts of Ctenaria are not homologous to rows of comb-plates of a ctenophore.

(3) Ctenaria is radially symmetrical, while a ctenophore is biradially symmetrical.

(4) The development of gonad is different. In Ctenaria, the gonads develop from manubrium but in Ctenophora gonads develop from meridional canals.

(5) Absence of characteristic aboral sense organs in Ctenaria.

Hydractinia, a narcomedusan, shows some closer resemblances with Ctenophora.

These resemblances are:

(1) Possession of two tentacles with sheath situated between the margin and the apex of the bell.

(2) Possession of aboral sense organ. But the presence of swimming-plates in Ctenophora and the presence of velum in Hydractinia remain as important differences to visualize a close relation among them.

3. Resemblance with Anthozoa:

The ctenophores also possess certain anthozoan features which are as follows:

(1) Ciliated ectoderm of Anthozoa is probably a forerunner of the ciliated band of Ctenophora.

(2) Presence of well-developed stomodaeum.

(3) The gut in embryos of both is four-lobed, thus, presenting a biradial symmetry.

(4) Mesogloea is cellular.

(5) The gonads develop in connection with the endoderm and the sexual elements passed out through the mouth.

Both the aboral sense organs and rows of comb-plates of a ctenophore have no parallel parts in an anthozoan. Lasso cells differ structurally from the nematocysts and tentacles are hollow in Anthozoa, while solid in Ctenophora.

4. Differences from Coelenterata:

The ctenophores differ greatly from coelenterates in the following points:

(1) Possession of oppositely placed tentacles suggesting a biradial symmetry.

(2) Presence of an aboral sensory region.

(3) Absence of nematocysts except in one or two cases.

(4) Presence of eight locomotory meridional ciliated bands of comb-plates over the body.

(5) Presence of colloblasts (special adhesive cells) over the tentacles.

(6) Presence of mesenchymal muscles.

(7) Presence of definite organisation of digestive system with anal pores.

(8) Presence of determinate type of development.

5. Affinities with Platyhelminthes:

The Platyctenea has been considered to be a connecting link between Ctenophora and the Bilateria. Besides, Ctenophora, in general, exhibits many structural similarities with the Platyhelminthes and particularly with the turbellarians.

The similarities are as follows:

(1) Ciliation of the body.

(2) Dorso-ventrally flattened body.

(3) Crawling mode of life.

(4) Origin of the so-called mesoderm is more or less similar.

(5) The dorsal polar nerve of Turbellaria can be compared with the statocyst of Ctenophora.

(6) Ctenophora exhibits both radial as well as biradial symmetries.

(7) Similar earlier stages of segmentation and gastrulation.

(8) Gelatinous mesenchyme with muscle fibres and cells. The view that the primitive Bilateria have evolved through Platyctenea has not been accepted. A careful thorough examination of the Platyctenea reveals that it is a ctenophore which has become extensively modified for sessile habits.

It can further be suggested that Platyctenea is a tissue- grade diploplastic animal whereas Polyclad is an organ-grade triploblastic form. Further it can be stated that amongst the Platyhelminthes, the Acoela is the most primitive group and not the polyclads. The Ctenophora, on the other hand, exhibits no close similarity with the Acoela.

Ctenophora exhibit many striking characteristics of their own so that it appears more justified to treat them a separate phylum rather than a class of the phylum Coelenterata.

They present certain advanced structural features that appear to look forward to the Bilateria:

(1) Prominence of an apical organ.

(2) Mode of origin of musculature from mesoderm.

(3) Presence of gonoducts.

(4) Determinate type of cleavage.

It appears that Ctenophora are intermediate between Radiata and Bilateria. They appear to have diverged very early from trachyline stem which also gave off the other three branches of Coelenterata. The Ctenophora themselves represent a blind offshoot. Not in the direct line of ancestry, the Ctenophora appear to indicate the structural advances along which the Bilateria have evolved from the ancestral stock.

Characters and Classification of Ctenophora:

Definition:

Ctenophores are free-swimming, transparent, jelly-like, soft-bodied, marine animals having biradial symmetry, comb-like ciliary plates for locomotion, the lasso cells but nematocysts are wanting.

General Characters:

1. Free-swimming, marine, pelagic and solitary animals.

2. Body gelatinous, transparent, pear-shaped, cylindrical or flat or ribbon-shaped.

3. Biradially symmetrical body along an oral-aboral axis.

4. Comb-like eight ciliary plates on the body for locomotion.

5. Nematocysts absent; mesogloea cellular with muscular elements.

6. Digestive system with mouth, pharynx and stomach; the stomach is highly branched to form a complex system of gastro vascular canals.

7. The digestive system terminates out at anal pore.

8. Colloblasts, also referred to as lasso cells, are special adhesive cells present on the tentacles which help in food capture.

9. Skeletal, circulatory, respiratory and excretory systems are absent.

10. Nervous system is diffused type and the aboral end bears a sense organ, called statocyst.

11. Monoecious (hermaphrodite); gonads endodermal situated on the walls of gastric canals.

12. Development direct with a characteristic- larva called cydippid.

13. Regeneration and paedogenesis are of common occurrence. Alternation of generations not found.

14. Body organization cell-tissue grade.

Classification:

Class 1. Tentaculata:

1. Adults nearly always with two long aboral tentacles.

2. In some only the larva has tentacles, while adults possess oral lobes.

3. Mouth narrow and pharynx small.

Order 1. Cydippida:

1. Simple, rounded or oval body.

2. Digestive canals terminate blindly; no anal pore.

3. Tentacles two long and branched.

4. Tentacles are retractile into sheath.

Examples:

Mertensia, Pleurobrachia.

Order 2. Lobata:

1. Laterally compressed oval body.

2. Adults with two large oral lobes.

3. Tentacles reduced and without sheath.

4. The gastrovascular canals are connected by a ring canal at the oral end.

Examples:

Mnemiopsis, Bolinopsis.

Order 3. Cestida:

1. Body elongated and compressed, ribbon like.

2. Comb plates in four rows but rudimentary.

3. Tentacles and tentacular sheaths along the oral margin reduced.

Examples:

Cestum, Velamen.

Order 4.

Platyctenea:

1. Greatly compressed body in oral-aboral

2. Two well developed tentacles with sheaths.

3. Adults often without comb plates.

4. Flattened creeping forms.

Examples:

Ctenoplana, Coeloplana.

Class 2. Nuda:

1. Body large, conical and compressed

2. Tentacles and oral lobes absent.

3. Wide mouth and large pharynx.

4. Voracious feeders.

Order 1. Beroida:

1. Since class Nuda has only one order Beroida, hence, class characters are the characters of the order.

Example:

Beroe.

Representative Types of Ctenophora:

1. Cestum:

Cestum (Fig. 37.5) is commonly known as Venus’s girdle. It inhabits warmer seas specially the Mediterranean. The body is greatly elongated horizontally in the sagittal, and compressed in the lateral plane so that it appears ribbon-like in form. It is usually green, blue or violet coloured. Out of eight comb-plates, four are very small and the other four are continued all along the aboral edge of the body.

The base of two principal tentacles are large and are enclosed in sheaths and numerous small tentacles spring from grooves and are continued the whole length of oral edge; the small lateral tentacles line the oral ridge. Mouth lies in the mid-oral edge.

The only sense organ, statocyst, is found on the aboral edge. Cestum swims mainly by graceful serpentine movements by the muscular undulations of the body as well as by the beating of comb plates.

2. Ctenoplana:

Ctenoplana (Fig. 37.6) is a small marine, solitary planktonic ctenophore reported once off the west-coast of Sumatra and once among the islands to the east of Papua.

Its body is nearly circular in outline, flattened dorso-ventrally. It measures about 6 mm in diameter. Its dorsal surface is mostly olive green, brown or reddish in colour. In the centre of aboral or dorsal surface of its body is a sense organ with a statolith surrounded by a ring of small ciliated tentacles.

The mouth lies in the centre of ventral or oral surface. Tentacles two, pinnate and retractile.

The gastro vascular canal system is devoid of meridional canals but comprises a set of branching and anastomosing peripheral canals. The organs of locomotion are eight small deeply sunk swimming plates. While swimming it draws downwards the edges of the disc so that it becomes somewhat halmet-shaped when viewed laterally.

3. Coeloplana:

Coeloplana (Fig. 37.7) is a marine solitary ctenophore found in the Red Sea and on the coast of Japan.

It resembles in most of the features to Ctenoplana. However, its body is oval and dorso-ventrally flattened but elongated in the tentacular plane. It measures about 60 mm in length. Mouth is ventral in position. Tentacles paired and retractile. Anastomosing gastrovascular canal system. Sense organ (statocyst) is dorsally placed.

Swimming plates are not found. It moves by creeping. Gonads are found in the walls of meridional gastrovascular canals. The fertilised eggs are attached to the oral surface of the mother by the sticky secretion and each developing into a typical cydippid larva with comb-plates. The cydippid larva after free-swimming for some time metamorphoses into adult Coeloplana.

4. Beroe:

Beroe (Fig. 37.8) is commonly known as sea mitres or mitre jelly-fish. It is found in great swarms and cosmopolitan in distribution. Its body is thimble-shaped and measures about 10-20 cm in height. It is usually pinkish in colour. The rounded aboral end bears the sense organ surrounded by polar fields. The oral end is truncated and is occupied entirely by a wide mouth.

The greater part of the body is occupied by the huge stomodaeum, infundibulum, per-radial and infundibular gastrovascular canals, etc., all these being crowded into a small space at the aboral pole.

The meridional canals send off branches which unite with one another, forming a complex network of tubes. At the oral end the four meridional canals of each side and the corresponding stomodaeal canal unite into a horizontal tube which runs parallel with the margin of the mouth.

Tentacles are not found in any stage of the life. A small stomach is placed near the sense organ. It is a voracious feeder, swallowing crustaceans and sometimes other ctenophores even larger than itself.