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The following points highlight the top three non-segmented coelomate groups. The groups are: 1. Phylum Sipuncula 2. Phylum Echiura 3. Phylum Priapulida.
Non-Segmented Coelomate Group # 1. Phylum Sipuncula:
Sipunculids:
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This group includes nearly 2350 species of which the largest is Siphonomecus multicinctus (nearly 51 cm). The other important genera are Sipunculus sp. (Fig. 17.47B), Golfingia sp. and Phascolosoma sp. These marine animals usually possess a body, divisible into trunk and a narrow anterior end. The anterior end can be involuted inside the trunk and thus is called introvert.
In certain forms the extended introvert may exceed the length of the trunk. Mouth is present at the tip of the introvert and around the margin of the mouth the introvert membrane is fringed and funnel-shaped in Sipunculus but tentacle-like in other (Golfingia).
The other surface is covered by a chitinous cuticle and is iridescent in Sipunculus. Beneath the epidermis the body wall is represented by circular, diagonal and longitudinal muscle layers within which epidermal gland cells are embedded. Ciliated coelomic epithelium lines the spacious coelom which extends within the trunk, tentacles and within the cuticle in Sipunculus as cuticular canal.
The fluid within coelom is in constant circulation by the movement of cilia and it includes—wandering leucocytes, disc-like haemerythrin containing cells, reproductive cells and excretory cells. The extension of introvert and tentacles are caused by regulating the flow of coelomic fluid.
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The oesophagus is ciliated and the remaining part of the canal first runs posteriorly and then coils in anterior direction to open through anus which is near the base of introvert.
Chloragogen cells and a pair of metanephridia are responsible for excretion. The nephridia are present within the anterior end of the trunk and open near the anus through a separate opening, called nephridiopore. The nephridia also convey matured reproductive cells.
The chloragogen cells are present on the lining of the coelom over the intestine. After collecting excretory products these cells drop in the coelomic fluid. Locomotion includes crawling and swimming. Both are caused by the contractile action of introvert and longitudinal body muscles.
Nervous system includes a two- lobed brain and a single median posteriorly directed ventral nerve cord. Brain is present near the mouth. The skin acts as respiratory surface and the haemerythrin in disc-shaped coelomic cells is responsible for binding oxygen.
Sexes are separate and the reproductive organs are present in the coelomic epithelium of the anterior trunk region. Fertilization is external and larva resembles the trochophore.
Phylum Sipuncula:
(Gk. sipunculus = a little tube); Approx. 250 species (commonly called Peanut worms)
Taxonomic Retrospect:
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1. First Linnaeus included sipunculans in the Systema Naturae (12th ed.) under the Vermes and considered to be the relatives of holothurians.
2. De Blainville first introduced the taxon Sipunculida.
3. Quatrefages placed both sipunculans, echiurans and priapulids, creating a group Gephyrea in 1847.
4. Hyman (1959) separated the above- mentioned three groups and placed under separate phylum and discussed their polyphyletic nature.
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Characteristic Features of the Phylum:
1. Elongated, bilaterally symmetrical, un-segmented body with swollen posterior end.
2. Anterior end of the trunk is called proboscis or introvert which is retractable into the body cavity, supported by compensation system.
3. Parapodia or setae absent.
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4. Mouth is surrounded by fringed lobes or ciliated tentacles, help in food collection.
5. U-shaped gut and anus anterior and dorsal.
6. Spacious coelom not divided by septa. Two coelomic cavities, one in tentacles and another in the trunk.
7. Blood vacular system absent.
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8. Respiratory pigment haemerythrin which occurs in coelomocytes.
9. Brain with a single ventral nerve cord but no evidence of segmentation.
10. Paired sac-like metanephridia.
11. Sexes separate.
12. Coelomic gonads.
13. Cleavage spiral and determinate.
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14. Coelom formation schizocoelous type.
15. Trochophore larva in some species or a larval stage-Pelagosphaera is found in some.
16. No fossil record.
Habitat:
They live exclusively in marine habitat that burrow in sand or mud, or in coral beds.
Examples:
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Sipunculus, Phascolosoma, Golfingia, Phascolion, Siphonosoma, Cloeosiphon.
Affinities:
With Annelida:
The sipunculans show relationship with annelids.
The relationship is based on the following similarities:
1. Dermo-muscular body wall.
2. Central nervous system is of annelidan type.
3. Presence of nephridial excretory system.
4. Cleavage spiral type.
5. Origin of coelom is schizocoelous type.
6. Larva is typically trochophore.
Dissimilarities:
1. Absence of body segmentation in both larva and adult.
2. Presence of retractile introvert into the body cavity with marginal ciliated tentacles around the mouth.
3. Spacious coelom is not divided by the septa.
4. Absence of setae or parapodia in Sipuncula.
5. Anus is situated near the introvert.
6. A single ventral nerve cord without segmentation.
7. Nephridia are only one pair.
Phylogenetic relationship:
Though there are a number of dissimilarities between the two groups yet the sipunculans seen a closely evolutionary relationship to the annelids. Developmental features and molecular analyses suggest that they are closely related to molluscs.
Non-Segmented Coelomate Group # 2. Phylum Echiura:
Echiurids:
This group includes 150 species, of which the largest is Ikeda taenioides (185 cm). All are marine and possess a long proboscis, spacious coelom, metanephridia and fairly simple vascular system.
The features of a few interesting members of this group are given below:
Bonellia:
It exhibits extreme sexual dimorphism—female with large oval and green coloured body and minute male (3 mm) lives within the female (Fig. 17.48). Nearly 85 males are seen within the foregut of a female.
A few reach up to the distal end of the metanephridia and as it acts as reproductive duct; these males get the opportunity to fertilize the ova. In female, the proboscis is very long (1 m) and its terminal end is bifurcated as arms.
The body of the female has numerous papillae, and integument has poisons for protection. The proboscis has a groove on its ventral side which leads into mouth near the anterior end of the body. The minute male has an outer ciliated covering, but proboscis, mouth and anus are absent.
Most part of gut is converted into sperm duct. Development involves the appearance of a free swimming larva with much yolk. According to their genetic makeup all the larvae are females in the beginning. One which comes in contact with a female becomes male; otherwise it develops as a female.
Echiurus:
Segmentation is superficial; trunk is barrel-shaped; prostomium forms proboscis and it exceeds the length of trunk (Fig. 17.47A); a broad ciliated band is present on the ventral side of the proboscis, it is called gutter; presence of two whorls of setae at the posterior end; presence of insignificant brain and a single ventral nerve cord; always live within U-shaped burrows which are dug by bending the proboscis and the anterior part of the trunk; can swim by spiral twisting and extension of trunk; food includes minute organisms like diatoms, protozoa, algae, etc.
While feeding, it extends the proboscis from the burrow and rolls it along the bottom of the water; ventral cilia brush up the microscopic food and mix it with mucus.
The margin of the ciliated band or gutter forms a funnel through which food is directed to the mouth; one pair of thin-walled anal sac with numerous ciliated funnels act as excretory organ which open in the rectum; sexes are separate; two metanephridia act as reproductive duct; larva is a plankton and resembles the trochophore.
Characteristic Features of the Phylum:
1. Un-segmented body. Body divided into an anterior proboscis and a posterior elongated trunk.
2. General shape oval or cylindrical.
3. Mouth anterior and anus posterior.
4. Long, overhanging mouth and solid flattend or grooved proboscis, formed from the prostomium except Saccosoma, used for feeding. They are detritus feeders.
5. Proboscis sensitive and mobile.
6. Body surface may be smooth or warty or a pair of ventral setae present near the anterior end of the trunk (except Saccosoma).
7. Coelom undivided.
8. Intestine much coiled and gut complete.
9. A siphon associated with the intestine at both ends.
10. A ciliated groove in the elongated siphon.
11. Coelomic cavity contains erythrocytes with haemoglobin.
12. Closed blood vascular system contains colourless blood.
13. Nervous system includes nerve ring in the prostomium with a single ventral nerve cord (constructed on an annelidan plan) without ganglia.
14. A pair of anal sacs as sole excretory organ which opens in the rectum.
15. Sexes separate with sexual dimorphism.
16. Cleavage spiral.
17. Trochophore larva in the life cycle.
Habitat:
They live entirely in marine environments, burrowing in sand or mud, or live under rocks or coral crevices. They are detritus feeders.
Examples:
Echiurus, Thalassema, Ikeda, Urechis, Anelassorhynchus, Prashadus, Bonellia, Acanthobonellia.
Affinities:
A. With Annelida
Similarities:
1. Presence of two whorls of seate at the posterior side of Echiurus.
2. Segmentation is absent in the adult but feeble segmentation present in the larval stage.
3. Presence of a closed blood vascular system.
4. Presence of nephridia in some species that serve as excretory and gonoducts.
5. A typical trochophore larva with pre- oral and post-oral circlet of cilia.
Dissimilarities:
1. Absence of segmentation in the adult stage.
2. Absence of parapodia and cephalic in echinurans.
3. Coelom not divided by mesenteries or septa.
4. A single ventral nerve cord without ganglia.
B. With Sipunculans
Similarities:
1. General structure of the skin and body wall.
2. Presence of proboscis or introvert in both.
3. Presence of hooks or bristles.
4. Presence of un-segmented coelom in both.
5. Digestive tract coiled with a ciliated groove.
6. Closed blood vascular system
7. Un-segmented and a single ventral nerve cord.
8. Spiral cleavage in both.
9. Presence of trochophore larva.
10. Origin of coelom is of schizocoelous type.
Dissimilarities:
1. Lack of segmentation in Sipunculans both in adult and larval stage.
2. Absenceofa pre-oral lobe in Sipunculans.
3. Absence of anal vesicles in Sipunculans.
4. Anus is terminal and posterior Echiurans but mid-dorsal and anlerior in Sipunculans.
5. Absence of true setae in Sipunculans.
Relationship:
External and developmental features suggest that the sipunculans and echiurans show a closer relationship with annelids.
Non-Segmented Coelomate Group # 3. Phylum Priapulida:
Priapulids:
This group includes only 17 species of which the largest is Priapulus caudatus. The body is divisible into a spiny anterior retractile introvert (proboscis) and a posterior trunk (Fig. 17.47C). The introvert bears recurved spines, mouth, pharynx and brain but is without tentacles. Double-layered cuticle is present which is made up of chitin and proteins.
Sensory papillae are scattered all over the body. Coelom is large and filled up with fluid-containing several cell-types. One or a few caudal appendages are present at the posterior end of the body to act as respiratory organs.
The intestine is straight and on each side of the intestine lies a urinogenital organ. Each urinogenital organ includes a protonephridial duct, at the one end of which lies numerous solenocytes for, excretion and the other end bears the gonads.
Entire structure is enclosed by a membrane and remains attached with the body wall. The urinogenital organ opens through a pore near the anus. Moulting is very quick and is completed within a minute. Fertilization is external and the development is little known but cleavage appears to be radial. A non- ciliated larva becomes adult after two years.
Phylum Priapulida:
(Gk. priapos = God of gardens and reproduction) Approx. 16 species.
Characteristic Features of the Phylum:
1. Un-segmented but superficial annulations and vermiform animals.
2. Beaded introvert proboscis with small hooked spines.
3. Mouth with chitinous teeth.
4. Tentacles absent. Caudal appendages, the unique feature of priapulans, serve for gas exchange.
5. Chitinous cuticle present and moulted periodically.
6. Straight alimentary canal.
7. Blood vascular system absent.
8. Nervous system intra-epidermal and consists of a nerve ring with a ventral cord.
9. Phagocytic amoebocytes and respiratory pigment-haemerythrocytes are present in the coelomocytes of the fluid of body cavity.
10. Body cavity may be either pseudocoel or coelom, still remains a controversial issue.
11. Presence of two protonephridia and two gonads that share a common duct.
12. Sexes separate (gonochoristic).
13. Cleavage radial.
14. Loricate larva in most species.
Habitat:
Marine, mostly found in buried condition in mud and sand in shallow and deep water.
Geographical distribution:
They occur in boreal (north-east coast of North America) and temperate seas. The large sized species have recorded in cold waters of Antartica, North American coasts and in the Baltic Sea.
Examples:
Priapulus, Maccabeus, Meiopriapulus, Tubulichus.
Phylogenetic Relationship:
The evolutionary position of priapulids is still uncertain because the development of priapulids requires further study. Some characters such as introvert proboscis with hooks, similar musculature and respiratory haemerythrin indicate a closer relationship with Sipunculids.
