In this article we will discuss about Phylum Entoprocta:- 1. Characters of Phylum Entoprocta 2. Classification of Phylum Entoprocta 3. Affinities 4. Phylogenetic Relationship.
Characters of Phylum Entoprocta:
This group includes a number of minute sedentaric animals. They are either solitary or colonial forms (Fig. 14.34A). Superficially they resemble the Hydroidea and Bryozoa. Formerly this group was included under Bryozoa (Ectoprocta).
But recent workers have excluded them from Bryozoa and placed them under the phylum Entoprocta. All the forms are marine excepting the fresh-water form, Urnatella. The representatives of this group are Pedicellina, Loxosoma, Myosoma and others.
The stalked cup-like body is called calyx. The calyx bears circlet of tentacles at the free edge. The cavity of the cup is called vestibule or atrium and it contains both mouth and anus. The alimentary canal is curved to form a U-shaped tube (Fig. 14.34B). The space between the body wall and the alimentary canal is filled up with parenchyma (acoelomate).
The excretory system is protonephridial type and consists of a pair of excretory organs which are ciliated intracellular tubes. Each excretory tube begins with a flame cell. The two excretory tubes may open separately or may unite to form a common duct opening into the cloaca. The nervous system is represented by a sub-oesophageal ganglion which is bilobed in Loxosoma.
The sexes may be united or separate (gonochoristic). In the solitary form (Loxosoma), the body is divided into a calyx and a stalk which possesses foot-gland at the base.
In the colonial forms (Pedicellina and Urnatella) the body of each zooid is separated from the stalk by a diaphragm. Asexual reproduction occurs by budding from any part of the body and the buds remain attached to the parent which leads into colony formation.
The development is indirect and proceeds through the spiral cleavage. The larva is ciliated and free-swimming. It is a planktotrophic trochophore type larva. After a brief swimming phase the larva attaches itself to the free surface of other animal with the help of postero-ventral attachment organ and transforms into the zooid form.
On the basis of development it is assumed that they are related to those groups that have trochophore larva in their development and probably have evolved from a common ancestor, share with the coelomate bryozoans.
Classification of Phylum Entoprocta:
Distinctive characters of the phylum:
1. Phylum Entoprocta are minute, sedenteric and most of the marine animals.
2. Solitary or colonial forms.
3. Bilaterally symmetrical, acoelomate and un-segmented animals.
4. Body of Phylum Entoprocta is divided into calyx, stalk and stolon.
5. Both mouth and anus open inside the circlet of tentacles, hence called Entoprocta or Endoprocta.
6. U-shaped alimentary canal.
7. Circulatory and respiratory organs are absent in Phylum Entoprocta.
8. Protonephridial excretory system.
9. Entoprocts are hermaphrodites.
10. Spiral, determinate cleavage.
11. Formation of mesoderm from the cell 4d.
12. Development leads to a ciliated planktotrophic trochophore.
Examples – Loxosoma, Pedicellina, Myosoma, Urnatella.
Affinities of Phylum Entoprocta:
With Ectoprocta:
Formerly Entoprocta and Ectoprocta were grouped under a single phylum— Ectoprocta because the two groups bear some similar features:
1. Presence of a crown of ciliated tentacles.
2. Presence of U-shaped alimentary canal.
3. The larva of Entoprocta superficially resembles the Cyphonautes larva of Ectoprocta.
Dissimilarities:
1. Entoprocts are acoelomates but ectoprocts are coelomate animals.
2. In entoprocts, the anus and mouth are situated within the circlet of tentacles but in ectoprocts, anus remains at the outside of circlet of tentacles and the mouth is only located within the crown of ciliated tentacles.
3. The protonephridia and gonoducts are present in entoprocts but both are absent in ectoprocts.
Remarks:
Considering the above features it can be concluded that the ectoprocts are a highly organised group than the entoprocts and their exclusion as separate phylum is more reasonable.
Brien and Papyn (954) concluded that the entoprocts differ decidedly from the ectoprocts in their embryology, metamorphosis and adult anatomy and therefore support their separation as two distinct phyla. Hyman (1951), Barnes (1980, 1987) support the above view.
Phylogenetic Relationship of Phylum Entoprocta:
Again colonial entoprocts show superficial resemblance with bryozoans. So some workers consider that bryozoans may have evolved from entoprocts. Molecular studies of 18S RNA genes indicate that entoprocts are protostomes.