Q.4. Discuss the affinities of comb jellies. Give an account of the affinities of Hormiphora.
Ans.4.
A. Affinities with Coelenterata
Ever since the time of Eschscholtz (1829-1833), ctenophores have been regarded as a class of phylum Coelenterata. Some writers still continue to group them as subphylum Acnidaria together with the subphylum Cnidaria in the phylum Coelenterata.
1. Resemblance with Coelenterata
Ctenophores are believed to be an offshoot from the ancestral medusoid coelenterate. They resemble the coelenterates in several characters, such as :
1. Biradial (tetramerous) symmetry and tentacles.
2. Arrangement of parts along an oral-aboral axis.
3. Two cell layers in the body wall, i.e., epidermis and gastrodermis.
4. Thick gelatinous mesogloea in between the two cellular layers,
5. Absence of coelom or other internal spaces except for the digestive cavity.
6. Endodermal gastrovascular cavity with branches.
7. A single aperture or mouth leads from the gastrovascular cavity to the outside.
8. Diffused nerve plexus.
9. Presence of statocyst.
10. Endodermal origin of gonads.
11. Absence of nephridia.
12. Presence of lasso cells similar to the nematoblasts of the coelenterates. The existence of nematoblasts in a ctenophore (Euchlora Rubra) is additional evidence of the coelenterate origin of the Ctenophora.
13. No excretory, respiratory, or circulatory systems are present.
On account of these similarities, they are considered by many zoologists to be a class of Coelenterata.
2. Resemblance with Hydrozoa
Attempts have been made to demonstrate affinities of Ctenophora with Hydrozoa. An anthomedusan form, Ctenaria, presents remarkable similarities with a cydippid, such as Hormiphora, in the following characters :
1. The general body surface of a ctenophore corresponds with the exumbrellar surface of a medusa.
2. The pharynx or stomodaeum of a ctenophore corresponds with the sub-umbrellar cavity of a medusa.
3. Both possess a thick, gelatinous mesogloea.
4. Presence of a simple gastrovascular cavity.
5. Presence of two opposite tentacles in the sheath.
6. Eight radial canals given off from the stomach, and corresponding with them are eight bands of nematocysts diverging from the apex of the ex-umbrella.
3. Resemblance with Anthozoa
The ctenophores also present certain anthozoan characteristics.
1. Ciliated ectoderm of Anthozoa is probably a forerunner of the ciliated band of ctenophore.
2. Presence of a well-developed stomodaeum.
3. Gonads develop in connection with the gastrodermis and sexual elements passed out through the mouth.
4. The gut in embryos of both is 4-lobed, thus presenting a biradial symmetry.
5. Mesogloea is cellular.
But the aboral sense organ and rows of comb-plates of a ctenophore (1) have no parallel parts in an anthozoan, (2) lasso cells differ structurally from the nematocysts, and (3) the tentacles are hollow in an anthozoan while solid in the ctenophore. (BSc 1st Year Lower Non-chordates Phylum Ctenophora Sample Model Practice Question Answer Papers)
4. Differences from Coelenterata
The ctenophores, however, differ greatly from Coelenterates in having :
1. Oppositely placed tentacles that suggest a biradial symmetry.
2. 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 special adhesive cells and the colloblasts over the tentacles.
6. Mesenchymal muscles. With the development of a definite mesoderm, the ctenophores may be considered triploblastic.
7. More definite organization of the digestive system with anal pores.
8. Development of determinate type.
B. Affinities with Platyhelminthes
The following resemblances between the ctenophores (Coeloplana and Ctenoplana) and the polyclad turbellarians are of considerable importance :
1. Dorso-ventrally flattened body.
2. Crawling mode of life.
3. Ciliation of the epidermis.
4. Lobed gastrovascular cavity, especially in the embryos.
5. Similar earlier stages of segmentation of gastrulation.
6. Gelatinous mesenchyme with muscle fibres and cells.
C. Conclusion
Ctenophora have undergone considerable specialization and bear many striking characteristics of their own, so it seems more logical to treat them as 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, such as:
1. Prominence of an apical region.
2. Mode of origin of musculature from mesoderm.
3. Presence of gonoducts.
4. Determinate the type of cleavage.
It implies that Ctenophora are intermediate between Radiata and Bilateria. They seem to have diverged very early from ancestral medusoid coelenterates, which also gave off the other three branches of Coelenterata. The Ctenophora themselves represent a blind offshoot.
BSc 1st Year Lower Non-chordates Phylum Ctenophora Sample Model Practice Question Answer Papers
BSc 1st Year Lower Non-chordates Phylum Ctenophora Sample Model Practice Question Answer Papers
BSc 1st Year Sample Model Practice Mock Test Question Answer Papers