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In this article we will discuss about the interrelationships between the different classes of phylum Cnidaria.
Class Hydrozoa represents the most primitive forms amongst the Cnidarians. The lowest hydrozoan form is exhibited by Hydra and the hydrula stage of some Hydrozoa.
The relationship between Hydrozoa and Scyphozoa is not very clear. In spite of this, it can be suggested that they have been derived from a common trachyline stem.
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Some of the scyphozoan characteristics, such as the marginal notches, gastric pouches, tentaculocysts, etc., have already been evolved in the Narcomedusae of class Hydrozoa. Nausithoe, belonging to the order Coronate possesses a branched larval form which resembles a hydroid colony very closely.
After their origin from the common stem, the hydrozoans and the scyphozoans have diverged greatly and the changes undergone by the scyphozoan ‘polyp’ (Scyphistoma) are the formation of mesenteries, gastric tentacles, etc., which are primarily for increasing the digestive surfaces in the coelenteron.
Amongst the scyphozoans, the Stauromedusae, for its polypoid characteristics and lack of tentaculocysts, are long being regarded as the ancestors of the other scyphozoans form. But this seems to be improbable if we consider the coelenterate stem to be a medusa. It is attested by many; that the Stauromedusae represent a post- larval stage.
They are not in the direct line of Scyphozoan evolution but lie very close to that line. Cubomedusae are the most primitive forms existing amongst the scyphozoans. To correlate them with higher order leads into difficulties, because its developmental dynamics are not fully known. The Coronatae, with all possibilities, are very near to the direct line of the scyphozoan ascent.
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The Actinozoans lack medusoid stage and the polypoid stage reaches its climax. The similarities of the Scyphistoma larva and the actinozoan polyp establish a common origin of the Scyphozoa and the Actinozoa.
The similarities are:
1. Gastric tentacles of Scyphistoma correspond to the mesenteric filaments of Actinozoa.
2. Gonads are endodermal in origin.
3. Septa have retractor muscles.
But difficulties in such assumption lie in the absence of stomodaeum in Scyphistoma and the symmetry of body forms (tetramerous symmetry in Scyphistoma and the octomerous bilateral symmetry in Actinozoa). It can best be suggested that the Scyphistoma and the ancestral form of Actinozoa have undergone parallel evolution after their origin from a remote common ancestor.
Of the actinozoans, three distinct evolutionary lines are recognisable:
1. The antipatharian-cerianthid line:
It constitutes the most primitive living groups of Actinozoa. They possess simpler orientation of septa and tentacles and have ill-developed mesoglea.
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2. The alcyonarians or octocoralline line:
According to Hyman (1940), octocorallines and hexacorallines probably had a common ancestry. The ancestor was a small polyp with eight tentacles and poor musculature.
3. The zooanthid-anemone or hexacorralline line:
The presence of Edwardsia stage in all the members of this group indicates that a form like this was probably the ancestor. It was devoid of basilar, sphincter and retracor muscles, but had “complete ectodermal muscle layer” as quoted by Hyman.
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The tendencies to form colony and skeleton have evolved independently amongst different members of Actinozoa and as such they have little phylogenetic significance.
