In this article we will discuss about the features and classification of ctenophora.
Salient Features of Ctenophora:
1. Body bi-radially symmetrical.
2. Presence of eight meridional rows of ciliary plates (comb plates).
3. Absence of cnidocytes.
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4. Presence of adhesive cells (colloblasts) in the tentacles.
5. Mesenchymal muscle cells present in the mesoglea between the ecto- and endoderm layers.
6. Development mosaic with cydippid larva.
Owing to its peculiar anatomical organisation and the common features shared with different groups of invertebrates, attempts have been made by a number of workers to establish its relationship with other invertebrate phyla.
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In spite of certain resemblances with Cnidaria, particularly Acanthomedusae (Hydrozoa), Actinozoa, and Scyphozoa, relationship of Ctenophora with Cnidaria is doubtful. In our present state of knowledge it is impossible to determine the affinity of Ctenophora with any group of Cnidaria.
Ctenophores have certain resemblances with Turbellaria (Platyhelminthes) and it has been suggested that Ctenoplanidae (Ctenophora) possibly represent an intermediate stage between Ctenophora and Platyhelminthes.
Classification: Phylum Ctenophora:
1. Exclusively marine, solitary, free-swimming, pelagic, carnivorous, phosphorescent, radially symmetrical, triploblastic, acoelomate metazoa.
2. Soft, delicate, transparent, gelatinous body, without segmentation.
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3. In some, body typically spherical, pear- shaped or cylindrical.
4. Absence of cnidocytes and gastric filaments.
5. Skeletal, respiratory, circulatory and excretory systems absent.
6. Eight meridional rows of comb-like ciliary plates are present on the surface. The ciliary combs retain ciliary movement throughout life.
7. A pair of long, solid and contractile tentacles usually project from blind pouches in opposite sides of the body.
8. The tentacles bear special adhesive cells, the lasso cells or colloblasts, which help in food capture.
9. The mouth, lying at one end leads into a large ectodermal stomodaeum connected with a series of endodermal gastro vascular canals.
10. Diffused nervous system; aboral end bears a sensory organ of equilibrium, the statocyst.
11. All are hermaphroditic, the testes and ovaries formed side by side from the endoderm of gastro vascular canal.
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12. Alternation of generations absent; development accompanied by metamorphosis with formation of a characteristic cydippid larva.
The phylum Ctenophora has been divided into two classes:
Class 1. Tentaculata or Micropharyngea:
The body in section is either circular or slightly compressed in lateral plane.
Adults with two long aboral tentacles, retractile in sheaths and un-branched meridional and stomodaeal vessel; in certain cases only larva has tentacles, while the adult has oral lobes.
Examples: Hormiphora, Pleurobrachia, Bolinopsis, Velamen, Ctenoplana, etc.
Class 2. Nuda or Macropharyngea:
1. Tentacles absent.
2. Mouth very wide, gullet occupies the greater part of the interior body.
3. The meridional vessels are produced into a complex system of anastomosing branches. Example: Beroe.