Ctenophora- singular ctenophore. These are commonly called sea walnuts or comb jellies or Venus’s girdles. Phylum Ctenophorehas a wide range of body forms, including the flattened, deep-sea Platyctenids, in which the adults of most species lack combs, and the coastal beroids, which lack tentacles and prey on other ctenophores by using huge mouths armed with groups of large, stiffened cilia that act as teeth.
Characteristics features of Ctenophora:
- Habitats: The members of Ctenophora are exclusively marine.
- Level of Organisation: Tissue level of organization.
- Germinal layer– Dipoloblastic organisms.
- Symmetry: The body is radially symmetrical. (Biradial symmetry)
- Locomotry organs: The body of organisms bears 8 rows of ciliated comb plate, which is situated externally. This works as a locomotory organ.
- Tentacles may be present or absent, are present two in numbers. They possess adhesive cells called colloblast.
- The brain or nervous system is not found
- Bioluminescence is found in ctenophores. ( Bioluminescence- it is the property of living organisms to emits light- e.g. Pleurobrachia). The presence of special sense organs called statocytes is a characteristic feature.
- Digestion: Both extracellular and intracellular digestion is found.
- Not separated sexes are found in Ctenophora.
- Only sexual reproduction is found in these animals.
- Fertilization occurs externally. The indirect development, an immature ciliated stage called cydippid larva.
- Some examples of Ctenophora: Pleurobrachia (sea gooseberry) and Ctenoplana, Cestum (the venus girdle), and Beroe (swimming eye of the cat).
Classification of Ctenophores
- Cydippid ctenophores have bodies that are more or less rounded, sometimes nearly spherical and other times more cylindrical or egg-shaped; the common coastal “sea gooseberry”
- The tentacles of cydippid ctenophores are typically fringed with tentilla (“little tentacles”), a few genera have simple tentacles without these side branches.
- Colloblasts are specialized mushroom-shaped cells in the outer layer of the epidermis with three main components:
- A domed head with vesicle (chambers) that contain adhesive
- A stalk that anchors the cell in the lower layer of the epidermis or in the mesoglea
- A spiral thread that coils around the stalk and is attached to the head and to the root of the stalk. The function of the spiral thread is uncertain, but it may absorb stress when prey tries to escape, and thus prevent the collobast from being torn apart
The lobata consisting a pair of lobes, and are muscular, cuplike extensions of the body which project beyond the mouth.
- The tentacles originate from the corners of the mouth
- Between the lobes on either side of the mouth, many species of lobates have four auricles, gelatinous projections edged with cilia that produce water currents that help direct microscopic prey toward the mouth, by which they are able to prey planktonic prey.
- Lobates comprising eight comb-rows, originating at the aboral pole which is usually not extending beyond the body to the lobes; in species with (four) auricles, the cilia edging the auricles are extensions of cilia in four of the comb rows
- No feeding appendages, large pharynx bears “macrocilia” at the oral end.
- Fused bundles of several thousand large cilia make pieces of prey that are too large to swallow as a whole.
- In species, Beroe a pair of narrow strips of adhesive epithelial cells on the stomach wall is found, which zip the mouth when the animal is not feeding.
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