Ctenophores “Comb Jelly”
Recently these organisms gained lot of importance in the world of evo devo for emerging as the most primitive animal group on this planet.The phylogenomics studies done by Dunn et al which appeared in Nature issue of April 2008 had strong data for placing Ctenophores at most basal level more lower then sponges ,who held this place for long.
The phylum Ctenophora commonly known as comb jellies(even though they are relatively more complex then jelly fishes) are marine predators, feeding mostly on plankton. These are diploblastic orgnaisms (lack mesoderm made up of only two germ layers in endoderm and ectoderm). However, below the ectoderm lies the mesoglea(like mesoderm), a highly gelatinous layer filled with fibers as well as muscle, nerve, and mesenchymal cells.So some scientists are of opinion they are true triploblastic organisms with three germ layers.
The symmetry of ctenophores is interesting with many parts showing bilateral symmetry but some like anal pores show asymmetry. The body consists of two transparent layers inner and outer, which is covered by slime layer. The inner layer surrounds a cavity which serves as stomach with only one opening in mouth .Mouth is connected to a long gullet(invagination into the cytoplasm of certain ciliates, used for food intake.)Stomach has opening on other end ends in the form of anal pores but they are rarely used as undigested food finds its way out through mouth itself.Many species have two retractable tentacles emerging from the midpoint of the body, which are used to catch prey.The tentacles have adhesive structures called colloblasts, or lasso cells. These cells open when prey comes in contact with the tentacle. Sticky threads released from each of the colloblasts will then capture the food.
Many species are bioluminescent(caused by scattering of light due to the beating of cilia, not because of bioluminescence). The name comb jelly comes from eight “comb rows” of fused cilia, called ctenes, which are arranged laterally along the sides of the animal and used for locomotion.
Comb jellies are hermaphrodite but some reproduce asexually.Ctenophores are capable of extraordinary regeneration; even if half of the creature is destroyed, often the remaining half can rebuild itself.
The body is spread by a simple net of neurons without a ‘brain’. These nerves are concentrated around the mouth, tentacles, ‘combs’ and statocysts and are connected with the muscular cells found in the mesogloea and the inner cellular layer of the ectoderm.
The best-known genera of ctenophore is Mnemiopsis ,like jelly fishes their body is also filled with mostly water.Due to their soft bodies fossils record are of least help and thus making it very difficult to track the history of these enigmatic creatures.
In the coming posts we talk about the recent data available from Martindale’s lab on Homeobox genes ctenophore Mnemiopsis leidyi which was published in recent issue of Development genes and evolution.
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