24 



Ctenopnores — ComL Jellies 

 or Sea Walnuts 



The ctenophores or comb bearers constitute a small phylum whose members 

 live in the surface waters of warm seas and ocean currents. They are commonly 

 taken for jellyfishes and were formerly classified with them. Their differences 

 from coelenterates, the absence of stinging cells and the peculiarities of sense 

 organs and radial bands, are now regarded as important enough to place them 

 in a separate phylum. They are transparent and glimmering, some pink or 

 bluish or orange, but many colorless except for the continually shifting coppery 

 bronze iridescence of their combs. All of them are luminescent and the millions 

 occasionally swarming through the ocean surface create fantastically beautiful 

 illuminations. 



General Features. Ctenophores are moderately small, often about the size 

 of a plum. One of the smallest, Pleurobrachia is no larger than a garden pea 

 (Fig. 24.1). The pale violet Venus's girdle (Cestum) is a ribbon 2 to 3 feet 

 long. It is usually oval or globular, sometimes pear-shaped. 



Its conspicuous distinguishing feature is the eight rows of combs that radiate 

 from the mouth at one pole of the animal and extend to the opposite one like 

 the ridges of a cantaloupe (Fig. 24.1 ). These rows are arranged in radial sym- 

 metry, but the long tentacles usually present are located one on each side of 

 the body and Venus's girdle is clearly bilaterally symmetrical. Ctenophores 

 are regarded as a higher group than coelenterates because of their tendency 

 toward this balance of two sides of the body. Another mark of progress is the 

 three-layered body wall, ectoderm, endoderm, and a middle layer closely ap- 

 proaching the cellular mesoderm of higher animals. In ctenophores whole cells 

 are muscular, not merely the processes as in the epithelio-muscular cells of 

 hydra. They have no stinging cells. Neither is there asexual reproduction nor 

 alternation of generations as in coelenterates. 



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