CHAPTER XI 



PHYLUM CTENOPHORA 



This is a group of exclusively marine animals, most of which are 

 pelagic (float near the surface). There is a limited number that 

 lives and moves about on the bottom. Ctenophora (te nof '6 ra — 

 comb-bearing), because of their similarity to coelenterates, are often 

 classified as a class in this phylum. There are only twenty-one Amer- 

 ican species representing this phylum, and they are commonly called 

 sea walnuts or comb jellies. Most of them swim by means of eight 

 rows of fused cilia, called swimming plates or combs. These animals 

 are quite clear and transparent, with a faint tint of pink, purple, or 

 blue. They are often phosphorescent. There are two classes in the 

 phylum: (1) Tentac^data, with a pair of tentacles present either in 

 the larva stage or throughout life. Mnemiopsis leidyi is a lumines- 

 cent, transparent form ; Pleurobrachia hachei has long tentacles on a 

 relatively short, oval-shaped body; and Cestus veneris, Venus 's 

 girdle, may be four feet long and only two inches in width, bandlike, 

 transparent, with an iridescence showing violet, blue, and green 

 colors. (2) Nuda, with no tentacles at any stage; Beroe ovata, about 

 10 to 12 cm. in length, conical in shape, and rather common, is an 

 example. 



Habitat and Behavior 



These are primarily surface-living forms with rather wide distri- 

 bution but most abundant in tropical seas. They move about very 

 slowly through the water with the oral end forward and the two 

 long tentacles trailing if tentacles are present. The tentacles have 

 adhesive or glue cells (colloblasts) which produce a secretion, and 

 with these they capture any small organisms making contact with 

 them. 



Anatomy 



The size of different individuals of this group ranges from five 

 millimeters to four feet in length, and the shape may be spherical, 

 pear-shaped, ribbonlike or cylindrical. The symmetry is said to be 

 biradial since there are eight rows of radially arranged paddles or 

 plates which are equally distributed on each side of the median 



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