PHYLUM CTENOPHORA. COMB JELLIES 



131 



ABORAL VIEW 



Oral end 

 SIDE VIEW 



Sense 



Mouth 



Figure 67. The structure of a typical ctenophore. Side view of Hormiphora. (After Chun.) 

 Aboral view of Pleurobrachia. (From Lankester's Treatise.) 



mesoglea; this "layer" of cells in the cteno- 

 phores resembles that in certain coelen- 

 terates but represents a higher grade of de- 

 velopment. All the systems are of tissue 

 grade in construction except that there are 

 indications of reproductive ducts in some 

 forms. 



Ctenophores differ from the coelenterates 

 in having ciliated bands, aboral sense organs, 

 mesenchymal muscles, more definite organ- 

 ization of the digestive system with anal 

 pores, pronounced biradial symmetry', and 

 no nematocysts. They probably evolved 

 from primitive coelenterate ancestors, but 

 can no longer be combined with that 

 phylum. 



Pleurobrachia pileus is white or rose- 



colored, ovoidal, about 2 cm. long, and 

 possesses tentacles about 15 cm. long; it oc- 

 curs from Long Island to Greenland, on the 

 Pacific Coast, and in Europe. Mnemiopsis 

 leidyi is a transparent, luminescent species, 

 about 10 cm. long, that lives along our east- 

 ern sea coast. It is often parasitized by a 

 minute sea anemone 1.5 mm. long. Some 

 bizarre forms occur among the ctenophores, 

 for example, Cestus veneris (Venus's- 

 girdle), headpiece, page 130, may be two 

 inches wide and over three feet long, trans- 

 parent, but showing green, blue, and violet 

 colors; it swims by muscular movements of 

 the ribbonlike body as well as by the beating 

 of the elongated swimming plates. It lives 

 in tropical seas but is sometimes carried 



