GENERAL ZOOLOGY 



Fig. 10.18. Hormiphora plumosa, a 

 tentaculate ctenophore closely re- 

 lated to Pleurobrachia, in its normal 

 swimming attitude. (After C. Chun, 

 1880, Die Clenophoren des Golfes von 

 Neapei) 



Representative Types, Common and representative genera to be found 

 along our North Atlantic seaboard are Pleurobrachia, with tentacles (Fig. 

 10.18), and Mnemiopsis, without tentacles. A very specialized type of cteno- 

 phore is Cesium veneris of the Mediterranean. In this form the body is com- 

 pressed laterally and elongated to produce a thin, narrow plate, sometimes as 

 long as 4 feet. The minute creeping ctenophores, Coeloplana and Ctenoplana, 

 represent another extreme type in which the body is flattened and otherwise 

 modified for a creeping existence. 



Ctenophores are unfamiliar to most students. It is practically impossible 

 to preserve them satisfactorily for examination in the laboratory, but they 

 present many interesting features for those fortunate enough to study them 

 alive at the seashore. 



Radiata and Bilateria 



As indicated in Chapter 7 (p. 221), the Coelenterata and Ctenophora con- 

 stitute the grade Radiata, standing apart from and below the grade Bi- 

 lateria which includes all higher Metazoa. Although roughly comparable to 

 the coelenterates in many ways, the ctenophores have progressed in evolu- 

 tion to a somewhat higher level than the coelenterates, but in a direction 

 which evidently does not lead toward the higher Metazoa. 



When the classification into radiates and bilaterals is interpreted in terms 

 of evolution, the Radiata must be regarded as a very ancient type of animal 

 life. They show great specialization along their own lines, but the funda- 

 mental body plan, the two-layered sac surrounding a cavity with only one 

 opening to the exterior, is remarkably simple. It may be significant from the 



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