CTENOPHORA 173 



Ctenophores are hermaphrodite ; the male and female gonads occur 

 close to each other in the subcostal canals. Self-fertilization probably 

 occurs. It is a remarkable fact that, if the first two segments of the 

 dividing egg of a ctenophore be separated a half larva will develop 

 from each segment. In the egg, therefore, the organ forming sub- 

 stances must be localized. If these half larvae be kept until generative 

 organs develop, the missing half is then regenerated. In contrast to 

 this behaviour in the Ctenophora, the separated blastomeres of the 

 cnidarian egg as far as the sixteen-celled stage will develop each into 

 a complete animal. 



The Ctenophora are divided into two orders: (i) Tentaculata, 

 possessing tentacles, to which the majority of forms belong; {n)Nuda, 

 without tentacles, to which belongs only the genus Beroe. 



Most of the Tentaculata have the ovoid shape, similar to that seen 

 in Pleurobrachia^ but some are flattened in a peculiar manner. Cestus 

 Veneris, Venus' Girdle, is flattened laterally and the body is drawn out 

 into a narrow band, two inches wide and nearly a yard long. It is 

 found in the surface waters of the Mediterranean. 



The Platyctenea, a group of Tentaculata to which belong the forms 

 Coeloplana and Ctenoplana, are flattened dorsoventrally. The flatten- 

 ing is produced by the expansion outwards of the stomodaeum so 

 that the whole of the ventral surface corresponds to the stomodaeum 

 of the normal types. Ctenoplana lives in the surface waters of the 

 sea and retains traces of the swimming plates, but Coeloplana 

 crawls over the rocks and seaweed, and resembles a turbellarian. 

 It has lost the swimming plates and developed pigment, but it still 

 retains the sense organ and the two tentacles. The gut system is 

 irregularly branched and the muscular system is highly developed 

 for crawling purposes. One member of the group, Gastrodes, is a 

 parasite in the body of Salpa. Its chief interest, however, is in the 

 larva, which is a planula, found nowhere else among the Ctenophora, 

 and thus provides the strongest piece of evidence for the close 

 relationship of the Ctenophora with the Coelenterata. 



