196 THE INVERTEBRATA 



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 ; (ii) 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 Cteno- 

 phora with the Coelenterata. 



