206 



COELENTERATA. 



period of larval life that the completely mature form is attained by the unequal 

 growth of the swimming plates and their canals, the outgrowth of the tentacle- 

 like processes, and the formation of two lobe-like projections round the mouth 

 from those halves of the body which correspond to the longer rows of swimming 

 plates. The Lobatac present a peculiar phenomenon, which has been called by 

 Chun, dissogony. The Cydippe-like larva develops sexual cells on its four sub- 

 stomach vessels, and becomes sexually mature during the hot period of the year. 

 The eggs are fertilised and develop normally into larvae smaller than those 

 produced from adults. After the sexual activity has continued for some days, 

 the larva loses its generative organs, undergoes a complicated metamorphosis, and 

 develops into the adult, with generative organs in all of its eight meridional 

 vessels. The phenomenon of dissogony is therefore characterised by the fact 

 that sexual maturity occurs twice in the same individual, and that the two 

 sexual periods are separated by a sterile period in which there are no generative 

 cells, and in which a complicated metamorphosis occurs. 



Tr 



4 



-M 

 m 



FIG. 164. Boroe ovatus. Ot 

 sense-organ, at its sides are 

 the small tentacles of the 

 polar areas ; Tr funnel. 



PIG. 165. Callianira bialata (after Chun). 

 mouth. 



The Ctenophora live in the warmer seas, and, under favourable 

 conditions, often appear in great quantities at the surface. They 

 feed on marine animals of various size, which they capture with their 

 tentacles. Many, as the Beroidae, which do not possess tentacles, 

 are compensated for this deficiency by the possession of an unusually 

 large mouth (Fig. 164), by means of which they are able to receive 

 relatively large bodies, even fishes, into the wide oesophageal tube, 



