THE PELAGIAL 



231 



prevails among the Hydromedusae and Scyphomedusae and in the 

 swimming bells of siphonophores. It has been convergently developed 



Fio. 56. — Cirrothauma murrayi, a pelagic octopod. After Chun 



Fig. 57. — Pelagothuria nutans, a pelagic holothurian. m, mouth; a, anus; t, 

 tentacle; r, ampullae; d, swimming membrane between the ampullae. After 

 Ludwig. 



among pelagic cephalopods by web-like membranes connecting the 

 arms (Amphitretus, Cirrothauma, Fig. 56), and among pelagic holo- 

 thurians by similar webs between the tentacles (Fig. 57). The con- 



