314 



INVERTEBRATE MORPHOLOGY. 



animals are propelled through the water. In their general 

 form much diversity is observable. In accordance with their 

 pelagic habits the majority are more or less transparent ; and 

 some, the Gymnosomata, e.g. Pneumoderma, Clione, etc., are 

 entirely destitute of a shell, mantle, and, except in Pneuino- 

 derma and its allies, of a brauchia. Others, the Thecosomata, 

 possess these structures, however the shell in Limacina being 

 spirally coiled, the mantle-cavity situated in front of the 



FlG. 140. Hyalea complanata (after GECENBAUR, from HERTWIG). 



m = mantle. 



a = anus. 



br = branchiae. 

 c = heart. 



O = reproductive organs. 

 Jt digestive gland. 



oe = oesophagus. 

 re = nephridia. 

 v = stomach. 

 II = pedal ganglion. 



visceral hump being without a brauchia ; iii Sfyliola the shell 

 is not coiled, but is cone-shaped and bilaterally symmetrical, 

 the mantle- cavity containing a gill ; while in Cyuibuliopsis the 

 original shell is replaced by a cartilaginous case formed by 

 the subepidermal tissues of the mantle, and the voluminous 

 mantle-cavity contains no gill. The head of the Gynmoso- 

 mata carries a non-retractile proboscis, at the extremity 

 of which is situated the mouth, and it may furthermore bear 

 in addition to the tentacles usually present peculiar tentacle- 

 like processes, sometimes provided with suckers and perhaps 



