314 



INVERTEBBATE MOBPHOLOOT. 



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 Oymnosomata, e.g. Pneumoderma, Glione, etc., are 

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

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

 possess these structures, however — the shell in Limacina being 

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



Fig. 140. — Hyalea complanata (after GEr.ENBAnR, from Hketwio). 

 a = anus. m = mantle. 



ir = biaucbise. oe = oesophagus. 



c = heart. re — nephridia. 



G = reproductive organs. i) = stomach. 



7i = digestive gland. II = pedal ganglion. 



visceral hump being without a branchia ; in Styliola the shell 

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

 the mantle-cavity containing a gill ; while in Cymbuliopsis 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 Gymnoso- 

 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 



