MOLLFSCA. 159 



Class III. PTEROPODA. 



COPONAUT^E. 



Free pelagic mollusks swimming by means of two fin-like ex- 

 pansions developed from the anterior extremity. Hermaphrodite. 



The head, not well defined, is expanded on each side into a 

 large muscular fin [epipodium]. There is a small mouth, some- 

 times tentaculate and having a radula. The mantle may be 

 absent or only slightly developed, or it is only present in the 

 earlier stages. There is no proper respiratory organ, but there 

 is occasionally a ciliated branchial sac. 



These are small, active animals, gaily coloured, mostly provided 

 with thin, symmetrical shells, and found in large numbers on the 

 surface at night (Major Owen), or glistening in the sunshine 

 (Wyville Thomson). They " absolutely swarm " in the high seas ; 

 in the north Clio borealis and Limacina arctica are the chief food 

 of the whale. 



Pteropoda are Schmarda's twenty-sixth class. For Glaus they 

 are a subclass of Gastroppda. According to the former there are 

 scarcely 100 living, and about 150 fossil species. 



Without a shell GYMNOSOMATA. 



With a shell THECOSOMATA. 



Order I. GYMNOSOMATA. 



Head and foot distinct; no mantle. Shell absent. Larva 

 eventually with cilia. 



The fins are attached to the neck ; the head is tentaculate, 

 except in Cliodita, and in Pneumodermon the mouth also. The 

 latter has a rudimental shell placed at the bottom of the visceral 

 cavity. 



Cliid(g. Cliodita. Pneumodermida. 



Cll ' 



Order II. THECOSOMATA. 



Head rudimentary. Shell always present, but internal in 

 Cymbuliida3. Larva without cilia. 



In these animals the hinder part of the body is protected by a 

 light transparent or semitransparent shell, variously shaped, but 

 spiral in Limacinidse. 



