PTEROPODA. 



269 



CHAPTER XXIII. 



CLASS PTEROPODA. 



THE Pteropoda are defined by being free and pelagic, swimming 

 by means of two wing-like appendages (epipodia), developed 

 from each side of the anterior extremity of the body. The 

 flexure of the intestine is neural. 



As to the position of the Pteropoda in the Molluscan scale, 

 they must be looked upon as inferior in organisation to any of 

 the Gasteropoda, of which class they are often regarded as the 

 lowest division. They permanently represent, in fact, the tran- 

 sient, larval stage of the Sea-snails. 



The living Pteropods are all of small size, and are found 

 swimming at the surface of the open ocean, often in enormous 

 numbers. Locomotion 

 is effected by two wing- 

 like fins (fig. 241) devel- 

 oped from the sides of 

 the head. In some cases 

 the body is naked and 

 unprotected ; but there 

 is commonly a symmetri- 

 cal glassy shell, either 

 consisting of a dorsal and 

 ventral plate united, or 

 forming a spiral. 



The Pteropoda are divided into two orders, termed Thecoso- 

 mata and Gymnosomata; the former characterised by possess- 

 ing an external shell and an indistinct head ; the latter by 

 being devoid of a shell, and by having a distinct head, with 

 fins attached to the neck. 



The Gymnosomatous Pteropods, in which there is no shell, 

 as a matter of course, are wholly unknown in the fossil con- 

 dition. The Thecosomatous Pteropods, in which there is a 

 shell, are divided into two families the Hyaleida and Lima- 

 cinidce. The latter comprises forms in which there is a small 

 spiral shell, which is sometimes provided with an operculum ; 

 but it is unrepresented in a fossil state. The former family 

 comprises forms in which the shell is symmetrical, straight or 

 curved, globular or needle-shaped, and it is represented by a 

 considerable number of fossil forms, most of which are ex- 

 tremely unlike any known living examples of the class, being 

 often of comparatively colossal dimensions. The fossil forms 



241. Pteropoda. a Cleodora pyramidata ; 

 "uvieria columnella. (After Woodward.) 



