47 



CHAPTER XXYI. 



PTEROPODA. 



Class III. Pteeopoda. — The Ptcropoda are defined by being 

 free and j)elagic, swimming 'by means of two iving-like appen- 

 dages {epiijodia), developed from each side of the anterior co> 

 tremity of the hody. Tlie 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 repre- 

 sent, in fact, the transient 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 enor- 

 mous numbers. Locomotion is effected by two wing-Kke 

 fins (figs. 439, 440) developed from the sides of the head. 



Fig. 439.— Pteropoda. a, Cleodora pyramidata , 

 b, Cnvieria columnella. (After "Woodward.) 



Fig. iiO.— Eyalea tridentata, show- 

 ing the shell and the lateral fins at- 

 tached to the sides of the head (/, /). 



In some cases the body is naked and unprotected ; but 

 there is commonly a symmetrical glassy shell, either con- 



