48 



Class PTEROPODA. 



Head more or less distinct ; eyes none ; mouth often 

 furnished with cup-shaped appendages. Fins two on the 

 sides of the mouth ; or two, or rarely four, on the side of 

 the body between the head and abdomen, often furnished 

 with a small intermediate lobe between them, apparently 

 the rudiment of the foot of Gasteropods. Body ovate or 

 roundish, often enclosed in a thin, conical, cylindrical, or 

 subglobular shell, with a transverse contracted mouth. In- 

 dividual unisexual ? Animal free, floating on the surface 

 of the sea by the assistance of its fins. Nocturnal or cre- 

 puscular. 



The Pteropods inhabit the high seas, floating constantly 

 in the water by means of a pair of lateral fins. They are 

 extremely vivaceous in their movements, and are frequently 

 associated together in prodigious numbers. The form of 

 their shells is very varied, being globular, subulate, hemi- 

 spherical, pyramidal, or spiral, but always thin, glassy, and 

 transparent. Loven compares the fins on the side of the 

 head to the fins on the side of the head of the foetal or 

 first-hatched fry of Gasteropods, which are sometimes 

 retained in the form of a fringe on each side of the body 

 in the adult animal, as in Turho and Trochus. In the 

 cup- shaped disks or suckers which many of them have 

 affixed to the arms that surround the mouth, they show 



