PTEROPODA. 



207 



Class III. — PTEKOPODA. 



The head of the animals of this class is more or less 

 distinct, and at times is without eyes. (Gray falsely 

 denies them eyes altogether.) Two, rarely four fins are 

 situated between the head and the body, and are wing-like 

 organs of locomotion, hence the name ; sometimes a third 

 lobe is found between both these, which is apparently to 

 be regarded as a rudimentary foot like that of Gasteropoda. 

 The body is elongated, oval, or globular, sometimes naked, 

 sometimes enclosed in a shell of very varying nature. 



Naturalists are not agreed as to the situation of these 

 Mollusca in the system : Blainville and Souleyet include 

 them with the Gasteropoda, and place them near Bulla, 

 &c. ; others place them with the Cephalopoda, on account 

 of the suckers on the tentacula of Pneumodermon and 

 Spongiabranchia. 



The Pteropoda inhabit all seas, the Equatorial as well 

 as the Polar : they belong to deep seas, seldom approach- 

 i ing the shores, and are nocturnal, or at least twilight 

 , Molluscs, not to be met with, therefore, in the sultry heat 

 of the day; each species has its peculiar hour for ap- 

 pearing. The two fins are usually in constant and rapid 

 i motion ; by their means the animal moves in a horizontal 

 direction, rises or descends, while the body is kept in a 

 1 perpendicular position, or slightly inclined; some species 

 ! keep turning round in the same place, at the same eleva- 

 i tion, without making any perceptible advance. On the 

 appearance of any foreign body, or the violent shaking of 

 the vessel that holds them, they lay the fins together, 

 i withdraw completely into the shell, and the animal sinks 

 to the bottom. The Cavolinia swim faster than the Clio ; 



