88 PTEROPODA. 
Ciass PTEROPODA. 
Mollusk naked, or protected by an external or internal, testa- 
ceous or membranaceous shell of variable form, with or without 
operculum. They are essentially pelagic, furnished with a foot 
dilated on each side into a large aliform expansion suitable for 
swimming; or having only a rudimentary foot, but with accessory 
locomotive organs, represented by two lateral swimming disks. 
In swimming the body i is nearly reversed in position, the abdomen 
being uppermost. The more or less distinct head has one or two 
pairs of tentacles. Mouth terminal or subterminal, with lingual 
armature, and sometimes organs of prehension and of mastica- 
tion. Branchie, either external or contained within an interior 
cavity. The sexes are united in the same individuals, but the 
male organ is separated from the female. Carnivorous. 
The pteropods are all small mollusks, some of them even 
microscopic; they are commonly known as sea-butterflies and 
whale-food. The first of these names has been given on account 
of the form and incessant movement of their swimming lobes ; 
the second because they form a portion of the food of the 
Balzena and other cetaceans, as well as of a great number of 
fishes. The pteropods live at a certain depth beneath the surface, 
and only approach shores by accident, or when carried by storm 
or current. 
The Pneumodermon, Clio and large species of Cleodora usually 
appear at night only, and some only when the night is very 
dark; and VOrbigny supposed that it is only when the degree 
of obscurity at the surface approximates to that which the 
animal habitually perceives in its daylight habitation, that it 
rises at all; certain it is, says he, that so soon as the sun ap- 
pears, not a pteropod is to be seen. Later observers, however, 
have established the fact that specimens may be obtained from 
the surface of the ocean at all hours of the day, although they 
are mainly crepuscular in habit. 
These little animals are eminently sociable, forming consider- 
able masses in the regions which they inhabit. They occur in 
all seas, but most of them are found in temperate and tropical 
latitudes, whilst a few forms are restricted to the Arctic seas. 
Contrary to the usual fact among mollusca, the Arctic species 
are here the most highly colored ; due to the transparence of 
their shell, which partly shows the viscera. 
Pteropods live upon microscopic animals, and possibly small 
mollusks, such as Atlanta and crustaceans. A few of them 
possess organs of prehension, but it is difficult to indicate the 
means by which the most of them seize their prey. 
Among the pteropods some have an external or internal shell 
