PTEROPODA. 



Class 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 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. Branchiae, 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 

 Balsena 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 usuall}'^ 

 appear at night onl}^, and some only when the night is veiy 

 dark ; and d'Orbigny 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 



