THE PTEROPODS. 117 



ceaseless movements similar to those of the wings of the 

 butterfly. These fins are rowed with remarkable ease and 

 quickness, and according to the direction of the motion, 

 the creatures advance horizontally, rise or descend, while 

 the shell is kept in a vertical position or slightly inclined. 

 At times the Pteropod will turn itself without altering its 

 place, or poise itself still and motionless at the same depth; 

 but this immobility is to be observed only in a few species, 

 while all make habitually the papilionaceous movements. 

 If, during their course, any strange body comes in contact 

 with them, or a sudden shock agitates the vessel in which 

 they are kept, they fearfully fold their fins upon themselves, 

 or, in some species, withdraw them in the shell, and allow 

 the body to fall to the bottom of the vessel. It is probable 

 that on a similar alarm in a state of liberty, they would 

 unfold anew their fins, and stop their downward fall, when 

 the body had sunk to a depth which placed it beyond the 

 reach of danger. Rang has asserted that some of the 

 Creseis* do occasionally cluster on the gulf-weed, where 

 they rest themselves by embracing the leaves and stalks 

 with their fins, but D'Orbigny has never been witness of 

 such an occurrence, which is probably accidental, for the 

 species of Pteropods of which it is related become, he says, 

 rare in the gulf- weed fields ; and as the fins are not calculated 

 for prehension, they might too easily be driven amid the 

 crowded weeds where their thin brittle shells could hardly 

 resist the shocks to which they would be exposed. f 



The Pteropods are most numerous, both in species and 

 individuals, under the torrid zone, but a few " swarm 

 populous" in the arctic seas, whose prolific waters are in 

 strong contrast with the sterility of their shores and barren 

 lands. The Clio borealis, in especial, fills them in some 

 seasons with such teemful fecundity, that the whale cannot 

 open his mouth without necessarily engulphing myriads of 

 them. The Limacina, (Fig. 13) is not less profuse in the 

 same seas, of whose habits the worthy Otho Fabricius has 

 given a lively account, which loses, however, much of its 

 spirit in my translation. " The shell is its boat, which the 

 snail rows admirably through the water by the regularly- 

 timed strokes of the raised fins. In this act the open 

 extremity of the shell is the prow, the opposite end occupies 

 the place of a poop, and the margin of the body-whorl 

 resembles and performs the office of the keel. I have often 

 seen it with admiration and pleasure, — " quod saepius 



* Manual, 21. 



t Ann. dcs. Sc. Nat. Zoologic, n. s. iv. 189—192. 



