PTEROPODA. 49 



their affinity to the Cephalopods. The Pteropods, like 

 many of the Cephalopods, are crepuscular in their habits, 

 sinking at sunrise into the bosom of the deep, and coming 

 again to the surface on the approach of evening. Accord- 

 ing to the observations of M. D'Orbigny, the smaller spe- 

 cies of Cavolina first appear as evening advances ; small 

 species of Clio then rise in company with other Gavolinee 

 and Atlantce ; after sporting a few hours the lesser species 

 descend and disappear, the larger follow, and towards mid- 

 night hardly one remains; after sunrise not a single Ptero- 

 pod can be seen. Each species has its own time of rising 

 and going down, which causes M. D'Orbigny to infer that 

 each species dwells habitually in the water at a depth pe- 

 culiar to itself. As the sun rises the Pteropod sinks lower 

 and lower, until it has reached its greatest descent, but as 

 the sun goes down the Pteropod passes gradually upwards 

 until the surface is gained. M. D'Orbigny gives the fol- 

 lowing as the result of his observations of the geographical 

 distribution and habits of these animals. Of the twenty- 

 nine species which he has observed, fourteen are equally 

 common to all the seas, at least to the Atlantic and Pacific 

 Oceans ; eleven have been observed only in the Atlantic, 

 and four in the Pacific. Seventeen of the twenty-nine 

 species are entirely nocturnal and eleven are crepuscular. 

 The Pteropods swim rapidly by ceaseless movements of 

 their wing-like fins, which has caused them to be styled the 

 butterflies of the deep ; they are carnivorous, feeding on 

 minute Crustacea and medusa3. In their zoological ar- 

 rangement they are naturally divided into those with, and 

 those without, a shell, or Thecosomata and Gymnosomata; in 

 the former, the head is not distinct and the gills are inter- 

 nal; in the latter, the head is distinct and the gills external. 

 The true position in a natural arrangement of this curious 



VOL. I. H 



