1869.] 259 



A Contribution to our knoivledge of Pelagic 3Iollusca, by Captain 



G-. E. Fryer, Madras Staff Corps. 



[Received 10th March, read 2nd June, 1869.] 



Having during the last ten years made four voyages round the Cape 

 of Good Hope, and enjoyed some opportunities of studying the habits 

 of Pteropods ; and understanding that information regarding them, 

 would not be unacceptable to the Bengal Asiatic Society, I have the 

 pleasure to furnish the following particulars. Thinking also that a 

 few directions may be of use to intending collectors, I have appended 

 some information regarding this point at the end of this paper. 



Pteropods are essentially pelagic animals, rarely found in the neigh- 

 bourhood of coasts. Some are naked, while others have their bodies 

 enclosed in a shell which, wherever it exists, is exceedingly light and 

 delicate. They are taken singly or in pairs, or in shoals. They vary 

 from the size of a hazel-nut to that of a pin's head. They jerk along 

 the surface of the water by the aid of two muscular appendages, from 

 whose wing-like character the name of the class is derived. They 

 generally rise to the surface after sunset, and disappear with daylight, 

 not as some have supposed for the purpose of breathing, since, for 

 the aeration of their systems, the majority possess a special branchial 

 cavity into which the external water has free access. So regular, 

 however, is their appearance after dark, that D'Orbigny regarded it as 

 the evidence of design. 



All observers of Pteropods are aware that, although for the most 

 part they are blind,* their susceptibility of light is very great. 

 When captured and placed alive in a tumbler of sea-water in a dark 

 spot, they protrude their fins, and flap them away vigorously ; but 

 no sooner is the light of a lantern turned on them, than they collapse, 

 sink and lie motionless at the bottom. 



I suspect then, with M. D'Orbigny, that light controls the move- 

 ments of these interesting animals. It is probable that as light 

 dawns, each sinks to that depth suited to its individual sensibility, 

 and sustaining itself in the fluid by its own specific gravity ; it ad- 

 justs its position as light recedes, by instinctively rising until it finds 

 * Clio excepted. 



