PTEROPODA. 113 



l^igny imagined that eacli kind occupies a different 

 bathy metrical zone. Souleyet and other naturalists, 

 however, dispute these assumptions ; and there is no 

 doubt that every known species has been found on the 

 surface, and by day as well as by night — as any obser- 

 vant traveller, who has crossed the Equator, can testify, 

 their capture, with a towing-net or ship^s bucket, being 

 an agreeable diversion, and relieving the tedious mono- 

 tony of a long sea-voyage. Their food consists of 

 minute Entomostraca and other animals. Considerable 

 attention has been paid of late years to the physiology 

 and development of the Pteropods. According to Vogt, 

 Gegenbaur, and Krohn (all first-rate zootomists), the 

 larva has a circle or crown of strongly marked vibratile 

 cilia, which are absorbed or fall off in a subsequent 

 period of growth. They swarm within the tropics, but 

 are scarce in northern seas, whither some exotic kinds 

 are now and then wafted by the south-west winds and 

 consequent drift, arising from the Gulf-stream. Their 

 occasional appearance on our own coasts, and their 

 claims to be indigenous, rest on the same grounds as 

 the case of Ianthin(B ; and naturalists must please them- 

 selves as to considering such accidental wanderers 

 British or foreign. Two kinds at least have been found 

 alive in our seas. 



It is very doubtfal whether this group, although pecu- 

 liar, ought to constitute a distinct class, or be included 

 in the Gastropoda, to which it is certainly related, as 

 regards the foot-lobes, through Bulla and Aplysia. 

 Nevertheless its connexion with the Cephalopoda can- 

 not be remote, because two genera [Pneumodermon and 

 Spongiobranchia) have arms furnished with sessile cup- 

 shaped disks or suckers. It may be better, on the 

 wholCj to retain the classification of Cuvier. The order 



