PTEEOPODA. 



347 



In strncttire. tlie Pteropoda are most nearly related to the 

 marine tiniyalves, but miicli inferior to them. Their nervous 

 ganglia are concentrated into a mass helow the oesophagus ; they 

 have auditory vesicles, containing otolites ; and are sensible of 

 light and heat, and probably of odoui-s, although at most they 

 possess very imperfect eyes and tentacles. The true foot is 

 small or obsolete ; in deodora it is combined with the fins, but 

 in Clio it is sufficiently distinct, and consists of tvro elements ; 

 in Spinalis the posterior portion of the foot supports an oper- 

 culum. The fins are developed from the sides of the mouth or 

 neck, and are the equivalents of the side-lappets {epipodia) of 

 the sea-snails. The mouth of Pneumodermon is furnished with 

 two tentacles supporting miniature suckers ; these organs have 

 been compared with the dorsal arms of the cuttle-fishes, but it 

 is doubtful whether their nature is the same.* A more certain 

 point of resemblance is the ventral flexure of the alimentary 

 canal, which terminates on the under surface, near the right 

 side of the neck. The pteropods have a muscular gizzard, armed 

 with gastric teeth ; a liver ; a pyloric ccecum ; and a contractile 

 renal organ opening into the cavity of the mantle. The heart 

 consists of an auricle and a ventricle, and is essentially opistlio- 

 hranchiate, although sometimes afi'ected by the general flexure 

 of the body. The venous system is extremely incomplete. The 

 respiratory organ, which is little more than a ciliated surface, is 

 either situated at the extremity of the body and unprotected by 

 a mantle, or included in a branchial chamber with an opening 

 in front. The shell, when present, is symmetrical, glassy, and 

 translucent, consisting of a dorsal and a ventral plate united, 

 with an anterior opening for the head, lateral slits for long fili- 

 form processes of the mantle, and terminated behind in one or 

 three points ; in other cases it is conical, or spirally coiled or 

 closed by a spiral operculum. The sexes are united, and the 

 orifices situated on the right side of the neck. According to 

 Vogt, the embryo Pteropod has deciduous vela, like the sea- 

 snails, before the proper locomotive organs are developed. 

 (Huxley.) 



From this it would appear that while the Pteropoda present 

 some analogical resemblances to the Cephalopoda, and perma- 

 nently represent the larval stage of the sea- snails, they are 

 developed on a type sufficiently peculiar to entitle them to rank 



* The figures of EycToux and Souleyet represent them as being supplied with nei-ve3 

 {rem the cephalic ganglia ; whereas the arms of the cuttle-fish, and all other parts or 

 r-iodiScations of the foot in the moUusca, derive their nerves from the pedal ganglw., 



