xcii Introduction. 



Sejoiadae, and the extinct Belemnitidae are referred; the Becapoda 

 cJwjidrojiliora, which do not possess a calcified shell, but a horny 

 ^pen"* or ^gladius/ and the Octopoda, which, with the exception of 

 CirrhoteutJds, have no internal shell. 



Class, Gasteropoda. 



Mollusca^ distinguished as a class from the Pteropoda and Cepha- 

 lopoda most obviously by the characters of their ' foot,^ which is 

 ordinarily flat and sole-shaped, and adapted for crawling-. Usually 

 the foot is not divisible into a propodium^ mesopodium, and meta- 

 podium, though the posterior part of the organ is nearly always 

 well developed, and even when no division exists to denote its 

 typically trifid character, the presence of an operculum frequently 

 enables us to differentiate its metapodial portion. In the Hetero- 

 l^oda however, and in the Strombidae, the three divisions of the foot 

 are very clearly distinguishable; and the epipodium is occasionally 

 recognizable, as in Aplijs'ia and Tiirho. The foot proper may be 

 longitudinally divided for crawling, as in Thasianella, or expanded 

 into lateral lobes for swimming, as in Gasteropteron and Bullidae ; 

 or it may be adapted for the purpose of swimming by being con- 

 verted anteriorly into a vertical fin, whilst it retains its ordinary 

 caudate shape posteriorly, as in the Heteropoda. Finally, the foot 

 may be merely rudimentary, as in Glancns, lanthina, and Yermetiis. 



The Gasteropoda count among their number the only representa- 

 tives of the Sub-kingdom which have attained to aerial respiration, 

 and they form by far the most numerous of all Molluscan, and, with 

 the exception of the Insecta, of all animal Classes. 



Their digestive tract is almost invariably more or less convoluted, 

 and with the exception of the parasitic Entoconcha mirabilis, and 

 possibly a few Apneusta (see Baur, Nova A.cta, 1864, p. 71), it is 

 always proctuchous. The mouth and anus are ordinarily near to 

 each other, but are never in the same median plane. In certain 

 Apmeusta and Nudihranchiata , the intestinal tract takes a straight 

 antero-posterior course, but is provided with lateral gastro-hepatic 

 diverticula, which give it much the appearance of the digestive 

 tract of one of the Dendrocoelous Planarian or Ti'ematode Vermes. 

 The heart, which has been supposed to be absent in the Gasteropoda 

 just mentioned, does not seem to be so in any member of the class 



