292 



Mollusca. 



margin of the mantle-fold. At the anterior end of the sac, the mantle- 

 furrow is much accentuated, forming a deep pouch, the mantle- 

 cavity, opening ventrally ; posteriorly the furrow is shallow, just as 



in the Chitons. Most of the 

 organs (alimentary canal, 

 liver, gonads, etc.) are con- 

 tained in this saccular region, 

 whilst the lower portion of 

 the body is almost without 

 viscera ; the sac is covered by 

 a calcareous shell. In con- 

 sequence of this peculiarity 

 of structure, the body of the 

 animal is naturally divided 

 into two portions, the soft 

 visceral hump, the lower 

 boundary of which is indicated 

 by the edge of the mantle, 

 and a ventral portion, in- 

 cluding the foot and head. 



The head is usually 

 fairly well defined; it bears 

 a pair of tentacles, which, 

 in terrestrial forms, can be invaginated like the finger of a 

 glove, and withdrawn into the head, but in other forms, are only 

 strongly contractile. In some Gastropods (Opisthobranchs), behind 

 these there is another pair of tentacles, which may often be concealed 

 within depressions on the head, and whose surface is frequently 



Pig. 239. Diagrammatic figure of a Gastro- 

 pod seen from the left side (the shell removed). 

 a anus, / foot, fc mantle cavity, m stomach, mu 

 shell muscle, o mouth, op operculum. Besides 

 the parts indicated by letters, certain portions of 

 the nervous system are also drawn, viz., cerebral 

 and pleural ganglia (seen above the oesophagus), 

 and pedal ganglion (below the cesophagus). The 

 dotted line indicates the boundary of the mantle 

 chamber. — Orig. 



Fig. 240. Diagrams of various forms of Gastropod shells seen from the 

 left side. A — B slightly curved ; C spiral shell in which the successive coils do not 

 touch ; D ditto, in which the coils lie closely upon each other (the common type). — Orig. 



much folded; they are regarded as olfactory organs. Besides these, 

 a pair of small eyes is generally situated upon the head ; they 

 are sometimes borne at the tips of special tentacle-like stalks (as in 

 the Common Snail), but are usually placed directly upon the head 



