Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 ALIMENTARY CANAL OE MOLLUSCA. 3G1 The mouth of the Gastropoda is surrounded by the_ lips, which meet infrout of the entrance into the pharynx. These lips are folds of the integument, which can be retracted or protruded with the pharynx. In some of the Prosobranchiata this arrangement is carried so far that the fold, which, as we have $ seen, ordinarily forms the lips, gives rise to a more or less elon- gated sheath, in which a proboscis, within which again is the pharynx, lies freely. When
Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 ALIMENTARY CANAL OE MOLLUSCA. 3G1 The mouth of the Gastropoda is surrounded by the_ lips, which meet infrout of the entrance into the pharynx. These lips are folds of the integument, which can be retracted or protruded with the pharynx. In some of the Prosobranchiata this arrangement is carried so far that the fold, which, as we have $ seen, ordinarily forms the lips, gives rise to a more or less elon- gated sheath, in which a proboscis, within which again is the pharynx, lies freely. When this proboscis is protruded, the inner wall of the sheath, which encloses it, is gradually everted (Do- lium, Cassis, Conus, Voluta, Buccinum, Harpa, Murex, etc.). It is clear then that this, the most anterior portion of the intesti- nal tract, may be very highly specialised. § 278. Fig. 192. A Pharynx of a Gastropod (PI euro- branchus) ; vertical longitudinal section. B Trans- verse section of the same, taken along the vertical line drawn in A. oe (Esophagus. I Lips, r Radula. k Cartilage. C Pharynx of a Cephalopod (Loligo) ; vertical longitudinal section. t Arms. m Upper, m' Lower jaw-piece, y Tongue, r Radula. oe (Esophagus. Behind the pha- rynx in the Gastro- poda the fore - gut forms the oesophagus, and part of it widens out into a stomach ; this is succeeded by the mid-gut, which often traverses the visceral sac in the form of a single loop ; it passes on to the hind-gut, from which it is indistinctly separated. The canal is modified in character by the enlargement of various parts of the oesophagus; in this way a special portion is formed which functions as a crop. This is either spindle-shaped, as it is in many of the Prosobranchiata (it is very long in the Heteropoda), or it forms a unilateral diverticulum, which may develop into a cascal appendage (Lymnasus, Planorbis, Buccinum). Parts of the fore-gut may also widen out into stomachal enlargements, which are sepa
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