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 separated 

 off from the neighbouring tracts by constrictions. These form 

 divisions, set one behind the other. 



This separation distinctly corresponds to a division of function ; 

 this is also shown by the varying characters of the cuticular struc- 

 tures of the different portions. For example, in Aplysia, one portion 



