362 COMPARATIVE ANATOMY. 



is beset with pyramidal pieces of cartilaginous consistency, and 

 another is provided with firm horny hooks. Similar kinds of hooks 

 are found in the simple stomach of Tritonia ; in Scylhea there is a 

 broad girdle of sharply-pointed plates in the same region, while 

 in the stomach of those Pteropoda in which the mouth-parts are 

 rudimentary, there are strong radulaa. The presence of these organs 

 shows that these portions of the gut merely function in preparing 

 the food for digestion. 



The widened, mid-gut is no less modified, both in form and in 

 the differentiation of its various parts. In many it is faintly dis- 

 tinguishable. In others it forms a stomachal caecum, so that the 

 cardiac and pyloric ends get to lie close together ; this is the more 

 common form. The stomach may be broken up into several 

 divisions. For example, the cardiac and pyloric portions are often 

 separated by a longitudinal fold, which projects into the stomach 

 (Littorina). 



Of the peculiarities of the rest of the enteric tube, the frequent 

 enlargement of the hind-gut must be mentioned. In many Nudi- 

 branchs the whole gut undergoes more marked modifications 

 (^Eolidias) ; in proportion as the liver takes on its functions it 

 becomes more or less degenerated, the increased size of the liver 

 compensating for the great abbreviation of the gut (vide infra, 

 p. 365). 



In many of the Gastropoda there are glands connected with the 

 anus ; these are sometimes of some size (Murex, Purpura) ; but it is 

 not yet known what their significance is. 



The anus is lateral or dorsal in position according to the size of 

 the shell and the development of the mantle-cavity. When the 

 shell is absent, and a mantle-cavity also, the anus may be on the 

 dorsal surface, and even in the middle line of that surface, as it is 

 in some of the Nudibranchiata (Doris) (Fig. 200, a). In others it 

 retains the lateral position which it acquired owing to the earlier 

 possession of a shell (iEolidia). 



§ 279. 



In the Cephalopoda the pharynx (Fig. 199, ph) is continued into 

 a narrow oesophagus, which, after passing through the cephalic 

 cartilage, is either continued, without any change of diameter, into 

 the stomach (Loliginidae), or it is provided somewhere along its tract 

 with a crop-like enlargement, which is often of considerable size 

 (Nautilus, Octopoda). A stomach (Fig. 193, v) is formed by an 

 oval or rounded enlargement, which is sometimes (Nautilus, Octopus) 

 provided with strong muscular walls. On each side of the stomach 

 there is a layer of muscles, which arc arranged in a radial manner, 

 and in the centre of which there is a tendinous plate; this is 

 especially noticeable in the Nautilus. 



The pylorus, which is placed close to the cardia, leads into the 



