ALIMENTARY CANAL OF MOLLUSCA. 359 



tlio anus is, relatively speaking, least altered. It may still lie 

 more or less in the middle line. When the branchial cavity is more 

 anterior in position, and asymmetrical — as it is in most of the shelled 

 Gastropoda — the anus approaches it, inasmuch as its function is 

 here least interfered with. 



The enteric tube is divided into separate portions in just the 

 same way as in Vermes. 



Although embryological inquiries have not yet arrived at any 

 very definite results, this much seems certain : the mid-gut is 

 derived from the endoderm, and the fore-gut from the ectoderm. 

 The hind-gut is laid down with the mid-gut, and has therefore the 

 same history as it has. 



§ 270. 



As to the characters of the various divisions, we find that in the 

 Placophora the digestive tube is coiled several times; but the anus 

 still retains its aboral position, inasmuch as the above-mentioned 

 causes, which lead to a change in position, do not obtain in this group. 



The Lamellibranchiata likewise retain the most primitive relation 

 of parts. The mouth, which is placed in the Dimyaria between 

 the foot and the anterior adductor, is continued into a short 

 portion which functions as the oesophagus; this passes into a 

 widened portion, or stomach. The efferent ducts of the liver 

 open into this gastric mid-gut. In many Lamellibranchiata this 

 stomach is remarkable for the possession of a cascal diverticulum, 

 which is often of some size, and can be shut off by a valve; 

 this is placed in the pyloric region. In many forms we meet 

 with a peculiar structure in the cosca, or, when they are absent, 

 in the enteric canal itself; this, which is known as the " crystal- 

 line style/' is to be regarded as a secretion from the enteric 

 epithelium. The hind-gut, which forms by far the largest portion 

 of the whole tract, makes one or more coils, and then passes towards 

 the back of the animal ; as a rule it is of the same calibre through- 

 out, but it is sometimes differentiated into narrower and wider tracts. 

 It is closely surrounded by the other organs (liver, generative 

 glands) of the visceral sac ; its terminal portion passes underneath 

 the middle-line of the shell as far as the hinder margin of the body ; 

 in a large number of Lamellibranchiata it traverses, as it does so, the 

 pericardium and heart (Fig. 176, i) } and then ends, behind the 

 posterior adductor, at the anus ; this is placed on a process at the 

 aboral end of the body, which projects freely into the mantle-cavity 

 (Fig. 181, r). Here again the position of the anus is correlated with 

 the characters of the shell, wmich is in the form of two lateral valves. 



§ 277. 



In all Mollusca, except the Lamellibranchiata, part of the fore- 

 gut is differentiated into the so-called pharynx or buccal mass; 

 this is generally a large organ, the structure of which is well adapted 



