ALIMENTARY AND NUTRITIVE SYSTEM. 
ti45 
ALIMENTARY SYSTEM. 
The Alimentary or digestive system is well developed in the mollusca, 
and may be described as a long tube of variable width, convoluted 
within, but attached by strands of connective tissue to the walls of 
the primary body cavity or coelom, and which, in the last or rectal 
tract of its course, may, in certain groups, traverse the pericardium 
or secondary body cavity, and sometimes even pass through the 
ventricle of the heart also, performing different functions in the 
various portions of its tract or course, all of which have for their 
objects the extraction of nourishment from the ingested food and its 
assimilation by the animal for the growth or reparation of its various 
organs or tissues and the subsei|uent excretion from the body of 
w'aste or hurtful substances.. 
The whole enteric tract may be conveniently divided into three 
regions, an anterior, a median and a terminal portion, according to 
the position and chief function of its various parts. 
Fig. 496.— Alimentary canal of Helix (Miill.), showing the proposed regional division 
of its tracts (modified after Howes). 
f.g. fore-gut or anterior region of the alimentary tract ; m.g. mid-gut or stomachal region and 
hind-gut or intestinal region. 
b.c. buccal cavity, showing radula, radula sac and jaw ; cr. crop ; f. foot ; h.g. hermaphrodite 
gland or ovoteslis ; LI. anterior lobe of liver ; cc. oesophagus ; p.g. pedal gland ; p.r. pharyngeal 
or buccal retractor ; r. rectum ; r.L posterior lobe of liver ; ^.^/.salivary duct; s.g. salivary glands ; 
st. stomach ; ty. typhlosole. 
The Anterior region of the alimentary canal or fore-gut is of 
ectodermic origin and has an ingestive and comminatory function. 
It comprises the buccal bulb, with its specialized developments, and 
the oesophagus or gullet. 
The Median or digestive portion of the tract is of endodermic 
origin and constituted chiefly by the stomach, a dilatation or enlarge- 
ment of the mid-gut, which in some species contains a chitinous or 
cartilaginous rod and, with a considerable part of the hind-gut, is 
