xn PHYLUM MOLLUSCA 709 



serving for the support of the whole apparatus, and is capable 

 of being extruded, with the radula which it bears, through the 

 opening of the mouth, by the contraction of sets of protractor 

 muscular fibres. Muscles inserted into the odontophore also 

 effect the movements of the radula by means of which it 

 produces a rasping effect on the food, and probably the radula 

 itself is capable of a certain degree of to-and-fro movement on the 

 membrane (sub-radular membrane) which lies beneath it. The 

 entire buccal cavity is capable of being drawn forwards towards 

 the mouth opening, or backwards into the introvert, by the con- 

 traction of strands of muscular fibres passing from its wall to the 

 wall of the body. 



From the buccal cavity runs backwards a long narrow tube 

 with sacculated walls the oesophagus (Figs. 623 and 625, ces.). 

 Posteriorly this opens into a large ovoid sac the crop (Fig. 623, 

 crop). The outer surface of the crop appears marked with 

 numerous close-set fine lines, transverse or oblique in direction ; 

 and, when the cavity of the organ is opened, it is found that these 

 correspond to numerous delicate folds which extend far inwards, 

 and almost completely block up the lumen. On either side of the 

 crop is a large gland the salivary gland (Fig. 623, 1. sal. gl., Fig. 627, 

 r. sal. gl.)- partly composed of a compact glandular substance, 

 partly of spongy tissue in which the secretion collects. The two 

 salivary glands are unlike in size and shape, that on the left-hand 

 side being much longer than that on the right. Each has a narrow 

 duct (sal. du.) which runs forwards and inwards to the dorsal aspect 

 of the oesophagus, where the two come into close apposition, becom- 

 ing embedded in the wall of the oasophagus, along which they run 

 forwards to open into the buccal cavity. 



From the crop leads backwards and to the left a narrow cylin- 

 drical tube the posterior ossophagus. On this follows a stomach 

 (Fig. 621, stow.) which is in the form of a U-shaped tube partly 

 embedded in the substance of the digestive gland or " liver" the 

 hepatic ducts from which open into it. The tubular stomach is 

 followed by a somewhat narrower tube the intestine (Fig. 623, int.}. 

 This enters the cavity of the nephridium, round the interior of which 

 it bends ; and, leaving it at its right-hand side, runs forwards in a 

 straight course as the rectum (rect.), embedded in the glandular 

 wall of the mantle-cavity, to near the anterior end, where it 

 terminates in a short, freely projecting, spout-like portion, with 

 the anus (an.) at its extremity. 



The digestive gland or "liver" forms a mass of reddish-brown 

 glandular follicules which compose the greater part of the bulk of 

 the visceral coil. 



Vascular System. Close to the base of the ctenidium, behind 

 it and a little to the right, is the heart, lodged, like that of the 

 fresh-water Mussel, in a cavity, the pericardium, lined by a trans- 



