726 ZOOLOGY sect, xii 



a main stem, with which are connected a row of delicate flexible 

 laminae set at right angles to it : these are broadest in the middle, 

 becoming smaller towards the ends. 



The osphradium (osph.) lies close to the ctenidium on its right- 

 hand side, i.e. nearer the middle of the body. It presents a central 

 axis, connected at right angles with which are two rows 

 of close-set delicate lamellae. Like the ctenidium, it is closely 

 applied to the wall of the mantle-cavity throughout its length. 

 The laminae of the osphradium are supplied with nerves which 

 ramify over them, and its function seems to be to ascertain the 

 condition of the water that enters the mantle-cavity. 



Digestive System. — The mouth, situated at the anterior end 

 of the introvert, leads into a large chamber with muscular walls, 

 the buccal cavity (hue). At the sides of the entrance to this cavity 

 the investing cuticle is thickened to form two distinct horny plates 

 — the jaws (Figs. 613, 614, and 615, jaw). The jaws are flexible, 

 and on examination under the microscope are found to be com- 

 posed of numerous rows of minute bodies, the denticles; the 

 anterior edge is minutely denticulated. From the floor of the 

 cavity rises an elevation, the odontophore /(Fig. 613, od., Fig. 614, 

 odont.), which is somewhat elongated in the direction of the long 

 axis of the body and compressed laterally. Over the summit of 

 the odontophore runs longitudinally a narrow strap-like body, the 

 radula or lingual ribbon (Figs. 614, and 615, rad.), beset with 

 numerous minute horny teeth arranged in transverse rows. Pos- 

 teriorly this toothed ribbon passes into a narrow curved pouch 

 — the radular sac (Fig. 613, rad. s, Fig. 615, rad sac.) — extending 

 backwards from the posterior and lower aspect of the buccal 

 cavity. Anteriorly it does not extend beyond the odontophore- 

 prominence. The latter contains cartilages (Fig. 615, cart.) 

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

 of being extended, 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. 613 and 615, oe&.). 

 Posteriorly this opens into a large ovoid sac — the crop 

 (Fig. 613, crop). The outer surface of the crop appears marked 

 with numerous close-set fine lines, transverse or oblique in 



