676 ZOOLOGY- SECT, xn 



and reproductive apertures. The wall of the cavity is much 

 folded and plaited, and contains a quantity of glandular tissue, 

 the plaits being most numerous on the right-hand side in front 

 of the anus. 



The ctenidium or gill (Fig. 576, den.} is closely applied to the 

 wall of the mantle-cavity to the left of the middle. It consists of 

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

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

 becoming smaller towards the ends. 



The osphradium (osph.) lies close to the ctenidium di| 

 hand side, i.e. nearer the middle of the body. It presenf 

 axis, connected with which are two rows of close -se1 

 lamellae placed at right angles to it. Like the ctenidil 

 closely applied to the wall of the mantle-cavity througl 

 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 (buc.). At the sides .of the entrance to this cavity 

 the investing cuticle is thickened to form two distinct horny plates 

 the jaws (Figs. 576, 577, and 578, jaw). The jaws are flexible, 

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

 posed of numerous rows of minute bodies, the fli-nt'u-lcs; the 

 anterior edge is minutely denticulated. From the floor of the 

 cavity rises an elevation, the odontoplwre (Fig. 576, od., Fig. 577, 

 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. 577 and 578, rad.), beset with 

 numerous minute horny teeth arranged in transverse rows. Pos- 

 teriorly this toothed ribbon extends into a narrow curved pouch 

 the radular sac (Fig. 576, rod. s, Fig. 578, 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. 578, 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 protractoi 

 muscular fibres. Inserted into the radula itself are sets of bane 

 of muscular fibres by which it can be drawn backwards and foi 

 wards over the odontophore as over a pulley, the effect being 

 rasping of any hard substance against wty&h it is pressed. The 

 entire buccal cavity is capable of being drawn forwards towai 

 the mouth opening, or backwards into the introvert by the coi 

 traction of strands of muscular fibres passing from its wall to th< 

 wall of the body. 



