706 ZOOLOGY 



SECT, xn 



shell is a flat surface elongated in the antero-posterior direction. 

 The wall of the body in this region is composed of a dense mass 

 of muscular fibres : this is the principal part of the foot (pro- 

 podium and mesopodium combined) ; the posterior portion (meta- 

 podium) is a thick process projecting behind this and bearing 

 the operculum on its surface. The foot is highly contractile, and 

 it is by means of contractions passing over it in a succession of 

 undulations that the animal creeps along, dragging after it the 

 rest of the body enclosed in the shell. In the middle line of its 

 flat surface, nearer the anterior than the posterior end, is a slit- 

 like aperture leading into a cavity lined with unicellular glands 

 the pedal gland. 



When the remainder of the body has been removed from the 

 shell, it is found to be twisted up into a coil the visceral spiral, 

 corresponding to the spire of the shell within which it was lodged. 

 This is unsymmetrical, the axis of the spiral being directed not 

 straight backwards, but backwards, upwards, and to the right. 

 The external asymmetry of the body is not strongly marked in 

 the part which is capable of being protruded from the shell, but 

 is still recognisable ; and an examination of the internal organs 

 shows a marked excess of development on the left-hand side, 

 i.e. the side which corresponds with the longer outer side of the 

 spiral of the shell. The surface of the part of the animal which 

 is capable of being pushed out from the shell is covered with a 

 thick integument, which is darkly pigmented except on the lower 

 surface of the foot. Over the visceral spiral the mantle forms 

 a thin, delicate, colourless layer. Anteriorly the mantle becomes 

 thickened and pigmented, and at the posterior limit of the pro- 

 trusible part gives rise to a thickened ridge, the collar (Fig. 621, 

 coll.), forming a semicircle over the dorsal and lateral regions. 

 In the middle the collar is not in close contact with the body, 

 but leaves a large cleft leading into a very wide space extend- 

 ing backwards for a considerable distance. This space, which 

 is formed by an infolding of the mantle, is termed the mantle- or 

 pallial-cavity. In it are to be found the ctenidium, the osphradium, 

 and the anal, excretory, 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. 623, cten.) 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 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 outer 

 side, in the natural position of the parts (reversed in the figure 

 owing to the reflection of the wall of the mantle-cavity). It 



