xii PHYLUM MOLLUSCA 725 



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 (ineta- 

 jxx/ium) 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 asym- 

 metry of the body is not strongly 

 marked in the part which is capable 



Of being protruded from the shell, but FIG. 12. Diagram of the introvert of 

 ... -IT i Triton, in longitudinal section, 



is still recognisable ; and an examina- as it appears when almost com- 

 tion of the internal organs shows a ^J^fS^ 6 ^ 11 ^ 

 marked excess of development on the p^ e rt fc {h^^Ho?thnntI^v C e h rt d 



left-hand side, i.e. the Side which COr- the dotted line marks the position 



j .,1.11 -i of the opening through which the 



responds With the longer OUter Side iutrovertpasses. intro. introvert ; 



of the spiral of the shell. The sur- J^gST* 

 face 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 protrusible part gives rise to a 

 thickened ridge, the collar (Fig. 611, 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 extending backwards for a consider- 

 able 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. 613, den.) is closely applied to the 

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



