xii PHYLUM MOLLUSCA 675 



longitudinally; this is the penis. Running back from its base 

 is a narrow groove with prominent lips the sperm groove, con- 

 tinuous with that on the penis; in the female these parts are 

 not represented. 



Foot. On the side of the body (ventral) which the animal 

 applies to the surface of the ground when it extends from the 

 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 

 >f mJ^kir fibres: this is the principal part of the foot (pro- 

 11 mesopodium combined) ; the posterior portion (meta- 

 a thick process projecting behind this and bearing 

 tlum on its surface. The foot is highly contractile, and 

 ians of contractions passing over it in a succession of 

 Tons 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 FIO. 575. Diagram of the introvert of 



, -, 7 7 7 / Triton, in longitudinal section, 



tne peCLCU gland. as it appears when almost com- 



T-amm'nrlAT' r\f fVio "KnrKr pletely extended. The black lines 



remainder 01 tne body represent the waii of the ail- 

 has been removed from the Shell, mentary canal ; the cross-hatched 

 .... part the wall of the introvert ; 

 it IS found tO be twisted Up into a the dotted line marks the position 



coil the visceral spiral, correspond- i^ov7r?pe g s thr ugh 



. 



ing to the spire of the shell within ; * lumen of 



which .it was lodged. This is unsym- 

 metrical, 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 this becomes 

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

 trusible part gives rise to a thickened ridge, the collar (Fig. 574, 

 cofl.), forming a semi-circle over the dorsal and lateral regions. In 

 the middle this is not in close contact with the body, but leaves a 

 large cleft leading into a very wide space extending backwards for 

 a considerable distance. This space, which is formed by an infold- 

 ing of the mantle, is termed the mantle or pallial cavity. In it are 

 to be found the ctenidium, the osphradium, and the anal, excretory; 



x x 2 



