Glass 2. Gastropoda. 



293 



itself or on the side of the tentacles. The foot is generally a 

 flat, very contractile disc, occupying the whole veutral surface of 

 the lower portion of the body. 



The soft, thin-skinned visceral sac is covered by a tubular shell, 

 open at one end, closed at the other, and becoming gradually wider 

 towards the open end. Only in rare instances is the tube straight or 

 slightly curved, usually it is a spiral, the concavity of which 

 corresponds, in Gastropods, with the ventral side of the visceral sac 

 (see Fig. 239). The individual coils of the shell almost invariably 

 touch one another, and are, indeed, closely united. Some discoidal 

 shells form a flat coil, like a watch-spring ; usually, however, the 

 closed end of the tube is drawn out on one side, so that the axis of 

 the shell describes a spiral round a cone. The form of most 

 Gastropod shells is, therefore, conical, although many are widely 

 aberrant from this, on account of the very different form of shell- 

 tube. If a shell be placed so that the axis of the cone stands 

 perpendicular, with its apex (the closed end) uppermost, and the 

 mouth towards the observer (Fig. 241), the aperture then either 



Pig. 241. 



Fig-. 242. 



Fig. 241. The shells of two examples of a tropical 

 land Snail (Bulimus perversus). A left-handed spiral. 

 B right-handed spiral. — After v. Martens. 



Fig. 242. Shell of a Snail (Paludina) in which a 

 large part of the wall towards the observer is broken 

 away, n umbilicus, s columella. — Orig. 



lies to the right of the axis, and the shell is said to be a right 

 handed spiral ; or it lies to the left of the axis, giving a left-handed 

 spiral. The shell is borne by the animal in such a way, that its 

 point, if a right-handed spiral, is towards the right (and points 

 upwards and backwards) ; or if a left-handed spiral, towards the left 

 (upwards and backwards). Right-handed spirals are much more 

 common than left ; in some species the one kind occurs, in others, the 

 other ; as individual variations, examples of the left-handed spiral may 

 be found among forms in which the right is normal ; but it is very rare 

 to find both forms equally common in the same species. In some 



