UNIVALVE MOLLUSCANS. 293 



violet-coloured shell of this little animal is re- 

 markably thin, which facilitates its excursions 

 on the surface. It is singular that under this 

 fragile vesicular float a little line of pearly fibres 

 may be perceived, to which are attached its 

 eggs ; in some species they are contained in 

 little membranous bags or sacs. It is thought 

 that the young animals, when liberated from 

 these bags or chambers, ascend their mother's 

 float, and so are transported to the surface. 

 Fishes are enabled to rise to the surface of the 

 water by means of their air-bladders, and some 

 radiaries by a vesicle which surmounts them, 1 

 but neither of them are more singular than these 

 outriggers by which the vessel of the violet-snail 

 is kept both buoyant and steady. 



The foot of the Molluscans, when we first 

 observe it, seems to us merely an organ of loco- 

 motion, nothing remarkable in its structure, and 

 incapable of any multifarious action, but when 

 we study the history of this and the preceding 

 snail, we see that it is a most important organ, 

 and which performs a greater variety of opera- 

 tions than almost any organ of any other animal. 

 We have seen that it spins a fine silk and 

 thread ; that it secretes a fluid serviceable for 

 several purposes ; that it can form a float, as 

 in the present instance ; that it can be used as a 



1 See above, p. 195. 



