98 



THE FOOD OF ANIMALS 



Shell). Unaided, however, this structure does not seem powerful 

 enough for the purpose, and even if it were, it is difficult to see how 

 a ribbon-shaped organ could be employed to bore a round hole. 

 The matter has been carefully investigated {by Schiemenz) in 



a species of Natica (A^. Jose- 

 phina) (fig. 380), a sea -snail 

 possessed of a very large foot, 

 by means of which it burrows 

 in the sand in pursuit of the 

 bivalves upon which it chiefly 

 feeds, and which are commonly 

 found buried in an oblique 

 position, with the posterior end 

 projecting at the surface. The 

 Natica appears to detect its 

 prey by means of the sensi- 

 tive front-end of its foot, which 

 is then folded back so as not to interfere with the action of the 

 proboscis, and at the same time co-operates with the rest of the 

 foot in grasping firmly the front end of the unfortunate bivalve. 

 The prpboscis is now protruded, and close to the under side of 



its tip the true boring 



Fig. 380. — A boring carnivorous Sea-Snail [Natica. 

 yosepkina) 



The front of the foot is folded back, and above it is seen 

 the projecting snout with boring organ {a). 



as 



Fig. 38i.~Bivalve Shells bored by Natica 



organ is seen 

 circular disc, upon the 

 surface of which a 

 large number of little 

 glands open. This 

 structure is applied to 

 the shell to be per- 

 forated, and an acid 

 fluid is poured out from it which dissolves the shell over a 

 small circular area of corresponding size and shape. As soon as 

 a hole has been made, it would seem that the radula helps to 

 enlarge it, and is afterwards used for rasping the flesh of the prey 

 through the breach made in its defences (fig. 381). The exact 

 mode of procedure in Whelk, Purple-Shell, &c., has not yet been 

 determined, but there is little doubt that an acid secretion is 

 brought into play. It is well known that the so-called salivary 

 glands of a number of the predaceous sea-snails contain an ap- 

 preciable quantity of free sulphuric acid, and it is suggested that 



