68 THE LIFE OF THE MOLLUSCA 



or the bark of trees, etc. At times these feed- 

 ing tracks present quite remarkable patterns 

 (Plate XXVII.). 



A large number of molluscs enjoy a mixed diet. 

 Nearly all the Slugs are fond of animal food, and 

 some will invade the house in search of fats and 

 soap, and even milk. Many of the Helicoids, 

 especially those allied to our own Cellar Snail 

 (Polita cellaria), as well as many of the Water Snails, 

 are also mixed feeders. 



The Bivalves, too, and fixed species like Vevmetus 

 (Plate X., Fig. 17), Hipponyx, Magilus (Plate XXVIII., 

 Fig. 1), and probably the adult Rhizochilus (Plate XL, 

 Fig. 1), subsist on the microscopic animals and 

 plants brought to them by the currents which their 

 ciliary apparatus perpetually excites. 



Nearly all the siphonostomatous Gastropods are 

 carnivorous, and feed on either dead carrion or 

 on other shellfish. The Whelks (Buccimm, etc.), 

 Purpura, Nassa, and the holostomatous Natica, by 

 means of an acid secretion either in the saliva or 

 from a special gland (as in Natica) and the aid of the 

 radula, will drill holes through the shells of Bivalves 

 or the upper whorls of Gastropods, and, inserting 

 their extensile snout, devour the animal within. 



In some of those species of Murex that have a 

 fringe of spines round the mouth of the shell, one 

 spike will be seen directed inwards instead of out- 

 wards (Plate XXXIL, Fig. 1). This is employed by 



