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 cellavia),as well as many of the Water Snails, 
are also mixed feeders. 
‘The Bivalves, too, and fixed species like Vermetus 
(Plate X., Fig. 17), Hipponyx, Magilus (Plate XXVIII, 
Fig. 1),and probably the adult Rhizochilus (Plate XI., 
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 (Buccinum, 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 XXXII., Fig. 1). This is employed by 
