202 THE SEAS 



live in mud, spending their days slowly ploughing through 

 it, swallowing, as they go, large quantities from which they 

 extract such nourishment as it contains, just as the earth- 

 worms do on land. The burrowing sea urchins feed in 

 somewhat the same manner, but they pick up particles by 

 means of special grasping tube-feet, which surround the 

 mouth. Other animals of which the Shipworm is an ex- 

 ample, obtain their food by boring ; while others again scrape 

 off the various plants and animals which form a crust over 

 the surfaces of the rocks. Of the latter type of feeders, the 

 common sea urchins are examples, they hold on to the rock 

 with their tube feet and bite off the food by means of five 

 long teeth, which are supported in an intricate skeleton 

 known, on account of its discoverer and shape, as Aristotle's 

 lantern, which is shown in Plate 79. The muscles of the 

 lantern force the teeth downwards so that they all come 

 together beneath the mouth, biting off a circular piece of 

 food, which is automatically pushed into the mouth. 



Other animals which scrape their food from the surface 

 of stones are the common shore snails, such as the peri- 

 winkle and the limpet, which crawl about by means of their 

 big muscular feet. In common with other members of the 

 snail family they possess a very characteristic feeding 

 apparatus consisting of a long horny ribbon, made up of 

 many rows of fine teeth, and known as the " radula " 

 or lingua] ribbon (Fig. 44). This is supported by a strength- 

 ening framework over which it is drawn backwards and 

 forwards like a rope over a pulley. The whole mechanism 

 can be withdrawn into the mouth when the animal is not 

 feeding, but when in use is pushed out against the food 

 which it rasps away by continuous backward and forward 

 movements, each piece being pulled back into the mouth 

 and further broken up by the help of jaws before it is 

 swallowed. The radula is constantly being worn away, but 



