MOLLUSCS. 



101 



The slugs, so familiar in woods and in gardens, are also air-breathers, 

 and, like the snails, are injurious to agricultural interests. Frequently 

 the gardener finds some favorite plant cut down, and leading to it is a 

 glistening track, which at once proclaims who did the damage. Appar- 

 ently the slugs (Limax) have no shell ; in reality they have a small inter- 

 nal one, which much resembles a fish-scale in appearance. 



The limpets (Patella, Acmcea, etc.) are all marine, and, like all the 

 remaining molluscs, they breathe by means 

 of gills. In the tropical forms the shell 

 shows no traces of a spiral, but is rather 

 a low conical roof, covering the soft parts 

 of the body. Limpets are everywhere com- 

 mon, adhering to rocks and shells. If we 

 approach one carefully and suddenly try to 

 pick him from his seat, we will be success- 

 ful ; but if he have warning, that broad, 

 flat disc shown in the cut will adhere so 

 strongly that the shell will break before the 

 hold can be loosened. 



A strange story is told of the limpets, 

 and yet there is good reason to believe its 

 truth. When the tide comes in, the limpet 

 wanders off to find pastures of delicate sea- 

 weed coating the rock, until in some way or 

 other he recognizes the fact that the tide has turned, and then he makes 

 his way back to his former resting-spot, where he stays with his shell 

 drawn close down until the tide again turns. How the apparently stupid 

 animal knows when to return in order not to get caught away from home 

 by the falling tide, and also how it finds its way back, are problems in 

 molluscan psychology which have not yet been solved. 



The ear-shells or, as they are called on our west coast, abalones, cling 

 to the rock like the limpets. In their shape they but distantly resemble 

 an ear, while a row of holes through the shell is a peculiarity by which 

 these forms may at once be recognized. These holes are for the admission 

 of water to the gills, and when in an expanded condition the gills can be 

 seen to protrude. The holes appear as if punched through from the 

 inside ; and as the shell grows, the older ones are closed up by a deposit of 

 lime, although the traces of the openings are not obliterated. Abalones 

 are eaten in various parts of the world, and are described as - exceedingly 

 nutritious, but indigestible." Their value lies not in their use as food, but 

 in their iridescent shells. On the outside they are rough and homely, but 



Fig. 95. — Under surface of a limpet (Pa- 

 tella) , showing the hroad foot (a), and the 

 gill (c) surrounding it ; natural size. 



