253 



SUBKIKGDOM MOLLTTSCA. 



ened, they excavate a burrow sufficiently large to receive the 

 Mg. la^. entire shell. 



The Teredinldse (borers) have on 

 their anterior part a pair of valves 

 which they use to bore into wood. 

 They line the cavity, as they proceed, 

 with shelly matter, often making a 

 tube two feet in length. They work 

 in various directions, but never en- 

 croach upon one another, though it is 

 difficult to comprehend how they be- 

 come aware of the proximity of their neighbors. 



One-half of a Cockle Shell, 

 showing lilies of growth. 



Mg. ItSS. 



ORDER ASIPHONATA. 



G-eneral Characteristics. — The Asiphonata (siphonless} 

 include all bivalves with open mantle and without a siphon. 



The Mytilidae (mussel-fish) anchor their shells to rocks 

 by means of gluey threads, that 

 seem either to grow or be spun 

 from the foot. This "byssus" 

 is in some species so long and 

 silken, as, for a curiosity, to be 

 woven into cloth. 



Ostrseidse. — The Common 

 Oyster, found along the 

 Atlantic coast, when young, is 

 free, but soon attaches itself to 

 rocks by the left or rounded 

 valve. It thus generally lies on its left side. By ciliary 

 movements, it produces currents which convey the water 

 through the open mantle to the gills for respiration, and then 

 hurry it on to the mouth, which lies back, near the hinge, 

 where the labial fingers extract the food it may contain.* 



* The hard part always cut in opening the shell is the addnctor muscle whose 

 business is to shut the Talves, These open naturally ahout one-half of an inch. The 

 mantles on the two sides of the shell meet at the aperture. Their edges are fringed 

 with cilia— the English call them the oyster's heard— which can be protruded from 



MyUhis edulis, with dyssus. 



