408 



ANIMAL LIFE-HISTORIES 



same spot, for it is able to cast off the mooring-threads and creep 

 away by means of its foot, which, however, is much smaller than 



in forms which lead a com- 

 paratively active life. Another 

 well-known bivalve ( Tridacna), 

 which often attains a great 

 size, also possesses a byssus 

 and a small foot, and is com- 

 mon in the fissures of coral 

 reefs with its under side facing 

 upwards. The two openings, 

 by means of which currents of 

 water respectively enter and 

 leave the mantle -cavity, are 

 conveniently placed about the 

 middle of the upturned mar- 

 gin, instead of being situated 

 at the back end, as is usually 

 the case. Some of the File- 

 Shells (species of Lima) con- 

 struct a sort of nest for them- 

 selves by fixing bits of stone 

 and other foreign objects to- 

 . , .. gether with byssal threads. 



Fig. 930. Edible Mussel (Mytilus edults). The arrows o / 



indicate currents of water entering and leaving the mantle- The CUHOUS Saddle - OyStCrS, 

 cavity. r 1 1 11 



of which one small species 



(Anomia ephippium) is common on our shores, settle down on their 

 right sides when they leave the larval state, and become very 



unsymmetrical, as is usually the case 

 when the two sides of the body are per- 

 manently exposed to conditions of dif- 

 ferent kind. The lower or right valve 

 is here flat, and the upper or left one 

 strongly curved. The flat valve is very 

 closely applied to the underlying surface, 

 to which it is firmly fixed by a hard 

 conical plug that runs from the body of 

 the mollusc through a hole in the shell, 

 and is no other than the calcified byssus. As might be expected, 

 the foot is very small, and its chief use is to act as a byssus- 



BYSSUS 



FOOT 



Fig. 931. Shells of Saddle-Oyster 

 (Anomia}. A, B, and c, Successive 

 stages of growth. 



