4o8 



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- 

 „,.,, ,, ,,,,,., ,,., T. g-ether with byssal threads. 



Fig. 930.— Edible Mussel [Mytilus eduhs^. The arrows t> / "-^^^ ^tj.^.j. 



indicate currents of water entering and leaving the mantle- J ]-^g CUriOUS Saddle - C^VStPrS 



of which one small species 

 {Anomia cphippiiim) is common on our shores, settle down on their 

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

 f^ unsymmetrical, as is usually the case 



when the two sides of the body are per- 

 manendy exposed to conditions of dif- 

 ferent kind. The lower or rig-ht 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- 



Fig. 931. — Shells of Saddle-Oyster 

 {AiiO}}iiai. A, IS, and c. Successive 

 stages of growth. 



