314 ELEMENTARY r.IOLOGV. [CIIAP. 



wall, which become plumose and subsequently unite to 

 form the adult gill-lamella3 ; the byssus is thrown off, and 

 the larval shell and adductor muscle are replaced by newly- 

 formed valves provided with two adductors. The young 

 Afiodonta at length drops off and fiills into its ordinary 

 habitation in the mud, a bilaterally symmetrical animal, 

 differing from its parent only in the absence of functional 

 reproductive organs. 



LABORATORY ^YORK. 



The contraction of the foot and consequent displace- 

 ment of the viscera, resultant upon death under chloroform, 

 may be overcome by killing the animal in water sufficient to 

 cover it — heat slowly to about 40" C. 



In an animal freshly removed from the water only the 

 shell or cxoskchton is visible, but if kiUed as above directed 

 this will be slightly open, the foot will be protruded as 

 in life and the edge of the membrane lining it (the jnantlc) 

 will be visible. Raise one valve of the shell, by separating 

 the mantle from it with the handle of a scalpel, and then 

 cut through two strong bodies {the adductor muscles)^ one 

 at each end of the animal, which pass from valve to valve 

 and prevent their separation. The two valves will now be 

 united only by their ligament, as the result of whose elas- 

 ticity they gape ; cut through this, thus liberating one 

 valve and work over — 



A. General external characters. 



I. In the animal now laid bare may be distinguished — • 

 a. A dorsal border turned towards the hinge of the 



