5 8 OUTLINES OF NATURAL HISTORY. 



The mantle, though forming in this way a closed bag, 

 with no other openings into it except those just mentioned, 

 really consists of two halves — a right and left half — and 

 each half produces upon its outer surface one of the 

 "valves " of the " shell " (see fig. 23, where m m are the 

 two halves of the mantle, and s s the two valves of the 

 shell). If, therefore, we take the animal in its living state, 

 we do not see any portion of the body, except the long 

 water-tubes already spoken of ; but we see the double or 

 "bivalve" shell. As the two halves of the mantle are 

 right and left, and as each produces one valve of the 

 shell, it follows that the shell consists of a " right valve " 

 and "left valve." We may therefore compare the 

 Gaper to a man enclosed within two great shields, one 

 placed upon his right arm and one upon his left arm. 



Lea\'ing the shell, however, for the present, let us now 

 examine the internal anatomy of the Gaper. In order to 

 do this it is necessary to remove one of the valves of the 

 shell, and we cannot do this without some violence to the 

 animal within. The valve, namely, is attached to the 

 mantle which produces it, and is also kept in firm con- 

 nection with the opposite valve by means of two strong 

 muscles, which are known as the "adductor muscles" 

 (Latin, adcluco, I lead towards or bring together). Hence 

 in taking off one valve, we have to cut the mantle along 

 the line where it is attached to the shell, and also to cut 

 the two adductor muscles. When this is done, and the 

 valve and mantle on one side are removed, we have the 

 appearances presented in fig. 24. In this figure, the cut 

 edge of the mantle is seen at m ; the letter a represents 

 the front adductor muscle, which has been cut through ; 

 and a' is the hinder adductor muscle, which has also been 

 divided. We know that a is the front adductor muscle, 

 because close beside it is situated the mouth (0) ; and the 

 mouth, of course, is placed on the front of the body. 

 The mouth is surrounded by four long membranous leaf- 

 like processes or feelers (/j). Immediately above the 

 mouth, and occupying the greater portion of the centre 

 of the figure, are two flattened membranous plates (h). 



