264 



CONCHIFERA. 



the mantle are conjoined so as to leave three 

 apertures, through one of which the " foot " is pro- 

 truded ; a second is foi" the entrance and expulsion of 



Fig. 200.— mussels. 



the water required for respiration, while through the 

 third effete materials are rejected. These two last 

 openings are not prolonged into a tube or respiratory 

 syphon. To this family belong 



The Clam Shells (Tridacne), the giants of the bivalve 

 race ; they live attached by their byssus to rocks, 

 shells, and corals. The valve of a large individual 

 forms a very pictm*esque basin for catching the clear 

 falling water of a fountain, which flows prettily 

 through its deeply indented edges. In Roman 

 Cathohc countries the valves of this huge shell are 

 sometimes employed as "benitiers," or vessels for 

 containing holy water. A pair so used may be seen 

 in the church of St. Sulpice, in Paris, wliich weigh 

 five hundred pounds, and are more than two feet 

 across. Specimens attain even larger dimensions 

 than these, and are the largest shells known. The 

 byssus is so thick, and its attachment to the rock 



