La/mellibranchia. 



43 



With these hooks a certain number of larvae succeed in fixing 

 themselves upon fishes, and if they light upon soft tissue such 

 as a gill-filament the irritation of the hook causes the flesh to 

 grow over the Glochidium and to form a cyst round it. The 

 larval Mussel develops within the cyst and lives parasitically 

 on the fish until it is liberated by the breaking up of the cyst. 



Fig. 33. 



•> -.. 



Common British Cocklo (Cardium edvle). 



a. foot : h. exhalant siphon ; c. branchial or inhalant siphon ; d. edge of 



mantle ; e. ligament ; /. umbones or beaks of the shell. 



Fig. 34. 



Left valve of the Giant Clam (Tridacwa giga-?). 

 Length. 30 inehes. Weight. 154 lb. ; weight of the two valves, 310 lb. 



The Tellinidae (Case 179), Veneridae (Case 185), and the Cockles 

 (Cardiidae. Case 192) are all remarkable for the diversity of 

 colour and sculpture that their members exhibit. 



An interesting phase of Lamellibranch evolution is found at 

 its maximum among many of the Eulamcllibranch families now 

 to be considered. There is a tendency in the class for the mantle 

 edges to become fused at one or more points, and the resulting 

 orifices at the posterior end may be very much elongated and 



