CONCHIFERA. 



265 



so strong, that it is frequently necessary to cut it 

 with a hatchet in order to obtain the animal. 



FlG. 201. CLAM SHFLL. 



These are the shells alluded to by Captain Flinders, who observes : 

 " Many enormous cockles were scattered upon different parts of the 

 reef. At low water this cockle seems most commonly to lie half 

 open ; but frequently opens with much noise, and the water within 

 the shells then spouts up into a stream three or four feet high. It 

 was from this noise, and the spouting of the water, that we discovered 

 them, for in other respects they were scarcely to be distinguished 

 from the coral rock. A number of these cockles were taken on 

 board the ship, and stewed in the coppers, but they were too rank 

 to be agreeable food, and were eaten by few." 



The fourth family of Conchiferous Mollnsks em- 

 braces 



The Cockles (Cardiacea). These are distinguished 

 by the mantle being open in front, and, moreover, by 

 having two separate apertures, one serving for respir- 

 ation and the other for the discharge of effete mate- 

 rials : these are prolonged into two tubes (Fig. 202), 

 which are sometimes distinct from each other, but oc- 

 casionally conjoined; and, as a general rule, those 

 species provided with long tubes burrow into mud or 

 sand. As examples of this family, we may instance 



