MYA; SAXICAVA; PHOLAS. 415 



rapidly down to such a distance, as to render pursuit of this kind 

 useless. 



1041. This group is connected with the preceding by the 

 Mya, Lutraria, and other genera, which are common on our 

 coasts ; and which burrow into sand or mud. Mya arenaria, 

 and M. truncata, are found abundantly in the sand all round our 

 coasts ; the former, which is the largest, often burrows to the 

 depth of a foot. Their siphons are very long. Some, such as 

 the Saxicava, burrow in rocks. 



1042. The remaining members of this group are amongst the 

 most interesting of the Bivalve Mollusca, both as regards their 

 habits and the curious varieties of structure which they present 

 The valves of the shell never cover the whole surface of the ani- 

 mal, and the branchial siphons are always exposed. They bur- 

 row not only in sand or mud, but also in wood and rock. In 

 the Pholas, which presents the greatest resemblance to the ordi- 

 nary Bivalves, the shell is formed of two principal valves, which 

 leave a considerable space between them at each end when they 

 are closed ; and of supernumerary pieces, the number and position 

 of which vary considerably. The foot issues at the opening 

 through the larger end ; and the respiratory siphons, which are 

 very long and extensible, pass out by the other. Some Pholades 

 form their cells in mud or clay ; but many in rocks, and others in 

 wood. It is evident that a fleshy foot can be of little use in the ex- 

 cavation of a stony mass ; and the organ here appears to serve a 

 different purpose. The boring operation seems to be performed 

 by the shell itself, which has a rasp-like surface, and which is 

 renewed by vital action as fast as it is worn down. In order to 

 make the valves rotate backwards and forwards (like a surgeon's 

 trephine), the foot is affixed to the bottom or end of the hole, 

 and becomes a fixed point from which the muscles can act. 

 They seem to commence this operation almost as soon as they 

 quit the egg ; the young beginning to bore the rock on which 

 they are cast, and enlarging their cell, which they never volun- 

 tarily quit, in accordance with their own increase in size. They 

 possess a very curious means of freeing the tube from the rasp- 

 ings of the rock produced by their penetration. The siphon 



