140 THE BOOK OF NATURE STUDY 



does not merely close its shell like most bivalves, but seeks to 

 make its escape by flapping the valves together, with the result 

 that it is jerked rapidly through the water. It has a slender 

 foot, and is capable of spinning a slight attaching byssus. One 

 of its relatives, Lima hians, generally found in deeper water, 

 which has a white shell but a bright red mantle, has a similar 

 power of swimming, but uses its byssus to weave together particles 

 of stone and weed into a nest, within which it finds shelter. 



Another common bivalve which is readily kept in captivity 

 is the Carpet shell (Tapes pullastra), which has a solid shell, 



rhomboidal in form, prettily 

 marked by numerous close-set 

 bands crossed by longitudinal 

 striae. It has the mantle folds 

 prolonged into two tubes or 



* ^^^gsl^^&^^r-S siphons, of which the one near 



the hinge is inhalant and the 

 FIG. 66. The carpet shell (Tapes puiiastrd). O ne near the free margin of the 



presence of these siphons the 



carpet shell is able to live buried in sand or mud so long as 

 the tip of the tubes protrudes at the surface. A very little 

 observation on the shore will show that there are a considerable 

 number of bivalves furnished with siphons, which live buried 

 in sand, mud, or, in the case of forms like Pholas and Saxicava, 

 in rocks. The former is found especially in limestone rocks, where 

 it forms deep burrows and may be recognised by the bright- 

 red siphons which it protrudes. The species of Pholas occur 

 in various kinds of rocks, especially in shale, and have beautiful 

 pure white shells of delicate texture, marked by prickles which 

 are supposed to be used in the process of boring. 



Before leaving the bivalves it may be noted that it is possible 

 to tell from the appearance of the empty shell whether the animal 

 has had siphons or not, i.e. whether it has been a burrowing or 

 a surface form. Any bivalve shell shows near the margin of 

 each valve a line where the mantle was attached to the shell. 

 If this line follows uniformly the margin of the shell, the animal 

 had no siphons. If, on the other hand, it has a deep bay or sinus 



