MOLLUSCS 



409 



carrier. The perforation of the shell is at first a notch, as will 

 be seen by reference to fig. 931. 



The Scallops (species of Pecteit) afford another example of 

 bivalves with a very small foot, useless for locomotor purposes. 

 Some of the species swim by opening and shutting their shells, 

 while others are attached by means of a byssus, and rest on their 

 right sides. As in Saddle-Oysters, there is well-marked asym- 

 metry in such cases, but here, as usu- 

 ally, the valve which is kept below 

 is strongly curved, while the other 

 one is flat and forms a kind of lid 

 as it were. This is an obviously 

 advantageous arrangement, for the 

 opening and closing of the shell are 

 effected more easily than if the up- 

 wardly facing valve were convex and 

 comparatively heavy, as, curiously 

 enough, it is in the Saddle-Oysters. 

 Another character which generally 

 distinguishes bivalves that are per- 

 manently fixed on one side is the 

 rounded or irregular shape of the 

 shell, which loses the elongated form 

 seen in so many active species. 



The Thorny Oysters iSpondylus) 

 are closely related to the Scallops, 

 but all of them are fixed to firm 

 objects by the substance of the right 

 valve, which is relatively very large 

 and thick. The foot is much reduced, and there is no byssus. 



The Oysters (species of Ostred) are the most familiar examples 

 of fixed bivalves, and are attached by the substance of the left 

 valve. Not only is there no byssus, but the useless foot has 

 entirely disappeared, as an adaptation to the sedentary habit. 



Those bivalves which bore into hard substances present a 

 number of very interesting characters related to their mode of life. 

 Some account has elsewhere been given (p. 221) of the habits 

 of the Piddocks [Pkolas, fig. 932) in this connection, but some 

 interesting facts regarding them may be added or repeated here. 

 There can be little doubt that burrowing is effected by means of 



Fig. 932. — Piddocks [P/iolas] a, Burrow of 

 young individual ; b, b, phosphorescent areas ; c^ 

 young piddock; d, foot; e, shell. 



89 



