446 



PELECYPODA 



and since they are nearly equidistant from the axis of motion, i.e. 

 from the hinge, they do an equal amount of work, and are about 

 equal in size. But in a form like Modiola, where the growth of 

 the shell is irregular in relation to the hinge-line, the anterior 

 muscle is brought nearer and nearer to the umbones, where its 

 power to do work, and therefore its size, becomes less and less. 

 But the work to be done remains the same, and the posterior 

 muscle has to do it nearly all ; hence it moves farther and farther 

 away from the hinge-line, and at the same time gains in size. 

 In shells like Ostrea, Pecten, and Vulsella, the anterior muscle, 

 having drawn into line with the hinge and the posterior muscle, 

 becomes atrophied, while the posterior muscle, having double 

 work to do, has doubled its size.^ 



The development of the foot, again, largely depends upon 



Fig. 300. — Illustrating changes in the position and size of the adductor muscles accord- 

 ing to the shape of the shell: A, Mya ; B, Modiola ; C, Vulsella. The upper 

 dotted line shows the hinge-line, the lower connects the two muscles. 



habits of life. It is well developed in burrowing forms, while in 

 sessile genera (Ostrea, Chama, Spondylus^ it becomes unnecessary 

 and aborts. Even in Pecten, which does not become sessile, but 

 has ceased to use the foot as an organ of progression, a similar 

 result follows. Forms which burrow deeply often "gape " widely, 

 sometimes at one end only, sometimes at both. Venus, Donax, 

 Tellina, Mactra, which are shallow burro we rs, do not gape ; Solen, 

 Lutraria, and to a less degree Mya, burrow deeply and gape widely. 

 In order to burrow deeply the foot must be highly developed, and 

 the larger it becomes, the more will it tend to keep the valves 

 apart at the place where it is habitually protruded. Burrowing 

 species always remain in communication with the surface by means 

 of their siphons, the constant extension of which tends to keep 

 the valves apart at the end opposite to the foot. Burrowing 



i Compare Jackson, Amer. Nat. xxv. p. 11 f. 



6 J 



i 



