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 al)Out 

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

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

 muscle is hrought 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 lias 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, 

 l)ecomes atrophied, wdiile the posterior muscle, having dou1)le work 

 to do, has doubled its size.^ 



The development of the foot, again, largely depends upon habits 



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, Vuhella. The upper 

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



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

 genera {Ostrea, Chama, Spondylas) 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 sindlar result 

 follows. Forms which burrow deeply often " gape " widely, some- 

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

 Mactra, wliich are shallow burrowers, 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 surfixce by means 

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

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



■• Compare Jackson, Amer. Nat. xxv. p. 11 f. 



