The Cambridge natural history . eshell, with the hinge about midway between them. In thisposition they are best placed for effectually closing the valves. 446 PELECYPODA and since they are nearly equidistant from the axis of motion, ,from the hinge, they do an equal amount of work, and are al)Outequal in size. But in a form like Modiola, where the growth oftlie shell is irregular in relation to the hinge-line, the anteriormuscle is hrought nearer and nearer to the umbones, where itspower to do work, and therefore its size, becomes less and the work to be done remains the same, and
The Cambridge natural history . eshell, with the hinge about midway between them. In thisposition they are best placed for effectually closing the valves. 446 PELECYPODA and since they are nearly equidistant from the axis of motion, ,from the hinge, they do an equal amount of work, and are al)Outequal in size. But in a form like Modiola, where the growth oftlie shell is irregular in relation to the hinge-line, the anteriormuscle is hrought nearer and nearer to the umbones, where itspower to do work, and therefore its size, becomes less and the work to be done remains the same, and the posteriormuscle lias to do it nearly all ; hence it moves farther and fartheraway from the hinge-line, and at the same time gains 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 workto 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 upperdotted Hue shows tlie hinge-line, the lower connects the two muscles. of life. It is well developed in burrowing forms, while in sessilegenera {Ostrea, Chama, Spondylas) it becomes unnecessary andaborts. Even in Pecten, which does not become sessile, but hasceased to use the foot as an organ of progression, a sindlar resultfollows. 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. Inorder to burrow deeply the foot must be highly developed, andthe larger it becomes, the more will it tend to keep the valvesapart at the place where it is habitually protruded. Burrowingspecies always remain in communication with the
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
