Molluskan shell growth

Most mollusks have a spiral shell. Each group has evolved a small range of the possible spiral shapes that can be generated, adopting those that function best for their mode of life. Spiral shells can be described simply using four variables (Fig. 9.2). Understanding the underlying symmetry of apparently very different mollusk shells is important because it gives an insight into the basic similarity of all of these forms.

Gastropods usually have a low whorl expansion rate (low W) and a high rate of translation along the axis of growth (high T). Their shell aperture can have a complicated shape, and the rest of the shell is formed out of this pattern, like toothpaste squeezed from a tube.

Ammonites sometimes had no translation rate at all, and formed planispirals. Ammonites with high expansion rates and a short distance between the aperture and the coiling axis (low D) are involute, folded up so that their later whorls hide the earlier ones; those with low expansion rates are evolute, with each whorl visible.

Bivalves have shells with very high whorl expansion rates (high W). This is necessary because the two valves have to be able to open. If bivalve shells coiled more, the spires would interfere with one another and the shell would lock shut. However, bivalve shells do translate along the axis of coiling, which is why their plane of symmetry falls between the valves.

Four parameters describe any spiral. Think of this as a generating curve growing around a coiling axis:

W = whorl dimension S = shape of the aperture D = distance of generating curve from coiling axis T = translation rate along coiling axis

W2-W1 = whorl expansion rate

D2-D1 = rate of migration of generating curve

Coiling axis

S = shape of the aperture, and hence of the tube it generates

Growth direction

D2 = distance of generating curve from coiling axis after 360° of rotation

Growth direction

D2 = distance of generating curve from coiling axis after 360° of rotation

S = shape of the aperture, and hence of the tube it generates

T = translation rate, along axis of coiling

Laminado Frio Herramienta

W2 = whorl dimension after 360° of rotation

W1 = whorl dimension

D1 = distance of generating curve from coiling axis

Fig. 9.2 How to describe mollusk shells as spirals.

W2 = whorl dimension after 360° of rotation

T = translation rate, along axis of coiling

W1 = whorl dimension

D1 = distance of generating curve from coiling axis

Fig. 9.2 How to describe mollusk shells as spirals.

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  • hiewan
    What is a coiling axis in mollusks?
    9 years ago

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