Page:Natural History, Mollusca.djvu/235

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duction, with a milky fluid containing multitudes of small granules of a whitish colour. These are the eggs; and in many of the family they are not at the time of their exclusion abandoned at once, but are deposited between the membranes of the gill-leaves, where they undergo a kind of incubation. In some, the shell is developed in the egg before it quits this receptacle. This fostering of the eggs seems to be analogous to the gestation of the eggs in the Crustacea and the Pipe-fishes.[1]

I have hitherto spoken of but a single adductor muscle, but in a large number of species there is a second, placed near the front part of the animal. This variation naturalists have used to divide the Class into two Orders, denominated Dimyaria and Monomyaria, or respectively Double and Single-muscled Bivalves. These characters can be determined at a glance by looking at a single valve of any shell; for the place of attachment of the adductor muscle is marked by a distinct sunken impression in the interior of each valve. From this circumstance also, these impressions, by the differences in their form, position, and dimensions, afford excellent characters for the discrimination of genera.

The accompanying figure, representing the interior of one of the valves of a common shell, will serve to illustrate the appearance of the muscular impressions, as well as of some other parts that are commonly mentioned in technical description. The oval mark on the left-hand side of the figure is the front muscular impression; the pear-shaped mark on the opposite side is the hinder one: the bending line which connects them is

  1. Penny Cyclop, vii. 432.