. Zoology. Zoology. THE OYSTER 69 but next to the heart; one end is secretorj'-, communicating with the pericardial cavity, while the other is excretory and opens into the cavity of the body. The nervous system can be, with care and patience, worked out in the clam or fresh-water mussel. In the clam {Mya arenaria, Fig. 76) it consists of three pairs of small ganglia, one above (the "brain") and one below the oesophagus (tlie pedal ganglia) connected by a commissure, thus forming an (Esophageal ring; and at the middle of the mantle, near the base of the gills, is a tlurd pair of gangl
. Zoology. Zoology. THE OYSTER 69 but next to the heart; one end is secretorj'-, communicating with the pericardial cavity, while the other is excretory and opens into the cavity of the body. The nervous system can be, with care and patience, worked out in the clam or fresh-water mussel. In the clam {Mya arenaria, Fig. 76) it consists of three pairs of small ganglia, one above (the "brain") and one below the oesophagus (tlie pedal ganglia) connected by a commissure, thus forming an (Esophageal ring; and at the middle of the mantle, near the base of the gills, is a tlurd pair of ganglia (parieto-splanchnic), from which nerves are sent to the gills and to each division of the siphon. This last pair of ganglia can be usually found with ease, without dissection, especially after the clam has been hardened in alcoliol. The ear of the clam Is situated in the so- called foot; it bears the name of otocyst, and is connected with a. Fig. 77.—Mytilus eduUs, common mussel, with its siphons expanded, and an chored by its bjssus. nerve sent off from the pedal ganglion. It is a little white body found by laying open the fleshy foot through the middle. Micro- scopic examination shows that it is a sac lined by an epithelium, resting on a thin nervous layer supported by an external coat of connective tissue. From the epithelium spring long hairs; the sac contains fluid and a large otolith. The structure of this otocyst may be considered typical for Invertebrates. The ovaries or testes, as the sex of the clam may be, are bilaterally symmetrical, blended with the wall of the vi-ceral or liver-mass, and are yellowish. The openings for the exit of the eggs lie near the base of the foot. In the oyster the two shells are unlike, the lower shell being usually larger than the upper. A single oyster may produce over a million young. In six hours after develop- ment begins, the ciliated germ swims about in the Please note that these images are extracted from scanned page im
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