. A manual of zoology. PHYLUM MOLLUSCA 279 move from place to place, and in the case of some species is permanently fixed to some rock or other solid body by the substance of the larger valve, has no foot. The inhalant and exhalant siphons are sometimes absent, sometimes much longer than in the fresh-water mussel, as in the clam (Mya arenaria). Posterior to the foot there is in many Pelecypods a gland termed the byssus gland, secreting silky threads which serve to attach the animal temporarily or permanently, as, for example, in the sea mussel {Mytilus) (Fig. 166). In most Pelecypoda the gills
. A manual of zoology. PHYLUM MOLLUSCA 279 move from place to place, and in the case of some species is permanently fixed to some rock or other solid body by the substance of the larger valve, has no foot. The inhalant and exhalant siphons are sometimes absent, sometimes much longer than in the fresh-water mussel, as in the clam (Mya arenaria). Posterior to the foot there is in many Pelecypods a gland termed the byssus gland, secreting silky threads which serve to attach the animal temporarily or permanently, as, for example, in the sea mussel {Mytilus) (Fig. 166). In most Pelecypoda the gills (ctenidia, p. 271). Fig. 166. — Mytilus edulis, attached by byssus (By) to a piece of wood. F, foot; S, exhalant siphon. (From the Cambridge Natural History.) are simpler in character than in the fresh-water mussels. In one group, Protobranchia (Nucula, etc.), they take the form of a pair of plume-like organs, and are primitive in shape and structure. A remarkably modified member of this class of molluscs is the ship-worm, Teredo, which is very destructive to ships' timbers, piles of jetties, etc. The valves of the shell are extremely small, and the general surface of the mantle secretes a continuous shelly tube lining the burrow in which the elongated worm-like body of the mollusc lies. Highly. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Parker, T. Jeffery (Thomas Jeffery), 1850-1897; Haswell, William A. (William Aitcheson), 1854-1925. New York, The Macmillan Company; London, Macmillan & Co. ,Ltd.
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