. Principles of economic zoo?logy. Zoology, Economic. 74 BRANCH MOLLUSCA pass the food into the mouth. From thence the foodi passes into the stomach and to the long coiled intestine which passes through the pericardium, usually perforates the ventricle, and ends dorsal to the posterior adductor muscle. The Pelecypoda are sexual and sometimes hermaphroditic. There is a metamorphosis, there being usually a trochosphere stage. The sea mussel (My'Ulus) is an example of this class. Great clusters of this edible mussel are found just below low-tide marks. ^ The shell is generally of a purple or dark
. Principles of economic zoo?logy. Zoology, Economic. 74 BRANCH MOLLUSCA pass the food into the mouth. From thence the foodi passes into the stomach and to the long coiled intestine which passes through the pericardium, usually perforates the ventricle, and ends dorsal to the posterior adductor muscle. The Pelecypoda are sexual and sometimes hermaphroditic. There is a metamorphosis, there being usually a trochosphere stage. The sea mussel (My'Ulus) is an example of this class. Great clusters of this edible mussel are found just below low-tide marks. ^ The shell is generally of a purple or dark color. The long slender foot (Fig. .53} throws out yellowish horny fibers (the byssus), by which the mussel attaches. Fig. .53.—Mytilus edulis: O, Mouth; S, labial palps; P, foot; B, byssus secretion; Br, gills; M, thickened edge of mantle. (After Claus.) itself to foreign objects. If food becomes scarce or conditions unfavorable, it can detach itself and slowly move to another position by stretching out the threads of the byssus and attaching them ahead or above, and then drawing itself up to them, hence it is sometimes called the " climbing ; Ani/mia, of the same order as ]]Iy'tihiK, is permanently fixed. The oyster (Os'lrea) is a member of this class, which in adult life is fixed to the sea bottom or to some forc-ign object—\-ery often the shell of another oyster. Great clumps (see Fig. 52, \>. 72) ma>' bo thus fastened together, but their union is not organic. Oysters \ary in size from a few inches to 2 or 3 feet, the largest being a Japanese .siiecies. The shell of the oyster (Fig. 54) is rougher than that of the clam, and the hinge is at the pointed end, which corresponds to the anterior end of the clam. Its two valves are not alike, but. the lower or left one is much larger and becomes deeji enough to contain the body, while the upper or right valve is flat and serves as a hd. There is but one adductor Please note that these images ar
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