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 

 mussel." 



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 muscle. 



