OYSTERS AND METHODS OF OYSTER-CULTURE. 279 



intestine, where there is a inarkefl constriction of the alimentary canal. It appears, 

 therefore, to be a sort of loose valve in the cavity of the gut ; its function may be to 

 prevent coarse particles of food from ])a.ssing or it may in some way assist digestion. 

 In specimens hardened in acid or alcohol this rod is destroyed, or at least disappears, 

 so that I have been unable to lind it. The greater portion of its substance is appar- 

 ently made up of water. 



Tlie peculiar double induplication of the wall of the intestine is described in 

 another place. The fecal matters are extruded in the form of a demi-cylinder, with 

 one side excavated in a groove-like manner. This shape of the fecal matters is due 

 to the presence of the double fold. The feces themselves are composed of extremely 

 fine particles of quartz or sand grains, the tests of diatoms, organic matters, humus, 

 cellulose, fragments of the chitinous coverings of some of the minute worms and 

 articulates, etc., which have been swallowed and digested by the animal. The anus, v, 

 is situated on the dorsal side of the great adductor muscle Avhere the intestine ends. 



The organs of sensation of the oyster, though not very highly developed, are of 

 sufficient importance to merit attention. The auditory sense, although I have never 

 been able to dissect out the auditory vesicles, I am satisfied exists, because one can 

 not noisily api)roach an oyster bank where the oysters are feeding without their 

 hearing so that instantly every shell is closed. The tentacles of the mantle are often 

 extended until their tips reach beyond the edges of the valves. If the animal in 

 this condition is exposed to a strong light the shadow of the hand passing over it is 

 a sufficient stimulus to cause it to retract the mantle and tentacles and to close its 

 parted valves. The mantle incloses, like a curtain, the internal organs of the crea- 

 ture on either side, and lies next the shell, and, as already stated, secretes and 

 deposits the layers of calcic carbonate composing the latter. The free edges of the 

 mantle, which are purplish, are garnished with small, highly sensitiye tentacles of 

 the same color. These tentacles are ciliated and serve as organs of touch, and also 

 appear to be to some extent sensitive to light. 



The nervous system of the oyster is very simple, and, as elsewhere stated, is to 

 some extent degenerate in character. It is composed of a pair of ganglia or knots of 

 nervous matter, plate i, fig. 1, sg, which lie just over the gullet, and from these a pair 

 of nervous cords, d, pass backward, one on each side, to join the hinder pair which lie 

 just beneath the adductor muscle, j> g. The mantle receives nerve branches from the 

 hiudmost gauglia or knots of nervous matter; these, as their centers, control the 

 contraction and elongation of the radiating bundle of muscular fibers, as well as 

 those which lie lengthwise along the margin ; the former contract and withdraw the 

 edges of the mantle from the margin of the shell, while the latter in contracting 

 tend to crimp or fold its edges. The tentacles are mainly innervated by fibers 

 emanating from the hindmost ganglia, while the internal organs are innervated from 

 the head or cephalic ganglia. The hind ganglia also preside over the contractions 

 of the great adductor muscle. The nerve threads which radiate outward from it to 

 the tentacles dispatch the warnings when intruders are at hand that it must contract 

 and close the shells.* 



Ryder, John A. ; Fishery Industries of the United States, pp. 714-715. 



