OF THE OYSTER. 201 



as before, lift up the thin, fringed, veil-like covering, (the mantle,) 

 and you expose the gills, (yl to gl b ,) which hang as represented 

 in this figure from the lower side of the animal. They are four 

 in number, and have the appearance of cross-ribbed bands which 

 stretch from one end of the body to the other. It is in their 

 inequality that the one-sidedness of the animal is most promi- 

 nently set forth. The two (gl, gl?, and gV-, gl*} on the right side 

 of the body are of unequal width, and both are narrower than 

 the left pair (gl~, gl 5 ) ; the latter extending much nearer to the 

 edge of the mantle than the former. In front of the gills are 

 four smooth, leaf-like bodies, (t, t 1 , t 2 ,} which hang in pairs below 

 and right and left of the mouth, (m,) and, being joined above, 

 form a sort of hood over it. They are generally considered as 

 organs of touch or prehension, and, like the gills, are of unequal 

 width ; the narrower (t) pendent on the right, and the larger (t 1 ) 

 and longer on the left. 



Near the posterior end of the gills, and above them, is the great 

 double muscle (ms, ms l ) which serves to keep the valves closed. 

 In the shells its position may be recognized by the chestnut- 

 colored, striped, agate-like spot, as big as one's thumb-nail. 



Immediately in front of the muscle is a hollow, which, in 

 freshly opened specimens, seems to be the theatre of very active 

 operations. On carefully removing the semi-transparent mantle 

 with a pair of sharp scissors, the cause of this phenomenon 

 becomes apparent. We find there a sort of double sac, (h, A 1 ,) 

 the two halves of which are constantly and alternately contract- 

 ing and expanding, in moderately rapid succession. This sac is 

 the heart. The anterior half, the ventricle, (A 1 ,) by contraction 

 forces the blood through the two vessels (ao, ao 1 } which go off 

 from its narrower end into the body, and thence into the minute 

 vessels (v, v l ) which branch through the mantle (mn, mn l ). 

 From these, by means of other minute currents, the circulating 

 fluid is brought back into the gills, (gl 1 tog 1 ? 5 ,) where aerification 

 takes place, and then the blood is returned, through the so-called 

 branchio-cardiac vessels, (pv,) to the posterior chamber, auricle, 

 (h,) of the heart. From the latter the blood is injected into the 



