OF MOLLUSCA. 297 



enclosed by the mantle ; the two chambers of the heart grad- 

 ually change their relative position until the receiving chamber 

 (auricle) lies obliquely to the right of and behind the distribut- 

 ing chamber (ventricle). 



All these changes bring the embryo into the condition which 

 is shown in this figure, (fig. y hv i h 



197,) a representation of the 

 young of our most common 

 native Lymneus a short time 

 before it is hatched. Here we 

 find in a profile view the fol- 

 lowing features, namely, the , 



upper region covered by an _ ^ === ^^, _ 

 asymmetrical, oval mantle (//, 

 ml 1 ) which extends at its in- 

 ferior edge into a thickened border; the head clearly separated 

 from the creeping disc, (f,f 1 ,) and furnished with an eye (ey] at 

 the base of each of its two lateral, triangular, tactile organs (t) ; 

 the creeping disc shaped like a flat-iron, with the broader end (f) 

 in front; the intestine in the form of an irregular slender tube 

 (in, in 1 ) strongly bent upon itself and arched over the upper sur- 

 face of the body and to the left of the heart (h, h 1 } ; the circu- 

 latory system divided into distinct chambers and vessels, and 

 the blood passing along the back in a current from the main 

 posterior longitudinal channel (hv, hv 1 ) through a double valve 

 into the auricle, (/*,) and thence through another valvular opening 

 into the ventricle, (A 1 ,) from whence the fluid is thrown into the 

 distributing channels to be carried to various parts of the body ; 

 and, finally, the region of the respiratory organ, or lung, 



Fig. 197. Lymneus elodes. Say. 100 diam. The young of one of our native 

 amphibious snails. A profile view. t, tentacle at the corner of the head; ey, the 

 eye ; ./",_/', the creeping disc ; ml, ml 1 , the mantle : pc, the lung cavity, just behind 

 the front edge of the mantle ; pv, the region of the lung ; in, in' 1 , the intestine ; 

 7<, the auricle, and A', the ventricle of the heart; hv, Jiv*, the posterior blood- 

 vessel ; y, yl, the right and left sides of the back ; n, the region of the stomach 

 and principal nervous ganglions. Original. 



