398 



CKUSTACEA. 



jaws (fig. 202, b, c, d) i 

 of these, two are im- 

 bedded among the bran- 

 chiae; while the third, 

 as represented in the 

 figure, extends in a cres- 

 centic form over the ex- 

 ternal surface of the 

 whole series of those or- 

 gans. The end answered 

 in this case is obviously 

 the same as that accom- 

 plished in the Lobster, in 

 a different and perhaps 

 more efficient manner. 



(1018.) In the lowest 

 Crustacea the heart is a 

 long dorsal vessel, not 

 very dissimilar in form 

 and disposition from that 

 of insects, but, of course, 

 giving off arteries for the 

 distribution of the blood, 

 and receiving veins 

 through which the blood, 

 having accomplished its 

 circuit, is returned. 



(1019.) In the Deca- 

 poda the organ becomes 

 more centralized ; and in 

 the Lobster (fig. 201, e) 

 the heart is found to be 

 an oval viscus, situated 

 in the mesial line of the 

 body, beneath the poste- 

 rior part of the cephalo- 

 thorax; it is composed 

 of strong muscular bands, 

 and contains a single 

 cavity of considerable 

 size. The contractions of 

 this heart may readily 

 be witnessed by raising 

 the superjacent shell in 

 the living animal. 



Fig. 203. 



Venous system of the Lobster : a, side view of the venous 

 sinus covering the stomach ; b, side view of the sinus sur- 

 rounding the heart (the letter is placed near one of the 

 valvular orifices by which the venous blood is admitted into 

 the ventricle) ; c c c c, superficial dorsal sinuses. 



