Cottbhll .—Vascular System of Siphonaria obliquata. 



375 



are formed by a thin tough transparent membrane ^e heart as usual. 

 Ttwo Xrnbered, the ventricle being situated to the left of the auricle, 

 sZex pointing to the left and downwards while the auricle lies 

 almost horizontally and transversely to the body. The walls of the 

 auricle are thin, transparent, and extremely delicate. When this chamber 

 fsdi tended with blood the bluish tinge of the latter may be noticed, 



Fig. 1. 



due to the presence of the respiratory pigment haemocyaiiincharac^r- 

 istic of the respiratory fluid of Mollusca and Arthropoda. In this cos 

 tended condition the auricle is of a more or less cylindrical tape the 

 long axis being transverse to the body. Two large veins discharge blood 

 !? this chamber, uniting just as they reach it. . From he aun dett. 

 blood or haemolymph passes between the two auriculo-ventricular valves 

 into the ventricle. Owing to the fact that it possesses thicker walls the 



