216 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



which, was formerly thought to be due to the imbibition of 

 water. The reverse process, viz., the emptying of the spaces, 

 is brought about chiefly by the strong muscular contractions 

 of the animal, the blood being forced out of the tissues by 

 compression. 



[Dissection. Remove the shell so as to expose the 

 kidney lying on the surface of the flattened visceral hump. 

 Dissect away the left anterior portion of the kidney : the heart 

 is situated directly beneath it. Trace the arteries from the 

 heart. The course of the arteries can best be followed by 

 injecting, drop by drop, a strong aqueous or gelatine solution 

 of Prussian blue into the three main trunks whose apertures 

 can be found by slitting up the large arterial trunk arising 

 from the ventricle of the heart. For a complete injection 

 use Prussian blue gelatine and inject either into the efferent 

 branchial vessel from the ctenidium or into the spongy tissue 

 of the foot.] 



The Pericardium (Fig. 14, pc~) is a spacious chamber, 

 representing a shut-off portion of the true coelom, which does 

 not contain Wood. It communicates with the cavity of the 

 kidney by means of a small reno-pericardial duct situated close 

 to the entrance of the renal vein into the auricle, i.e., on its 

 anterior wall. The walls of the pericardium are supported 

 by tough interlacing fibrous bands. The nerve supplv is 

 derived from the parietal ganglion. (Fig. 16, pc. n.) 



The Heart does not fill the pericardium. It projects from 

 its floor into the cavity and runs obliquely across the bodv, 

 the ventricle being slightly anterior to the auricle, and both 

 lving anterior to the ctenidium. (Hence the name Opistho- 

 branchiata.)* 



The Auricle (Fig. 13, aur.), which is on the right, is thin- 

 walled, delicate, and transparent. Fine interlacing strands 

 of muscle and fibrous tissue give it the appearance of beautiful 

 lace. The auricle receives blood from the ctenidium and the 

 kidney by two efferent branchial veins and a renal vein 



* Great contraction may cause the heart to appear almost transverse. 



