TlIK AVATOMY OK M ICU VI.ATllAC PU.S — K FCSTKVKV. 129 



freely with the hiemocuile of the body cavity, and is, in fact, a 

 prolongation of that cavity. Just anterior to the nepliridium 

 this sinus bifurcates, each branch lieing about half the size of the 

 main trunk. One, the pericardial l)ranch (y«. ce. per.), passes back 

 in the wall dividing the pericardium and nepliridium, anrl opens 

 into the former at the anterior end, below the nephri<lial gland. 

 The other, or nephridial l)ranch (ui?. ce. neph.) pierces tho wall of 

 the nephridium, and at once l)ecomes a verital)le vein. It passes 

 upwai'ds and along the dorsal edge of the glandular wall of the 

 nephridium, sending down branches which lie between those of 

 the vena- maxima, and, like them, end in fine capilliaries on the 

 glandular lamell;>?. 



The nena-cava [ve. ca.) is a short sinus not having distinct 

 walls of its own, but with a large and well defined cavity. It 

 lies along the lower edge of the nephridial yland, and continuing 

 forward in the wall dividing the nephridium and pericardium, it 

 passes through the mantle to join the branchial vein, a little 

 anterior to the auricle. 



The effurent branchial vehi {ve. hr.) also has no distinct walls of 

 its own, but lies in the mantle ; it is, nevertheless, a well- 

 defined vessel, and its course is as usual along the ctenidium to the 

 auricle. 



The vence pericardiales {ye. pe7-.) are two large veins which 

 might almost be regarded as posterior prolongations of the peri- 

 cardium. Without using any force the injection passed through 

 them along the axis of the coil for a short distance, and then 

 became diffused around the hepato-pancreas. These two veins 

 have no distinct Avails of their own, but lay in the tissues and 

 muscles of the axis of the coil. 



As previously stated, there is a possible reno-pericardial com- 

 munication by means of the two branches of the vena-cephalica, 

 but, for reasons stated below, it is regarded as extremely unlikely 

 that this communication or interchange of contents does take 

 place. 



Circulation. — It is evident from the arrangement of the vessels 

 of the system, that blood may pass from the aorta-cephalica to 

 the efferent branchial vein by means of lucunse, and also that it 

 passes direct from the nephridium, through the nephridial gland 

 and vena-cava, to the auricle, but the direction of flow of the blood 

 in the veins is not so evident. It seems that both the nephridial 

 veins are afferent vessels, and that all the purified blood passes 

 through the nephridial gland to the heart. The considerations 

 which lead to this conclusion are : — 



1. The vena-maxima is obviously an afferent nephridial vein. 



2. The greater portion of the blood which passes through the 

 heart is conveyed to the body by the aorta-cephalica, it therefore 



