428 " THETIS " SCIENTIFIC RESULTS. 



Plates xxxix., fig. 4, and xli., fig. 3, will explain their arrange- 

 ment better than words. 



The aorta-cephaLica[ao. ce.) passes forward above and is attached 

 to the cesophagus ; it gives olf no branches till it enters the body 

 cavity, where it immediately bifurcates ; the smaller branch 

 crosses to the right side of the cavity, and passes forward, giving 

 off branches in its course; it finally bifurcates, both arms turning 

 down into the foot. The larger branch continues forward along 

 the left side, and the greater number of its branches carry blood to 

 the tissues of the foot and the mantle on the left side, but one 

 particularly large branch crosses towards the right side of the 

 cavity, and turns down into the foot, below the posterior end of 

 the proboscis-sac. The extreme anterior end of this main bi'anch 

 was lost in the general decay. 



The aorta-visceralis (ao. vise.) passes backwards along the axis 

 of the coil ; it gives off several small branches in its course. 



Of the two, the cephalic artery is very much the larger ; their 

 coui'ses and branches were followed by injecting them with 

 prussian-blue, as were all the vessels of this system ; and, so 

 small is the visceral artery, immediately behind the pericardium, 

 that it could not have been followed without the injection. 



Some of the veinous sinuses are very distinct vessels, and one 

 at least has distinct walls of its own, and is a veritable vein. 



For the convenience of description and discussion it has been 

 found advisable to give these sinuses names. They are, there- 

 fore, termed — vena-maxima, vena-cephalica, vena-cava ,vence-pe7-icar- 

 dia/es, and efferent branchial vein. 



The vena-maxima [ve. mx.) is the lai'gest of the sinuses; 

 posteriorly it communicates freely with the lacunse of the visceral 

 coil, and passes forward along the lower or axial side of the nephri- 

 dium to communicate with the large rectal sinus (rect. si.), and a 

 smaller sinus lying between the uterus and body pi^oper. In the 

 nephridium it gives off large but short branches which pass up 

 the right wall or glandular mass of the nephridium towards the 

 dorsal side. These branches are self-contained, that is, they are 

 provided with walls of their own. The wall of the vena-maxima, 

 which separ-ates its cavity from that of the nephi-idium, may be 

 well regarded as serving that purpose only, so that in the nephri- 

 dium this sinus might almost be looked upon as a veritable vein. 



The walls of the rectal sinus (recL si.) and mantle adjacent to 

 it are of a spongy texture, owing to their being perforated by 

 innumerable small sinuses ; the largest of these pass below the 

 mucous gland towards the ctenidium. There is no large sinus 

 connecting any part of the vena-maxima with the vena-cephalica. 



The vena-cephalica (ve. ce.) is a large, well-defined sinus, not 

 having distinct walls of its own, but lying in the integument 

 connecting the visceral coil and body. This sinus communicates 



