686 THE MICROSCOPE. 



covers the entire surface of the tail. As the vein approaches 

 the root of the tail it lies superficially to the artery, and 

 diverges from it at the point of entering the abdomen. 

 Here it approaches the kidneys, sends off branches to 

 them, while the main trunk continues its course on- 

 ward ; and, passing upwards behind a coil of intestine, it 

 approaches the liver, and runs in a curved course along the 

 margin of that organ. This blood is seen to enter the 

 vena cava by numerous fine channels, which converge 

 towards the great vein as it passes in close proximity to 

 the organ. Beyond the liver the vena cava continues its 

 course upwards and inwards to its termination in the sinus 

 venosus or rudimentary auricle of the heart. This ter- 

 mination is the junction of not less than six distinct venous 

 trunks, incessantly pouring their blood into the heart. 

 The circulation in the fringed lips forms a most compli- 

 cated network of vessels, out of which proceeds a vein 

 corresponding to the artery already traced. This descends 

 in a direct course till it joins the principal vein of the 

 head, which corresponds to our ovrn jugular. 



Thus we have traced the blood through its main chan- 

 nels, and completed the circle of its course. 



The blood is driven by the heart into each inner gill 

 through three large blood-vessels, which arise directly from 

 the truncus arteriosus, and may be called the afferent vessels 

 of the gill. (See enlarged view of gill, Plate VII. No. 156). 



It will be seen that " each internal gill or entire 

 branchial organ consists of cartilaginous arches (Xo. 156), 

 with a piece of additional framework of a solidly tri- 

 angular form, stretching beyond the arches, and composed 

 of semi-transparent, gelatinous-looking material. These 

 parts, forming the framework of the organ, support upon 

 their upper surface the three rows of crests with their 

 vascular network, and the main arterial and venous trunks 

 which lie parallel with and between them. The three 

 systemic arteries arising, right and left, from the truncus 

 arteriosus, enter each gill on its cardiac side, and then 

 follow the course of the crests, lying in close proximity to 

 them. The upper of these branchial arteries runs alone 

 on the outside of the upper crest, and another branch 

 leaving the trunk and passing into the network of the 



