Spencern—7Zhe Blood Vessels of Ceratodus. 13 
(1) The hepatic artery giving off branches to the lobes of the liver (Zep. art.) 
(2) The exteric artery, which forms the largest of the three branches and 
supplies the alimentary canal (ext. art.) This appears to be equivalent to the 
anterior mesenteric vein as described by Parker in Mustelus, the cceliac branch of 
the latter being probably represented in Ceratodus merely by the hepatic artery. 
(3) The right anterior spermatic (or oviducal) artery.—This runs backwards 
through the right testis or ovary as the case may be. It differs from the corre- 
sponding artery of the left side in that it is not directly continuous with the posterior 
spermatic (or oviducal) artery, though small branches pass directly from one to the 
other (7. ant. sp. art.). 
The enteric artery as above said supplies the alimentary canal. Passing to the 
ventral side of this, it turns backwards slightly to the right of the median line and 
then pierces the outer muscular coat just anterior to the point at which the spiral 
fold commences. This part forms the widest portion of the canal. Where the spiral 
fold commences the artery breaks up into three branches :— 
(1) The dorsal intestinal: this follows the curve of the fold until it reaches the 
mid dorsal line and then runs backwards along this to the posterior end (figs. 9, 12, 
and 13, @. zx¢. art.) 
(2) The ventral intestinal: this passes to the mid ventral line and runs along 
this to the posterior end (figs. 9, 12, and 13, v. zezé. at). 
(3) The zxtra-tntestinal : as described by Parker in Scymnus and Mustelus this 
forms a vessel which runs along the free edge of the spiral fold, this free edge m 
Ceratodus being very much thickened. As noted by Giinther the spiral fold 
contains a remarkable organ of very doubtful nature which is filled with a shmy, 
dark sepia-brown substance. The presence of this renders it very difficult to dissect 
out the artery ; but the relative positions of the three vessels can be seen from the 
diagrammatic section drawn in fig. 12. 
The larger first portion of the intestine has its walls supplied by two special 
branches (fig. 9), of which one arises from the dorsal intestinal and the other from 
the ventral intestinal artery. 
Along their courses the dorsal and ventral arteries give off lateral branches 
corresponding in position to the points at which they cross the spiral fold. As this 
made some nine turns in the course of the intestine of the specimen figured (fig. 9), 
there are on each side nine branches, and those arising from the dorsal and ventral 
vessels may run into one another round the sides of the intestine. Each one also 
gives off branches both forwards and backwards and down into the spiral fold. 
