70 
the liver pancreatic tissue surrounding the bloodvessels 
is also present. 
The Pancreas is apparently absent in Plewronectes, 
but is really present.in a diffuse form. It will be noted 
that a perivascular tissue is stated to occur round 
all the visceral blood vessels and particularly round the 
factors of the hepatic portal system. ‘This tissue appears 
on naked eye examination as a band on either side of the 
vessel equal to or greater than the diameter of the latter. 
It lies of course within the thickness of the mesentery in 
which the blood vessels travel. It is particularly abun- 
dant round the vessels in the vicinity of the pyloric ceca, 
where it forms nodular masses which are also associated 
with fatty tissue. This diffuse perivascular tissue is the 
pancreas, which nowhere has the massive form charac- 
teristic of Hlasmobranch fishes. It recalls the extended 
form of organ characteristic of Mammalia, except that 
here the gland acini have a constant association with the 
blood vessels, forming an investment round the latter. 
No proper pancreatic blood vessels are accordingly present. 
Text-fig. 1, B. is a section through a small portion of the 
mesentery including two branches of the cceliaco-mesen- 
teric artery and a small factor of the hepatic portal system. 
It will be seen that the mesentery is greatly thickened 
round the blood vessels, and this thickening is really due 
to a mass of gland acini. No attempt is made in the 
figure to represent the structure of these acini, but their 
radiate grouping round the portal vein is indicated. Hlse- 
where in this section they had no definite arrangement. 
Nodular masses of pancreatic tissue are also present round 
the paired pyloric ceca, and from these some small ducts 
enter the ceca. Probably a number of such efferent ducts 
are present, but we have not determined this exactly. The 
gland acini have the structure characteristic of the 
