70 



the liver pancreatic tissue siirroundinrif the bloodvessels 

 is also present. 



The Pancreas is apparently absent in Pleuronedes, 

 but is really present in a diit'iise form. It will be noted 

 that a perivascular tissue is stated to occur round 

 all the visceral blood vessels and particularly roiind 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 caeca, 

 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 Elasmobranch 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, ]i. is a section through a small portion of the 

 mesentery including two branches of the cceliaco-mesen- 

 teric artery and a small factoi' of the hepatic joortal system. 

 It will be seen that the mesentery is greatly ihickened 

 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. Else- 

 where in this section they had no definite arrangement. 

 Nodular masses of pancreatic tissue are also present round 

 tlH> paii-ed ])yloric cieca, and from these some small ducts 

 enter the cseca. Probably a number of such efferent ducts 

 are present, but we have not determined this exactly. The 

 !>land acini have the structure characteristic of the 



