2 74 FISHES 



duct (e.g. Zejndosteits). Among the Dipnoi a well-developed 

 pancreas is present. jn Protojiterus} embedded in the wall of the 

 stomach and intestine, internal to the peritoneal investment of 

 these organs, and extending even into the first fold of the spiral 

 valve. The gland is traversed by fine ductules which unite 

 together and open into the bile-duct just before the latter enters 

 the intestine. In the remaining Dipnoi the existence of a 

 pancreas has yet to be ascertained. Developmentally, the pan- 

 creas resembles the liver, and, histologically, is very similar to 

 that of the higher Vertebrates, consisting of terminal glandular 

 alveoli continuous with intermediary tubular portions, and eventu- 

 ally with the finer ductules, which, by their union, form the 

 main efferent duct. 



The Pyloric Caeca. — These structures are caecal outgrowths 

 from the intestine, and are situated close to the pyloric extremity 

 of the stomach and the intestinal apertures of the bile and 

 pancreatic ducts. Wholly wanting in the Cyclostomata and 

 Dipnoi, and, unless represented by a pair of caeca opening into 

 the long, tubular, non-valvate anterior portion of the intestine in 

 the Greenland Shark {Laemargus borealis),- in the Elasmobranchs 

 also, they are very generally present in the Teleostomi, although 

 extremely variable both in number aixl arrangement in different 

 families. In Amia there is no trace of pyloric caeca. PolyiJterus 

 has a single short caecum with a thick muscular wall. In 

 Acipenser, Polyodon, and Lejndosteiis, on the contrary, pyloric 

 caeca are unusually well developed. In AcijJcnser the caeca are 

 not only numerous, but are so connected together by connective 

 tissue and blood-vessels, and so invested externally by the peri- 

 toneum, as to form a large, compact, gland-like mass, communicat- 

 ing with the intestine by a single wide duct. In Polyodon the 

 organ is essentially similar, but is lobed externally. In Lepidostevs 

 (Fig. 155, B, ^jy.c), the caeca are also very numerous, but relatively 

 short, and, although united into a compact mass, open by four 

 pit-like orifices into the intestinal cavity. In Teleosts the caeca 

 are subject to extraordinary variations in number, size, and 

 arrangement.^ In some families, and even in groups of higher 

 taxonomic value, they are entirely absent, as is the case with the 



' Newton Parker, op. cit. pp. 138-139. 

 '^ Turner, Journ. Anat. and Phys. vii. 1873, p. 233. ' 



' Stannius, op. cit. pp. 197, 198 ; Owen, op. cit. p. 428, et seq. 



