274 FISHES CHAP. 
duct (eg. Lepidosteus). Among the Dipnoi a well-developed 
pancreas is present in Protopterus, 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 extrennty 
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 and arrangement in different 
families: In Amia there is no trace of pyloric caeca. Polypterus 
has a single short caecum with a thick muscular wall. In 
Acipenser, Polyodon, and Lepidosteus, on the contrary, pyloric 
caeca are unusually well developed. In <Acipenser 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-lke mass, communicat- 
ing with the intestine by a single wide duct. In Polyodon the 
organ is essentially similar, but is lobed externally. In Lepidosteus 
(Fig. 155, B, py.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 
1 Newton Parker, op. cit. pp. 188-139. 
’ Turner, Journ. Anat. and Phys. vii. 1873, p. 233. 
% Stannius, op. cit. pp. 197, 198 ; Owen, op. cit. p- 428, et seq. 
rT 
