92 
BUIvI^ETlN OF THE BUREAU OF FISHERIES 
MUCOSA. 
Epithelial coat . — The epithelium of the pyloric caeca is a coat of one uniform type 
throughout, and consists of a single layer of slender cylindrical cells with interspersed 
goblet cells and wandering leucocytes. The epithelial coat reaches its greatest develop- 
ment in the normal adult animal, in which its total surface extent is eight times the 
outer surface of the caecum. This relatively enormous extent is secured by deep folds 
of the coat, which extend out into the lumen of the appendage. Cross sections of the 
pyloric appendages show all possible varieties of section of the epithelium. There are 
transverse sections of simple tongue-like folds extending from near the stratum com- 
pactum to approximately the center of the lumen. These are not villi, though they 
appear so. On the other hand there are regions where the epithelium is cut obliquely, 
or even in a plane parallel with the surface of the epithelial coat for some extent. There 
is in such a picture necessarily a great amount of irregularity in the boundary outlines 
of the epithelial surface. 
The picture just described is due to the fact that the epithelial coat of the mucosa 
is built on a simple but extremely flexible plan. There are no projections of its surface 
comparable to the villi of the small intestine in the mammalia, but there are many 
clefts and folds. If a king salmon’s caecum were completely relaxed as to its muscu- 
lature and distended from the inside to its maximum extent, both as to length and as to 
circumference, its epithelial coat would be greatly smoothed out and comparatively 
simple. Its irregularities of surface still observable would be chiefly low longitudinal 
ridges broken here and there by transverse furrows and partial clefts. Relaxation of 
the muscular walls and dilation of the caecum to this extent probably seldom occur in 
nature though ofttimes closely approximated. In a maximally stretched caecum the 
proportion between the surface extent of the mucosa and the outer wall should be 
lower than the minimum given in the preceding paragraph. If in such a case the caecal 
wall should contract in diameter without changing its length the epithelial coat would 
be thrown into deep longitudinal ridges and clefts that would extend from the base of 
the mucosa to the center of the lumen of the caecum. These ridges would appear in 
transverse section as simple uncomplicated folds, i. e., very long slender finger-like 
projections of mucous epithelium into the lumen of the caecum. These projections 
show, as might be expected, a uniform type of cell from tip to base, as shown in figure 
8, plate XXVIII. 
Now if the caecum contracted in length it would bend and twist these longitudinal 
folds, and at the same time interrupt their continuity by producing transverse ridges 
and pockets in the epithelial coat, giving rise to a net-like appearance as seen from a 
surface view, a view not unlike that presented by Bdinger ® in his figures 1 1 and 13b, and 
by Eggeling* in figure 156. A cross section of a caecum in this state always shows an 
extraordinary complexity of section throughout the epithelial coat, a complexity that 
was found extremely confusing until interpreted in the manner presented above. 
® Edinger, L.: op. cit., pi. 41. 
^ Eggeliug, H.: Dunndarmrelief und Emahrung bei Knochenfischen, Jenaische Zeitschrift, bd. 43, 1908, p. 416. 
