922 PROFESSOR W. C. M‘INTOSH AND MR E. E. PRINCE ON 
upper wall. Certain parts of the rectum, on the other hand, are most complexly folded 
(Pl. XXV. figs. 3, 4, 5, and 7), only a central area in the sections being devoid of ruge. 
The folds continue until the external aperture is reached (fig. 5). 
In the embryo of the salmon, forty days after fertilisation, the alimentary canal is 
closed in the cardiac region, and remains of small calibre throughout the rest of its 
extent; indeed, at the commencement of the segmental ducts, it is less in section than the 
notochord, and is very little larger than one of the segmental ducts. When thirteen days 
old, the alimentary canal in the region of the heart is more or less median in position, 
and presents two lateral slits. It then opens out, and again contracts to a small tube 
less than the girth of the notochord in the region of the liver, its epithelial coat forming 
a simple lining, without rug. The tube, for the most part, remains simple to the 
posterior termination. 
The young salmon, forty-five days old, artificially reared, has its cesophageal region 
less developed as regards size, and the mucous rug internally. The globular glands, 
however, have a similar arrangement, though they are proportionally larger. The 
basement-tissue is coarsely granular. The circular muscular coat is strongly developed. 
The canal is peculiar on account of the rapid narrowing that takes place, and the dis- 
appearance of the globular glands, the section of the entire canal in this region being 
only a little larger than that of the notochord. 
In the Gadoids and Pleuronectids the structure of the alimentary canal agrees in the 
main with the features seen in Anarrhichas—the cesophageal portion, of small calibre, 
having thick walls with complex internal ridges, while posteriorly the walls become 
thinner and saccular, the internal ridges much less prominent, and the diameter of the 
canal greatly increased. In Callionymus, 4 inch long, the mucous membrane of the oral 
cavity rises into very thick prominences or longitudinal folds, which are richly glandular. 
Swim-Bladder.—In many post-larval fishes the swim-bladder retains, until a com- 
paratively late stage, its embryonic character as a sac lined by a thick layer of large soft 
epithelial cells, often so well developed anteriorly as to reduce the lumen of the organ to 
that of a narrow tube. In the cod, 5%; inch in length, this is the condition, and the 
anterior portion is traversed by a large hemal trunk. The capacity of the bladder at a 
later stage enormously increases, and when the young Gadoid reaches the length of 3 
inch, this structure has thin membranous walls with a layer of thickened epithelium 
only on the inner surface. In the post-larval Clupeoid, ;°; inch long, it is large, and has 
a similar structure to that just described in the Gadoid, with, however, a less marked 
development of the epithelial coat. The epithelium of the imner surface in Callionymus, 
1 inch in length, rises into thick massive folds ; but the most remarkable development of 
this layer in the swim-bladder is seen in the gurnard, + inch long, the large pulpy cells, 
each with its nucleus placed excentrically nearest the free surface, forming thick ruge, 
and supported by a layer of smaller cells irregularly scattered as an outer tunic. The 
large epithelial cells just described also occur in the swim-bladder of the Gadoid, 
when 4 inch in length, and the deeply-stained nucleus is excentric in position ; but it 
