STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
425 
Close to the pyloric valve, on its right side, is a small papilla, on the apex of which 
the bile duct opens ( b.d '.). 
A small, apparently glandular, mass closely connected with the bile duct, in the 
position in which we have seen the pancreas in the larva (Plate 27, figs. 62 and 63, pi), 
is almost certainly a rudimentary pancreas, like that of many Teleostei; but its 
preservation was too bad for histological examination. We believe that the pancreas 
of Lepidosteus has hitherto been overlooked. 
The small intestine passes straight backwards for about 8 centims., and then presents 
three compact coils. From the end of these a section, about 5 centims. Jong, the walls 
of which are much thicker, runs forwards. The intestine then again turns backwards, 
making one spiral coil. This spiral part passes directly, without any sharp line of 
demarcation, into a short and straight tube, which tapers slightly from before back¬ 
wards, and ends at the anus. The mucous membrane of the intestine for about the 
first 3‘5 centims. is smooth, and the muscular walls thin: the rest of the small 
intestine has thick walls, and the mucous membrane is reticulated. 
A short spiral valve ( sp.v .), with a very rudimentary epithelial fold, making nearly 
two turns, begins in about the posterior half of the spiral coil of the intestine, extend¬ 
ing backwards for slightly less than half the straight terminal portion of the intestine, 
and ending 4 centims. in front of the anus. Its total length in one example was about 
4'5 centims. 
The termination of the spiral valve is marked by a slight constriction, and we may 
call the straight portion of the intestine behind it the rectum (re.). 
The posterior part of the intestine, from the beginning of the spiral valve to the 
anus, is connected with the ventral wall of the abdomen by a mesentery. 
The air-bladder (a.b.) is 45 centims. long, and opens into the alimentary canal by a 
slit-like aperture (a.b'.) on the median dorsal line, immediately behind the epipharyngeal 
teeth. Each lip of this aperture is largely formed by a muscular cushion, thickest at 
its posterior end, and extending about 6 millims. behind the aperture itself. A narrow 
passage is bounded by these muscular walls, which opens dorsally into the air-bladder. 
The air-bladder is provided with two short anterior cornua, and tapers to a point 
behind : it shows no indication of any separation into two parts. A strong band of 
connective tissue runs along the inner aspect of its whole dorsal region, from which 
there are given off on each side—at intervals of about 12 millims. anteriorly, gradually 
increasing to 18 millims. posteriorly—bands of muscle, which pass outwards towards 
its side walls, and then spread out into the numerous reticulations with which the air- 
bladder is lined throughout. By the contraction of these muscles the cavity of the 
air-bladder can doubtless be very much diminished. 
The main muscular bands circumscribe a series of more or less complete chambers, 
which were about twenty-seven in number on each side in our example. The 
chambers are confined to the sides, so that there is a continuous cavity running 
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