STRUCTURE OF FISHES. 413 
rating the trunk into a thoracic and caudal portion. To 
make up for the short intestine, its absorbing surface is 
greatly increased in all except the bony fishes by a peculiar 
fold called the “‘spiral valve.” The rectum always opens in 
front of the urinary and genital outlets; except when the 
latter communicates directly with the rectum, thus forming 
a cloaca. All fishes have a well-developed liver, usually a gall- 
bladder, with several gall-ducts ; and in general a yellowish 
pancreas. 
The heart consists of a ventricle and auricle, the latter 
branchial with a venous sinus (sinus venosus) ; while to the 
ventricle is added an arterial bulb, which subdivides into five 
pairs of arteries, one for each gill-arch. The Dipnot ap- 
proach the Amphibians in the possession of a second auricle 
as well as of genuine lungs, ?.¢., cellular air-sacs. The lungs 
are fundamentally comparable with the air-bladder or swim- 
ming bladder. It is generally situated below the back-bone, 
and is developed originally as an offshoot of the cesophagus. 
It is either free, not connected with the digestive tract, or its 
original attachment may be retained in the form of the 
‘‘pneumatic duct,” which, when persistent, opens into the 
cesophagus. In the sharks it is either absent or exists in a 
rudimentary state. 
The kidneys are two voluminous, dark-red lobulated or- 
gans, lying close together next to the back-bone, behind, 7. e., 
above the air-bladder, and occupying nearly the whole length 
of the abdominal cavity. The efferent ducts (ureters) either 
pass along in front of or by the side of the kidney, and some- 
times unite to form a single sac, the outlet of which is situ- 
ated either behind or below the generative orifice. It has 
been found that the minute structure of the kidneys of em- 
bryo sharks resembles somewhat the segmental organs of 
worms, the original kidney being composed of bundles of 
ciliated funnels, like those of worms, combined, however, 
with Malpighian bodies and renal lobules which do not exist 
in worms, while all these parts have a common duct, the 
ureter, which also does not exist in worms, being character- 
istic of Vertebrates. 
In the fishes the sexes are, with a very few exceptions, dis- 
