198 ZOOLOGY. 
matter, or else entirely wanting, are called adipose. A finof this character 
is found on the back of the trout (Salmo fontinalis) posterior to the main 
dorsal. 
The gills consist of bony or cartilaginous spines, arranged parallel to each 
other, like the teeth of a comb; over which run blood-vessels from the 
heart, for the sake of the purification to be experienced by contact wiih the 
oxygen dissolved in the water. Sometimes the gills, instead of being pecti- 
nate, are arranged in bunches. The gill-cover consists of four bones, of 
which the one immediately behind the orbit is called the pre-operculum. - 
Posterior to this are the operculum and inter-operculum, the former above 
the latter. Inferior to these, or slightly posterior, is the swb-operculum. 
Anterior to the lower part of the opercular bones is the branchial mem- 
brane, supported by the branchiostegous rays. In cases where the gill-covers 
are wanting or concealed, there are generally five to seven apertures in each 
side. Respiration in the fish is performed by taking in water through the 
mouth, and forcing it through the gill-openings by muscular contraction. 
By the contact of water with the venous blood in the gills, oxygen is 
imparted, and the requisite decarbonization effected. The entire circula- 
tion of the blood is as follows: Venous blood collected from the venous 
system, is accumulated in the single auricle. Thence it is forced into the 
ventricle, and this drives it into the gills. Here the blood is changed from | 
venous to arterial, and is distributed to the different parts of the body. 
The heart thus never contains any but venous blood, the arterial first pro- 
ceeding from the gills. 
The swimming bladder of the fish is the true homologue of the lungs in 
the higher vertebrata ; a fact well shown by its intermediate character in 
Amia and other fishes. | Whenever present, it appears to contribute, to a 
greater or less extent, to the function of respiration. It exhibits various 
forms, bi-lobed, bi-partite, &c., and in the young fish, or even in some adults, 
there is a distinct communication by a tube with the esophagus, answering 
to the trachea. It usually contains a gas, with oxygen in greater propor- 
tion than in atmospheric air. Some species are destitute, either entirely, 
of an air-bladder, or possess it in a very rudimentary state, as in Cobitis. 
Those without it are generally ground fish, which keep close to the bottom. 
Besides the use of the air-bladder in respiration, it serves an important pur- 
pose in enabling the fish to vary its specific gravity, and thus float at any 
desired elevation in the water. 
The body of the fish, with a few exceptions, is longer than broad, and 
compressed. There is little expression in the face, the features being on the 
same level, and the nose not projecting. The line of distinction between 
the head and body is difficult to draw, owing to the entire absence of 
neck. The heart is situated far forward, between the branchial apparatus. 
The simple brain does not fill the cavity of the cranium. The tongue is 
mostly cartilaginous, sometimes covered with teeth. The salivary glands 
are inconspicuous, and it is not probable that the fish possesses much sense 
f taste. There is no external ear, and the entire auditory apparatus is 
here at its minimum of vertebrate development ; although not so simple as 
402 
