ICHTHYOPSIDA. 233 
are suspended in cavities placed on the side of the neck, and in 
the ordinary bony fishes there is only one such cavity on each 
side. The water is taken in at the mouth by a process analo- 
gous to swallowing, and it gains admission to the branchial 
chamber by means of a series of clefts or slits which perforate the 
sides of the pharynx. Having passed over the gills and lost its 
oxygen, the effete water makes its escape behind by an aperture 
called the “ gill-slit,” which is placed on the side of the neck. 
The opening of the gill-slit is 
closed in front by a chain of flat 
bones which constitute the “ gill- 
cover,” and by a membrane which 
is supported upon a variable 
number of slender bony spines. 
This is the general mechanism of 
respiration in one of the bony 
fishes, but different arrangements 
are found in other cases, which 
will be subsequently noticed. 
The eart in fishes may be re- 
garded as essentially a branchial 
or respiratory heart, being con- 
cerned chiefly with driving the 
venous and impure blood to the 
gills. It consists in almost all 
cases of two cavities, an auricle 
and a ventricle (fig. 126). The 
auricle (a) receives the venous 
blood which has passed through 
all the various parts of the body, 
and propels it into the ventricle = 
(v). From the ventricle proceeds Fig. 126.—Diagram of the Circulationin 
a single great vessel (the “ bran- a Fish. The arterial system is repre- 
: ‘ sented black, the venous system is 
chial artery”), the base of which left light. @ Auricle, receiving the 
is usually developed into a mus- ahi aha ecco pau ne 
cular aging? the “bulbus ACC chial ee Le ears one 
riosus” (7), which acts as a kind blood to the gills (4); ¢ Great sys- 
of additional ventricle. By the ‘mic vessel, carrying the pure blood 
: <7 to the tissues. 
ventricle and Jdulbus arteriosus . 
the venous blood is driven to the gills, where it is subjected to 
the action of the water. The aerated blood is not returned to the 
