Introduction. FISHES. Structure. 1 07 
narrow isthmus of ossicles, which are frequently studded 
with haif-like teeth; and, rising in backward curves 
from thp isthmus, there is a series of successive pairs 
of branchial arches, whose upper ends are connected to 
the sub-occipital by membrane. Each arch consists of 
several pieces with joints, rendering it pliable and 
elastic ; and each has, a litUe above the middle of its 
limb, a larger joint, which enables it to fold up on itself 
when necessary. On the interior edge of each arch 
there are two rows of knobs, cones, plates, or lancet- 
shaped processes beset with fine teeth ; these are 
named ralcers, and act as sieves in straining the water, 
and preventing gross substances from passing between 
the arches. To the extero-posterior side of each arch 
are attached the gills, whose general aspect needs no 
description, but which present considerable modifica- 
tions of form and arrangement in particular groups of 
fishes. Generally there are four gill-bearing arches, 
but sometimes the gills are fewer. In the embryo fish 
six arches may be perceived, and even seven are 
indicated, but the first is atrophied before the sixth is 
developed, and only five are matured. The last arch 
does not in general carry gills, but is modified into a 
pair of pharyngeal jaws, being dentiferous on its sides, 
having two bony, dental plates or groups of plates 
below named the inferior pharyngeals, and very fre- 
quently similar superior plates lining the sub-occipital 
and called superior pharyngeals. The union of the 
inferior pharyngeals into a single bone is an essential 
character of the group of Pharyngognaths. 
In cartilaginous fishes the jaws and parts of the skull 
are so different as to need special descriptions. The 
gills of these Selachians, though adherent to the sides 
of gill-chambers, each of which opens externally, have 
essentially the same ultimate structure and the same 
function as the gills of fishes proper, which is to submit 
the blood extensively to the action of the oxygen 
contained in the waters that fishes inhabit. 
In osseous fishes the tips of the gills are free, and there 
is only one external opening on each side, by which all 
the water that bathes the gills flows out. This opening is 
regulated by a valve called the operculum or gillcover, 
which consists of four pieces — viz., thQ pre- operculum, 
a chevron-shaped or crescentic bone, bounding the 
cheek posteriorly ; the operculum, often quadrangular, 
and filling the space between the pre-operculum and 
upper part of the gill-opening; sub-opcrculum, lying 
as its name denotes under the operculum, and bordering 
the lower part of the gill-opening; and, fourthly, of the 
intcr-operculum, which lies before the sub-operculum, 
and runs forward under the lower limb of the pre-oper- 
culum towards the mandible. Between the sub-oper- 
culum and inter-operculum of the one side and those 
of the other a membrane named the brancJiiostegous 
stretches across the throat, and fills the interval between 
the limbs of the mandible. It is distended on rib-like 
bones called branchiostegals, which are articulated to 
the horns of the hyoid bone. Care is generally taken 
to note the number of the branchiostegals in the 
description of a fish, and in some groups the number is 
very constant, furnishing in that case a good character. 
When the gill-opening is large, and runs forwards to 
the root of the tongue, the branchiostegous membranes 
are narrow and are completely separated from each 
other. On the contrary, the membranes are continued 
into one another when the gill-opening is small and 
high up. 
The Heart. — In fishes all the blood is propelled 
through the gills by the heart, which, lying behind 
them, is protected by the coracoid arch. This central 
organ of the circulation consists of a single series of 
chambers — viz., in the osseous fishes, of the venous 
sinus, the auricle, the ventricle, and the arterial bulb 
or stem. There are generally two valves at the auriculo- 
ventricular opening ; but the Bays have three valves 
there, the Sun-fish four, and in the Sharks there is one 
large valve like a parachute with several marginal 
attachments. The numbers and arrangement of the 
valves in the arterial stem serve as a primary character 
of the Ganoid fishes ; and in the Sharks and Rays also 
the valves are numerous. 
Swim-bladder. — The swim-bladder, or air-bladder, 
is an organ peculiar to fishes, and lies in the belly near 
the spinal column. In some fishes it is closed com- 
pletely, and then the included air is found to consist 
chiefly of oxygen ; iii other fishes it opens through 
a tube directly into the stomach or oesophagus by a 
valvular orifice, and then it is found to be distended 
with azote and a small proportion of oxygen ; but in no 
case have its contents been found to be atmospheric 
air. In some groups of fishes the air-bladder is con- 
nected with the acoustic organs by chains of small 
bones. 
Hearing. — In the class of fishes the organs of hear- 
ing are wholly within the head and cannot be disco- 
vered except by dissection, the Plagiostomes alone 
having a fine tube leading to an external orifice from 
the interior labyrinth. This external opening is not 
easily detected, and it is not surprising that the sense 
of hearing has been denied to fishes by some authors ; 
but many facts may be cited both from ancient and 
modern writers, showing that when kept in stews fishes 
will come to the call of those that feed them, or may be 
assembled at the sound of a whistle or bell. It is 
probable that the senses are less acute in fishes than 
in animals higher in the vertebral scale ; but as they 
are furnished with nerves of touch, taste, sight, and 
smell, as well as of hearing, there is no reason to sup- 
pose that they are not endowed with all the five senses 
in greater or less perfection ; and as certain large nerves 
are in a few species supplied to galvanic or electric 
organs, we have an instance of a nervous function 
peculiar to this class. A notice of this galvanic power 
will be more appropriate when we have to speak of the 
groups in which it is displayed. 
With respect to intelligence, fishes in stews have 
been known to recognize those that feed them ; and 
we have seen one kept in a glass tank that, though 
very shy of strangers, evidently distinguished the lady 
who supplied it with food. The gregarious fishes that 
move about in large sculls cannot be expected to evince 
much individual attachment ; but some that live in 
pairs are known to have refused to eat, and to pine 
away when deprived of their companion. Many fishes 
make elaborate nests for their eggs, some instances of 
which will be alluded to hereafter. 
