1 OG Intkou ucTiON. FIS H ES. Structure. 
posterior, designing thereby to render the term more 
applicable as a distinctive character to his order of 
Acanthopteres, whose ventrals have a spinous ray in 
front, and need the firm support of the coracoids when 
used as a defensive weapon. Only in a very few 
instances are the pubic bones united directly to the 
spinal column as the hind legs are in quadrupeds ; they 
are almost always merely suspended in the soft parts. 
The term apodal, applied to fishes, indicates that the 
ventrals are absent; and ahrachial, in like manner, 
denotes the non-existence of pectorals. 
Lateral line. — Most fishes have pores in the skin 
from which a lubricating fluid is poured out, in some 
very copiously, as in the glutinous hag, for which it 
suffices to convert the surrounding water into a jelly. 
These pores are rarely absent from a row of dermal 
bones that run under the eye, and are therefore 
named suh-orhiials ; and they may generally be traced 
over the temples, in a chain of ossicles called sur- 
temporals, and onwards b\'^ the suprascapula to the 
side of the fish, which they traverse towards the tail, 
following the line of decussation of the two great lateral 
muscular masses. In this position the porous row 
acquires the name of the “lateral line,” and it is a 
character of which special mention is generally made 
in the description of the external aspect of a fish. The 
scales that it traverses most commonly differ from the 
others in size, figure, or in being perforated, or tubular, 
or notched, and they form the points of meeting 
between the oblique rows of dorsal and ventral scales. 
In many fishes which have no other scales, the ducts 
of the lateral line are jirotected by cartilaginous or 
osteoid cases. Sometimes the lateral line runs only a 
short way behind the pectoral, sometimes it goes to the 
base of the caudal rays, sometimes onward to their 
tips ; or it may be Interrupted, and recommence again 
on a lower level ; or, finally, it may be altogether 
imperceptible or absent, as in most sharks ; or there 
may be two, three, or many lateral lines, as in the 
Mullets and Chiri. 
Water-tubes. — Agassiz has discovered that in 
many fishes tliere exist external openings which have 
hitherto been mistaken for muciferous pores, but whose 
function is to admit water into the circulating system 
through a series of tubes that end in the blood-vessels 
near the heart. They are easily seen in the head of 
the common shad, but none have been detected in the 
Selachians; and they are generally few in fishes that 
live habitually in shallow waters. He considers them 
to be safety-tubes for balancing the system on sudden 
alterations of pressure. An analogous system exists 
in Molluscs, by which water can be admitted into the 
heart from the exterior.'*' 
Endo- skeleton. — The very numerous groups of 
fishes cannot be described or recognized without a 
more frequent reference to the parts of the internal 
skeleton than is requisite in characterizing the fewer 
divisions of the Mammals or Birds. But we are 
restrained in this work from lengthened anatomical 
descriptions, and must confine our brief notices of the 
bones to those pieces whose names enter into the 
definitions of the family groups of fishes. 
*■ Duthiers, Proceed. Hoy. Soc. x. No. 37, p. 193. Dec. IS.'iO. 
In the skull of an osseous fish five ridges are some- 
times very conspicuous. A mesial one, chiefly deve- 
loped on the super-occipital, which is the hindmost 
bone on the top of the skull, and is conspicuous in 
figure c of Plate 17. The distal point of this ridge is 
frequently called the “occipital spine.” This mesial 
ridge is often prolonged forwards on the mid-frortal 
between the orbits. Next follow the pair of intermediate 
ridges, one on each side developed on the par-occipi- 
tal, exteriorly to the super-occipital, and running for- 
wards over parietal and mid-frontal near the upper 
edge of the orbit. To the distal point of the interme- 
diate ridge one limb of the chevron-shaped supra- 
scapula is attached. Still more exterior or lower down 
on the side of the head run the external ridges, also a 
pair, one on each side. To the distal end of this ridge, 
which is a process of the mastoid bone, the lower limb 
of the suprascapula is attached ; the fore part of the 
ridge reaches to the hinder margin of the orbit on the 
post-frontal. Fishes with round heads like the loaches 
want these five ridges ; some, as the carp, have merely 
the occipital spine. In the Choetodons the mesial ridge 
is greatly elevated, and in the Sclerogenids the whole 
five ridges are more or less conspicuous. 
The upper profile of the face or snout is supported 
by the nasal hone; and on each side of it are the 
nostrils, opening by one or two small orifices into the 
pituitaiy sacs, but not penetrating to the cavity of the 
mouth. Under the nasal bone are the jaws, variously 
formed in various groups of fishes, but retaining their 
relative positions and connections pretty exactly. The 
premaxillaries, a pair of bones applied to each other 
on the mesial line, form the middle part of the upper 
jaw, and in a considerable number of osseous fishes 
border the entire upper half of the mouth. Articulated 
to the palatine, behind the premaxillary, lies the 
maxillary, whose lower end in many fishes comes for- 
ward to the corner of the mouth, and enters into the 
composition of its upper border. When the jaws are 
protractile, each premaxillary sends out a long pedicel, 
which glides backwards and forwards by the side of the 
nasal on a tubercle of the maxillary. The mandible, 
composed of two limbs, each consisting of two or more 
pieces, makes up the lower jaw. Underlying the nasal, 
within the roof of the mouth on the mesial line, is the 
vomer; and on each side of it is one of the pair of 
palatine hones. In the back part of the roof of the 
mouth, over the gullet, is the sub-occipital, which is 
confluent with the sphenoid that articulates anteriorlj^ 
and above with the vomer. All the bones here named 
as belonging to the orifice of the mouth or its roof are 
dentiferous in one species of fish or another. Interiorly 
between the limbs of the mandible there is often a 
lingual or tongue-hone, frequently bearing teeth, and 
opposed to the vomer. There are also in almost all 
cases teeth on the branchial arches or on parts of the 
hyoid bones connected with them. 
The Gills and Hvoid Apearatus.— -Descending 
from the mastoid bones of each side a series of pieces 
meet at the tongue, and form an elastic inverted chevron 
like the letter V. Running backwards in the lower 
angle of the V, and on the mesial line from the tongue, 
between the limbs or cornua of this arch, there is a 
