736 I C II THYOLOGY. 
bony or cartilaginous arch, with a double fringe, between 
, which the blood circulates in very fmall veflels, Th e gills 
are fattened within on each fide to two bones in the palate. 
■It is by means of the gills that fifties breathe. They draw 
the water by the mouth, at the fame time (hutting the 
aperture of the gills; by this means the gills are to fifhes, 
for the circulation of the blood, what the lungs are to 
Other animals, for the infpiration of air; for, if the fifli 
i'uddenly raifes up the covering of the gills, the water in- 
ftantly rufhes out, in the fame manner as air efcapes from 
the lungs by expiration. But there are fome fifh which 
have been fuppofed to poffefs a kind of lungs befides the 
gills; fucli are the lamprey, thornback, &c. This gave 
. Linnaeus the idea of forming them into a particular clafs 
of amphibious animals, famphibia nantcs.) But the very 
principle itfelf has been lately denied and refuted by 
Vicq. d’Azyr, a French writer. Thefe lpecies have an 
aperture on each fide; and the lower part, between the 
membrane of the gills and the aperture of the mouth, is 
called the throat. 
A peculiarity called the Jingle gill is met with in fome 
fpecies. Befides the four gills, each of which is compofed 
of a double row of fringes, feveral fifhes have a fifth, with 
only a fingle row of fringe; this has no bony arch, nor 
is it loof’e like the reft, but it fpreads along the infide of 
the front gill-covert juft to its. outer rim. Like the dou- 
, ble gills, it is arched, and receives a fmall branch of the 
artery which furnifhes blood to the other gills. It is not 
always proportioned to the fize of the other gills, being 
fometimes large, fometimes fmall, and in fome fifh there 
is only as it were the beginning or rudiment of a gill. 
Pallas fpeaks of a kind of fingle gill in a cartilaginous 
fifh, the Cyclopterus dentatus, in which fpecies lie ob- 
ferved three double gills loofe, and a fingle one, fixed, 
which lie calls adnata. Broufi'onet fpeaks of it under the 
name o f pfeudobranchia, or baftard-gill, having remarked it 
in the Pleuronebtes mancus, the Chattodon longiroftris, 
and the Clupea thrifia. Dr. Walbaum, in his new and 
enlarged edition of Artedius, gives it different denomi¬ 
nations, as branchia manea, branchia acccjforia, See. he found 
it in the fword-fifli. ProfefTor Monro has noticed it in 
Lis Account of the Circulation of the Blood, See. in 
Fifties: he calls it Jingle gill. But none of thefe authors 
lias given a latisfaftory account of its uf'es. Bloch took 
infinite pains to difeover the nature and ufe of the fingle 
' gill. He firft examined, whether perhaps, being an appen¬ 
dage to the lungs or to the gills, it might not; be peculiar 
; to the fifties of hot climates, as requifite to bring more 
cool air to the blood ? whether it is wanting in thole fifhes 
which die l'oon after they are taken out of the water? 
Whether peculiar to cartilaginous fifties, or to the bran- 
chioftegous ? to falt-water fifh, or to thole of frefh-water? 
to thole of lakes or thofe in rivers ? to the ftationary or 
the migratory; for he conceived, that fome of thefe cir- 
cumftances, being afeertained, might become the bafis for a 
claftification of fuch fifh ; as hereby we might aftign the 
proper country or haunts of a fifh before unknown ; or, 
in cafe of trail ('porting certain fifh, it would readily be 
known in what waters they would thrive belt. But all 
his enquiries were vain; he never could difeover why the 
fingie gill is necefiary to fome fifh, and why others can do 
without it. In the courfe of the alphabet, we have men¬ 
tioned the genera and fpecies, as they occur, which poffefs 
this peculiarity, leaving future naturalifts to draw their 
own conclufions. It certainly opens a wide field for/pe¬ 
culation, and no fatislfi&ory or analogical conclufions have 
yet been drawn from examining the qualities and proper¬ 
ties of thofe genera and fpecies in which it is found. Ac- 
‘ cording to Bloch’s clarification, the fingle gill is prefent 
in -iC) genera, (though not in all the fpecies of each,) in 
x 5 it is partly formed, and in \z it is entirely wanting. 
Among thofe which have it not, we find, the filure, the 
pike, the pond-loach, the eel, the perch, &c. which, liv- 
( ji«g often id ftill waters, might be luppoled to have colder 
blood than others, and therefore not to need an organ for 
cooling the blood, which fome have fuppoled this to be; 
and this is confirmed by fome of the mixed genera, as the 
Cyprinus, in which thofe fpecies which live in ftill water 
are without it, as the carp, tench, crucian, &c. while thofe 
are furnifhed with it which live in running ftreams, as 
the narrow carp, the bley, See. But this conjecture, thus 
fupported, falls to the ground, when it is obferved that 
the river-loach, which lives only in pure cool gravelly 
ftreams, is provided with it, and that it is wanting in moft 
fea-fifti. This gill might indeed be taken as a diftinftive 
mark to fubdivide the numerous genus of falmons and 
that of carps, as fome fpecies of each are furnifhed with it. 
Fifties, as before obferved, have no neck ; the head joius 
on to the body. The body of moft fifties is covered with 
fill all bright laminae of ghe nature- of horn, called Jcales. 
Some fpecies, as the turbot, fturgeon, Sec. have bony or 
cartilaginous protuberances inftead of feales; others are 
covered with tubercles, as the ftickleback; and others 
again have the (kin entirely fmootli, but covered with a 
vifeous gluey matter, as the loach, filurus, &c. The 
looking-glafs carp, with large feales, with which the body 
is however only covered in part, feems to hold a middle 
place between thefe extremes. 
The body, or trunk, confifts of the bread, the belly, 
and the tail. The bread is fhort, becaufe the lungs are 
in the head; it is feparated from the belly by a white 
ftiining membrane called diaphragm. The belly is the 
part between the bread and the tail. In l'ome fpecies the 
belly is thick: in others it is thin or carinated. In fome 
the back is quite round ^ in others only partly fo ; but in 
moft fpecies it terminates in a kind of fliarp edge, as the 
belly often does. 
We call the Jides the fpace between the belly and back. 
In moft fifties may be difeerned on each fide a line which 
ftretches from the head to the fin of the tail : this is call¬ 
ed the lateral line. 
Th t jins take their name from the parts they belong to, 
as the peftoral fins, the dorfal, ventral, anal, and tail, fins. 
The dorfal fin is fometimes fingle, as in the pike ; fome 
fifh have two, as the perch, See. and fome three, as the 
ftockfifli, Sec. Some fpecies, as the falmon, have a fecond 
fin, which however is only a lengthened membrane, called 
pinna adipofa. The peitoral fins are always two in num¬ 
ber ; they are placed near the opening of the gills; aud 
the fifh uf'es them as oars to advance in the water. In 
fome few fpecies thefe are fo lengthened, that the fifties 
can ufe them as wings, and fuftain themf'elves for fome time 
in the air. There are feveral fpecies which have no ventral 
fins; thefe are called apodes, or “ wanting feet;” fucli are the 
eel, f'word-fifh, &c. The ventral fins, when prefent, are al¬ 
ways two in number, and ferve like feet to fuftain the fifh 
when at the bottom of the water. They are of courfe placed 
under the body, but not always in the fame fpot; they are 
fometimes near the throat, fometimes at the breaft, and 
fometimes at the belly. In the firft cafe, the fifh are called 
jugulares ; luch are the ftock-rifli, eelpout. Sec. in the fe¬ 
cond cafe, they are called tboracici, as the perch, Sec. in 
the third, they take the name of abdominales, as the pike, 
the falmon, carp. See. The anal fin, which is placed be¬ 
tween the belly and the tail, is generally fingle, and, with 
the dorfal fin, ferves to keep the fifh in equilibrio. The 
tail-fin ends the external parts of the fifh ; it ferves them 
to advance, turn, and direct their motions. In fome it is 
round, in others ftraight. Sometimes there is a cut or 
dent which forms a fort of crefcent when it is not deep, 
and a fork when it is, as may be feen in the fword-fiih 
and the bream. All thefe varieties in the formation of 
the different parts are fo many diltinciive marks whereby 
to arrange the fifties into orders, genera, and fpecies. 
The fins confift of a membrane of greater or lefs height, 
breadth, and thicknefs, and fupported by cylinders move- 
able or not, more or lefs numerous, which are called rajs, 
becaufe they are difpofed fometimes like rays diverging 
from 
