66 '$ 
inferior extremities, as quadrupeds and fowls; for their 
progreflion is performed in a different way from either of 
thole fpecies of animals: for this purpole they are provi¬ 
ded with machines, properly confiding of a great number 
of elaftic beams, c'oi'inetled to one another by firm mem¬ 
branes, and with a tail of the fame texture; their {pine is 
very moveable towards the pofterior part, and thefirongelt 
rnulcles of their bodies are inlerted there. Their tails are 
fo framed as to contrail to a narrow fpace when drawn to¬ 
gether to either fide, and to expand again when drawn to 
a llraight line with their bodies; fo, by the alTiftance of 
this broad tail, and the fins on their tides, they make their 
progreflion much in the fame way as a boat with oars on 
its iides and rudder at its Hern. The perpendicular fins 
fituated on the fuperior part of their body keep them in 
dquilibrio , hindering the belly from turning uppermoll; 
which it would readily do, becaufe of the air-bag in the 
abdomen rendering their belly fpecifically lighter than 
their back; but, by the refiflance thefe fins meet with 
when inclined to either fide, they are always kept with 
their backs uppermoll. It may next be obferved, that 
thefe creatures have nothing that can be called a neck, fince 
they feek their food in an horizontal way, and can move 
their bodies either upwards or downwards, as they have 
occafion, by the contraction or dilatation of the air-bag; 
a long neck, as it would hinder their progreflion, w’ould 
be very difadvantageous in the element they live in. 
Tim abdomen is covered on the inferior part with a black- 
coloured thin membrane refembling our peritonaeum. It 
is divided from the thorax by a thin membranous partition, 
which has no mufcular appearance ; fo that we have now 
feen two different forts of animals that have no mufcular 
diaphragm. Thefe creatures are not provided with teeth 
proper for breaking their aliment into fmall morfels, as the 
food they ufe is generally fmall fillies, or other animals 
that need no trituration in the month, but fpontaneoully 
and gradually diffolve into a liquid chyle. Their teeth 
therefore ferve to grafp their prey, and hinder the food 
they have once caught from efcaping again. For the 
lame purpofe, the internal cartilaginous bafts of the bran- 
cliite, and the tv\ o round bodies fituated in the pofierior 
part of the jaws, have a great number of tenter-hooks 
fixed into them, in fuch a manner as that any thing can 
daftly get down, but is hindered from getting back. The 
water that is neceffarily taken along with their food in too 
great quantities to be received into their jaws in degluti¬ 
tion, paffes between the interftices of the branchiae and the 
flap that covers them. 
The cefophagus in thefe creatures is very fhort, and fcarce- 
ly dillinguilhed from their ftomach; and the food lies al- 
moft equally in both. The llomach is of an oblong figure. 
There are commonly found fmall filhes Hill retaining their 
natural form in the llomach of large ones; but, when 
touched, they melt down into a jelly. From this, and the 
great quantity of liquors poured into their flomachs, we 
may conclude, that digeilion is folely brought about in 
them by the diffolving power of a menllruum, and that 
no trituration happens here. The guts of thefe animals 
are very fiiort, making only three turns ; the lad of which 
ends in the common cloaca for the faeces, urine, and fe- 
men, fituated about the middle of the inferior part of 
their bodies. 
To that fufcftance which we call pancreas, fome give the 
name of intejlinula caeca. It confills of a great number of 
fmall threads, like fo many little worms, which all termi¬ 
nate at lad in two larger canals, that open into the firff 
gut, and pour into it a vifcous liquor much about the place 
where the biliary duds enter. That kind of pancreas 
formed of inteftinula cmca is peculiar to a certain kind of 
' filhes; for the cartilaginous, broad, and flat, kind, as the 
lkate, foie, flounder, &:c. have a pancreas relembling that 
of the former clafs of animals. Their intellines are con- 
neded to the back-bone by a membrane analogous to a 
mefentery. Their liver is very large, of a whitifli colour, 
and lies almoll wholly in the left fide, and contains a great 
[Comparative* 
deal of fat or oil. The gall-bladder is fituated a confide- 
rable way from their liver ; and fends out a canal, the cyf- 
tic duct, which joins with the hepatic duift juft at the en¬ 
try into the gut. Some fibres are ftretched from the liver 
to the gall-bladder ; but nobody has hitherto difeovered 
any cavity in thele cords : fo in this animal it fhould feem 
impollible that the bile can be carried into the gall-blad¬ 
der in the ordinary way; and confequently it mull either 
be fecreted on the fides of that fac, or regurgitate into it 
from the canalis choledoclnis. Here we may make the 
fame remark as upon the biliary duels of fowls, viz. that 
hepato-cyftic duds exift in the one as well as the other. 
This, for example, is very obvious in the falmon, where 
large and diftind duds run from the biliary duds of the 
liver, and open into the gall-bladder. 
The fpleen is placed near the back-bone, and at a place 
where it is fubjed to an alternate preflure from the con- 
ftridion and dilatation of the air-bag, which is fituated 
near it. Since, in all the different animals we have dif¬ 
fered, we find the fpleen attached to fome part that may- 
give it a conquaffation; as in the human fubjed and qua¬ 
drupeds, it is contiguous to the diaphragm; in fowls, it is 
placed between the back-bone, the liver, and the ftomach ; 
in fillies, it lies on the faccus aerius: and fince we find it 
fowell ferved with blood-veffels, and all its blood return¬ 
ing into the liver, we mull not conclude the fpleen to be 
an inutile pondus, only to ferve as a balance to the animal 
pro cequilibrio, but particularly defigned for preparing the 
blood for the liver. 
The only organs of generation in this animal, are two bags 
fituated in the abdomen uniting near the podex. Thefe 
in the male are filled with a whitilh firm lubftance called 
the milt ; and in the female with an infinite number of lit¬ 
tle ova cluftered together, of areddilh yellow colour, call¬ 
ed the roe. Both thefe at fpawning-time we find very 
much diftended ; whereas at any other time the male or¬ 
gan can fcarcely be dillinguilhed from the female ; nor fs 
there any proper inftrument in the male for throwing the 
feed into the organ of the female, as in other creatures. 
We fhall not take upon us to determine the way whereby 
the female fperm is impregnated ; but we find that the 
fpawn of frogs confills of fmall fpecks wrapped up in a 
whitifli glutinous liquor; thefe fpecks are the rudiments 
of the young frogs, which are nourilhed in that liquor till 
they are able to go in fearch of their food. Spallanzani 
has found, that the eggs of frogs, toads, and water-newts, 
are not fecundated in the body of the female; that the 
male emits his femen upon the fpawn while it is flowing 
from the female ; and that the feetus pre-exifts in the bo¬ 
dy of the female. But whether impregnation takes place 
in the fame manner in filhes, he has not yet been able to 
determine, though he feems to think it probable; for, in 
the fame way, the ova of fifties are thrown out and depo- 
fited in the fand, the male being for the mod part ready 
to impregnate them, and they are hatched by the heat of 
the lun. It is curious enough to remark with what rare 
they feek for a proper place to depofit their ova, by fwim- 
ming to the Ihallow, where they can better enjoy the fun’s 
rays, and Hum the jaws of other large filhes. The river- 
fillies, again, fpawn in fome creek free front the hazard of 
the impetuous ftreant. But whether this mixture be 
brought about in fillies by a fimple application of the ge¬ 
nitals to each other, or if both of them throw out their 
liquors at the fame time in one place, and thus bring about 
the defired mixture, it is not eafy to determine; the lat¬ 
ter, we think, feems mod probable. Thefe creatures are 
fo Ihy, that we cannot ealily obferve their manner of co¬ 
pulation, and we are confequently but little acquainted 
with their natural hiltory. Frogs, it is very evident, do 
not copulate; at lead no farther than to allow both fexes 
an opportunity of throwing their fperm. Early in the 
fpring the male is found for leveral days in clofe contad 
upon the back of the female, with his fore-legs round 
her body in fuch a manner that makes it very difficult to 
feparate them; but there is no penetration. At this time 
the 
ANATOMY. 
