990 
PISCES. 
Fig. 523. 
Lepidosiren. 
position of any contractile organs corresponding 
with the left side of the heart of Mammalia, 
immediately proceeds to give off branches to 
the trunk and abdominal viscera. Almost 
close to its commencement it sends into the 
abdomen the large visceral trunk, which sup- 
plies all the abdominal viscera with blood, 
ing distributed to the liver, to the stomach, 
to the intestines, to the spleen, to the generative 
organs, and to the swimming bladder. 
After giving off the above large visceral ar- 
tery, the aorta continues its course backwards 
beneath the bodies of the supra-abdominal ver- 
tebre ; but as soon as it reaches the post-abdo- 
minal vertebre, it enters the canal formed by 
their inferior arches (hemapophyses), through 
which it passes backwards to the tail. During 
its passage through the abdomen the aorta fur- 
nishes arteries right and left to the kidneys, 
between which it lies; but with this exception 
all its branches, which are given off opposite 
each vertebra like the intercostal arteries of 
the human subject, are distributed to the mus- 
cles of the trunk. 
Venous system.—All the arteries given off 
from the aorta are accompanied by correspond- 
ing venous branches, through which the blood 
is returned into two large veins, one above and 
the other beneath the vertebral column. The 
former running with the spinal cord in the su- 
perior vertebral canal, the other being in con- 
tact with the aorta through its whole length, 
these great veins frequently intercommunicate by 
means of inosculating branches, and the inferior 
one ultimately terminates in the great venous 
sinus that opens into the auricle of the heart. 
This sinus likewise receives through several 
different trunks the veins from the liver, from 
the generative organs, from the kidneys, from 
the pectoral and ventral fins, from the branchial 
organs and other parts of the throat, and like- 
wise from the head, the blood derived from the 
head having been previously collected in a 
large sinus. 
The highest form of the respiratory and cir- 
culatory apparatus met with in Fishes is found 
in the Lepidosiren, a remarkable genus found 
in the river Amazon and in the Gambia.* In 
this interesting creature the gills of the fish are 
combined with rudimentary air-sacs which per- 
form the office of lungs, and, in fact, the whole 
arrangement of its circulatory system approxi- 
mates so nearly that of the amphibious reptiles 
that, were it not for its otherwise completely 
icthyic characters, it might almost be regarded 
* Professor Owen, Description of Lepidosiren 
annectens. Trans. of Linnxan Society, vol. xviii. 
as belonging to that order of Reptilia with 
which it forms an interesting link of connec- 
tion. The branchie of the Lepidosiren consist 
of separate elongated filaments attached by one 
extremity to the branchial arches, which are 
four in number. These cartilaginous branchial 
arches are developed on each side in the sub- 
mucous tissue, and are not attached either to the 
os hyoides below or to the cranium above. The 
membrane covering the third, fourth, and fifth 
arches is minutely papillose, while the margins 
of the first and second are finely denticulated, 
and between these are five branchial apertures, 
or interspaces, through which bristles are repre- 
sented as passing in the accompanying figure, 
(ig 524, 1, 2, 3, 4, 5.) 
he gills do not form 
any external projection 
as in the gill-bearing 
Perennibranchian Am- 
phibia, but are con- 
tained in a moderately 
capacious __ branchial 
chamber, the ietes 
of which are formed by 
a mucous and muscu- 
lar stratum, the external 
outlet being a vertical 
slit situated immedi- 
ately anterior to the 
filamentary _ pectoral 
limb. 
Fig. 524. 
observes 
Professor Owen, “ al- 
though the organs of re- 
spiration through the 
medium of water cor- 
respond in all essential ° 
points with those of the 
true fishes, yet the gills 
approximate in their 
filamentary form to 
those of the Perenni- 
branchiate Reptiles. 
And again, although 
the gills are four in 
number on each side, 
as in the osseous fishes, 
yet the number of 4 
branchial —_ apertures 
and arches corresponds 
with that which cha- 
racterizes the higher 
cartilaginous fishes. So 
that, while we perceive, 
even in the organs for 
breathing water, a ten- 
larynx ; f, laryngeal or 
thyroid cartilage ; 1,2, 
3, 4, 5, interspaces be- 
tween branchial arches. 
