PISCES. 
dency towards the amphibious type, we find 
at the same time that the branchial as well 
as the osseous system manifests a most interest- 
ing transitional structure between the plagios- 
tomous and osseous fishes. We have next to 
consider that part of the respiratory system 
which is organized for breathing immediately 
the atmospheric air, or the lungs; for I do not 
know how otherwise to designate, according 
either to their physiological or morphological 
relations, those organs which in the technical 
language of the ichthyologist would be termed 
the swimming orair-bladder. The trachea, or to 
use the same technical and partial nomenclature, 
the “ductus pneumaticus,” is a wide short 
membranous tube, as in the Perennibranchiate 
Reptiles. The glottis (fig. 524, e,) opens 
near the posterior part of a long rudimental 
thyroid cartilage (f): a few lines posterior to the 
_ isthmus faucium the opposite end of the trachea 
dilates into a membranous sac, which commu- 
nicates by two large lateral apertures with the 
lungs. These are widest at their anterior extre- 
mities, and gradually decrease in diameter to 
the cloaca, behind which they terminate each 
Fig. 525. 
Respiratory and circulatory apparatus of Lepidosiren 
annectens, after Owen. 
a, auricle; 6. ventricle laid open to show the ter- 
mination of the vena pulmonalis, in which a 
black bristle is placed; ¢, bulbus arteriosus laid 
open; d, pericardium; e, vena cava abdomi- 
nalis; f, vena pulmonalis; 1, 2, 3, 4, 5, 6, 
branchial arteries of left side; m, pulmonary 
artery; ”, pulmonary vein. 
in an obtuse point (fig. 525). They are 
lodged in the dorsal angle of the abdominal 
cavity behind the kidneys, and are attached by 
cellular tissue to all the surrounding parts, and 
especially to the ribs, of which they bear the 
impressions on their posterior surface. The an- 
991 
terior part of each lung is divided into four or 
five small lobes, behind which it takes on the 
form of a simple compressed bag, and so con- 
tinues to its posterior extremity. The parietes 
of the lung present a moderate thickness 
throughout, and the whole of the internal sur- 
face is cellular, the cells having the same pro- 
portional size and form as in the respiratory 
portion of the lung of a serpent. The cells 
are largest and most subdivided at the anterior 
part of the lung, the livid colour of which, in 
the specimen dissected” by Professor Owen, 
“attested the great vascularity of the part.” 
In tracing the arrangement of the circulatory 
system of the Lepidosiren the same interme- 
diate type of structure is most interestingly 
conspicuous. The heart consists of an auri- 
cle, (fig. 525, a,) ventricle, (b,) and a bul- 
bus arteriosus. The vena cava (e) termi- 
nates in the right side of the auricle; it is 
joined by the two superior cave and by the 
single large pulmonary vein; this vein (f)) 
does not, however, communicate with the sinus, 
but passes along entire and adherent to the 
inner surface of the vena cava as far as the 
auriculo-ventricular aperture, where it empties 
its contents into the ventricle by a distinct 
orifice protected by a cartilaginous valvular 
tubercle. It needed only that the pulmonary 
vein should have been dilated before its termi- 
nation, in order to have established a bi-auri- 
cular structure of the heart, as in the Amphi- 
bious Siren. The same functional advantage 
is, however, thus secured to the Lepidosiren 
with a maintenance of the simple dicelous 
type of the heart of the fish ; this continuation 
of the pulmonary vein preventing the admix- © 
ture of the respired with the venous blood, 
until both have arrived in the ventricle. 
The ventricle (fig. 525, b) is extremely 
small; its parietes are thick and reticularly 
muscular; a small round orifice leads to the 
bulbus arteriosus (c). This is formed by a 
short spiral turn of the dilated aorta, which is 
concealed under a simple continuous fibrous 
coat. The area of this part of the vessel is 
almost entirely occupied by two continuous 
valvular projections or their processes, which 
are attached by one edge to the internal sur- 
face of the aorta, and have the opposite margin 
projecting freely into the arterial cavity. 
The aorta in this remarkable species fulfils 
at once the office of a systemic, a branchial, 
and a pulmonary artery ; it distributes on each 
side six vessels, (fig. 525, 1, 2, 3, 4, 5, 6,) 
corresponding to the six branchial cartilaginous 
arches. The mucous membrane is produced 
into a branchial fringe on the convex side of 
the first, fourth, fifth, and sixth branchial arches, 
and the corresponding arteries are minutely sub- 
divided before they are continued to the dorsal 
side of the pharynx ; these four pairs of vessels 
are therefore true or functional branchial arte- 
ries. The mucous membrane merely invests 
with a simple fold the second and third bran- 
chial arches; and the corresponding arterial 
trunks undergo no subdivision as they wind 
round them, but are continued entire (as in the 
Amphiuma and Menopoma) to their termina- 
