988 
through the exposed apertures. There is, more- 
over, another remarkable arrangement in the 
Lampreys, whereby, notwithstanding the sucto- 
rial character of their mouths, the members of 
this genus have the character of their respiratory 
apparatus approximated to that of other races 
of Fishes. This consists in the presence of a 
small tubular orifice situated in the middle of 
the back of the head, just in front of the eyes, 
which leads downwards into the pharynx (6,) 
into which it opens by the orifice c, so that 
water can enter by this passage while the 
mouth is kept immoveably fixed to the surface 
whereunto the Lamprey has attached itself. 
The respiratory system of Branchiostuma is 
highly curious in its arrangement. It may be 
said to consist of two portions, a hyoid appa- 
ratas of a very remarkable kind, and a respi- 
ratory or branchial sac enclosed by a series of 
thoracic ribs, both of which will require a par- 
ticular description before we proceed further 
with this part of our subject. 
Hyoid apparatus. — The hyoid apparatus 
supports the mouth and guards its entrance, 
the oral aperture, which is in the form of a lon- 
gitudinal slit, being bounded on each side by a 
series of hyoid cartilages, each of which consists 
of several pieces articulated together so as to 
form a continuous chain, which gradually 
diminishes in thickness towards its anterior 
extremity. To every one of these pieces is 
attached a cartilaginous arch composed of two 
cartilaginous stems, which are armed with 
delicate denticulations, and covered with a fine 
membrane, which is extended along the base of 
the posterior ones like a thin interdigital web, 
which forms the margin of the mouth. 
Branchial cavity.—This is situated behind 
the hyoid apparatus, and consists of a kind of 
thorax composed of upwards of a hundred 
transparent, cartilaginous, and hair-like arches, 
the upper extremities of which are fixed in two 
streaks of a soft white substance, which runs 
along on each side of the inferior surface of the 
chorda dorsalis on the sides of the inferior lon- 
gitudinal ligament. The inferior extremities 
of the ribs terminate in a more complicated 
manner. Each alternate rib bifurcates. The 
inferior branch on each side meets its fellow of 
the opposite side at an angle in the median 
line, while the superior branch likewise curves 
up and meets the corresponding division of its 
Opposite, while the non-bifurcated ribs, which 
are shorter, terminate in a line with the bifur- 
cation of the neighbouring pairs. There are, 
moreover, slender cross pieces joining the 
neighbouring ribs together, disposed in an 
alternating series like the joinings of stones 
-in masonry. The heart is situated at the 
posterior extremity of the branchial sac be- 
tween its posterior extremity and the com- 
mencement of the liver; it consists, accord- 
ing to Costa, of two delicate cavities situated 
transversely above and a little to the right of the 
intestine. These cavities (“ orecchiette’”) com- 
municate, and the blood passes from one to the 
other, being driven forward by rapid pulsa- 
tions. From the second cavity of the heart 
the blood is driven into the trunk of the bran- 
PISCES. 
chial artery, which is situated between the in- | 
testine and the point of attachment between | 
the branchial ribs and the spinal column, The 
course of the blood is at first rapid and 
distinct, owing to the size of branehi 
artery being considerable; but as it runs for- 
wards, it expends itself in numerous spiral ves- 
sels, which, accompanying all the thoracic arches 
and their transverse attachments, thus present » 
a considerable surface and constitute the bran- 
chie of the fish. Finally, towards the anterior 
extremity of the respiratory cavity the branchial 
artery has become so much reduced in size as 
to be no longer distinguishable. J 
In addition to the extensive surface furnished 
by the respiratory sac there are three vascular 
lamellae situated on each side of the ) 
to which Costa applies the name of o lar 
branchie, believing them to be analogous to the 
branchie attached to the opercula of the Stur- 
geon and other Fishes; these receive their sup- 
ply of blood from an arterial teank that sur- 
rounds the back part of the roof of the mouth. 
Heart.—The heart of Fishes (fig.522, C, A) 
is situated beneath the throat immediately be- 
hind the inferior terminations of the branchial 
arches, where it is lodged in a com 
rtitioned off from the cavity of the abdomen 
i a strong tendinous septum, which forms a 
sort of immoveable diaphragm. In the Lam-. 
preys the heart nee encased A 2 cartila- 
ginous capsule (/ig.521, m) formed by the 
rior faetine of the cartilaginous Cameraman 
surrounds the branchial apparatus. 
In the course of the circulation the heart of 
Fishes is interposed between the systemic 
veins and the organs of respiration, its office 
being to propel the venous blood received 
through the vene cave into the gills, whence it 
is conveyed by the arterial system to be distri- 
buted to the body without the intervention of 
any contractile cavity devoted to its i 
through the arterial trunks. Fora diagram of 
the course of the circulation in Fishes the 
reader is referred to fig. 319, vol. i. 
The heart of a fish is therefore at once distin- 
guishable from that of any other vertebrate ani- 
mal by the circumstance of its consisting ofonly — 
two cavities, an auricleand a ventricle, the former 
of which receives the blood from the body, while 
the latter diffuses it over the branchial surfaces. 
The auricle (fig. 522, C) is very capacious, — 
having its walls made up of intercrossing mus- 
cular fibres, which from their frequent decussa- — 
tions present internally a reticulate appearance. 
It receives the venous blood from a sinus (D), 
formed by the conjoined cave through a — 
orifice, which is guarded by two large semilu- 
nar valves, arranged so as to prevent the reflux 
of blood into the veins. te” 
The ventricle is considerably smaller tha 
the auricular cavity, with which it communi- 
cates by a wide lateral orifice. Its shape it 
the osseous Fishes is most usually somewhat 
four-sided, and its muscular walls, considerably — 
thicker than those of the auricle, present inter 
nallya strong fasciculated appearance. Theau- — 
riculo-ventricular opening is generally guarded 
by two strong valves ordinarily of a semilunar 
