ORGANS OF CIRCULATION.
The _Blood-corpuscles_ of fishes are, with one exception, of an
elliptic shape; this exception is _Petromyzon_, which possesses
circular, flat, or slightly biconvex blood-corpuscles. They vary much
in size; they are smallest in Teleosteans and Cyclostomes, those of
_Acerina cernua_ measuring 1/2461 of an inch in their longitudinal, and
1/3000 in their transverse diameter. As far as it is known at present
the _Salmonidæ_ have the largest blood-corpuscles among Teleosteans,
those of the salmon measuring 1/1524 by 1/2460 in., approaching those
of the Sturgeon. Those of the Chondropterygians are still larger; and
finally, _Lepidosiren_ has blood-corpuscles not much smaller than those
of Perennibranchiates, viz.--1/570 by 1/941 in. Branchiostoma is the
only fish which does not possess red blood-corpuscles.
[See G. Gulliver, “Proc. Zool. Soc.,” 1862, p. 91; and 1870, p.
844; and 1872, p. 833.]
Fishes, in common with the other Vertebrates, are provided with a
complete circulation for the body, with another equally complete for
the organs of respiration, and with a particular abdominal circulation,
terminating at the liver by means of the _vena portæ_; but their
peculiar character consists in this, that the branchial circulation
alone is provided at its base with a muscular apparatus or _heart_,
corresponding to the right half of the heart of Mammalia and Birds.
The _Heart_ is situated between the branchial and abdominal cavities,
between the two halves of the scapulary arch, rarely farther behind,
as in _Symbranchidæ_. It is enclosed in a _pericardium_, generally
entirely separated from the abdominal cavity by a diaphragma, which
is, in fact, the anterior portion of the peritoneum, strengthened
by aponeurotic fibres. However, in some fishes there is a
communication between the pericardial and peritoneal sacs, viz. in
the Chondropterygians and Acipenser, whilst in the Myxinoids the
pericardial sac is merely a continuation of the peritoneum.
[Illustration: Fig. 67.--Heart of Ceratodus.
_a_, Atrium; _b_, Conus arteriosus; _d_, Papillary valve
within the conus; _e-g_, Transverse rows of Ganoid valves;
_h_, _i_, Anterior arcus aortæ; _k_, _l_, Posterior arcus
aortæ; _v_, Ventricle.]
The heart is, relatively to the size of the body, very small, and
consists of three divisions: the _atrium_, with a large _sinus
venosus_ into which the veins enter; the _ventricle_; and a conical
hollow swelling at the beginning of the arterial system, the structure
of which forms one of the most important characters used in the
classification of fishes. In all _Palæichthyes_ (Figs. 66 and 67) this
swelling is still a division of the pulsating heart, being provided
with a thick muscular stratum; it is not separated from the ventricle
by two valves opposite to each other, but its interior is fitted with
a plurality of valves, arranged in transverse series more or less
numerous in the various groups of _Palæichthyes_. _Lepidosiren_ and
_Protopterus_ offer an example of a modification of this valvular
arrangement, their valves being longitudinal, each valve in fact being
formed by the confluence of several smaller ones situated behind one
another. This Palæichthyan type is called _conus arteriosus_.
In Cyclostomes and Teleosteans (Fig. 68) the enlargement is a swelling
of the artery, without muscular stratum and without contractility; with
the exception of the Myxinoids its walls are thick, fibrous, with many
trabeculæ and pouches, but it has no valves in its interior, and is
separated from the ventricle by two valves opposite to each other. This
Teleostean type is called _bulbus aortæ_.
[Illustration: Fig. 68.--Bulbus aortæ of Xiphias gladius, opened.
_a_, Section through part of the wall of ventricle; _b_,
Section through the bulbus; _c_, Teleosteous valves of the
ostium arteriosum; _d_, Accessory valves, of rudimentary
nature and inconstant; _e_, Trabeculæ carneæ of the bulbus.]
The sinus venosus sends the whole of the venous blood by a single
orifice of its anterior convexity into the atrium; two thin membranous
valvules turned towards the atrium, prevent the blood from re-entering
the sinus. A pair of other valves between atrium and ventricle have the
same function. The walls of the ventricle are robust, and, internally,
it is furnished with powerful fleshy trabeculæ.
The bulbus or conus arteriosus is prolonged into the branchial artery
which soon divides, sending off a branch to each branchial arch. On
returning from the respiratory organ the branchial veins assume the
structure and functions of arteries. Several branches are sent off
to different portions of the head and to the heart, but the main
trunks unite to form the great artery which carries the blood to the
viscera and all the parts of the trunk and tail, and which, therefore,
represents the _aorta_ of higher animals.
In the majority of Teleosteans the _aorta_ has proper walls formed
by its own membranes, but in the Sturgeons it is independent at its
commencement only, and replaced by a canal formed by hæmal elements of
the vertebral column, and clothed inside with a perichondrium. In many
Chondropterygians and some Teleosteans (_Esox_, _Clupea_, _Silurus_),
the aorta possesses its own firm membranes along its ventral side,
dorsally being protected by a very thin membrane only, attached to the
concavity of the centra of the vertebræ.
The circulatory system of _Branchiostoma_ and of the _Dipnoi_ shows
essential differences from that of other fishes.
_Branchiostoma_ is the only fish which does not possess a muscular
heart, several cardinal portions of its vascular system being
contractile. A great vein extends forwards along the caudal region
below the notochord, and exhibits contractility in a forward direction;
it is bent anteriorly, passing into another tube-like pulsatile trunk,
the branchial heart, which runs along the middle of the base of the
pharynx, sending off branches on each side to the branchiæ; each of
these branches has a small contractile dilatation (_bulbillus_) at
its base. The two anterior branches pass directly into the aorta, the
others are branchial arteries, the blood of which returns by branchial
veins emptying into the aorta. The blood of the intestinal veins is
collected in a contractile tube, the portal vein, situated below the
intestine, and distributed over the rudimentary liver. Of all other
fishes, only in _Myxinoids_ the portal vein is contractile. All the
blood-corpuscles of _Branchiostoma_ are colourless and without nucleus.
In _Dipnoi_ a rudimentary division of the heart into a right and
left partition has been observed; this is limited to the ventricle
in _Ceratodus_, but in _Lepidosiren_ and _Protopterus_ an incomplete
septum has been observed in the atrium also. All Dipnoi have a pulmonal
vein, which enters the atrium by a separate opening, provided with a
valve. The pulmonal artery rises in _Lepidosiren_ and _Protopterus_
from an arch of the aorta, but in _Ceratodus_ it is merely a
subordinate branch, rising from the _Arteria cœliaca_.