Modifications among the vertebrate classes. Fishes
The hearts of fishes show little modification from
the basic plan, except that lungfish hearts tend to
become subdivided. In them, the oxygenated blood carried
by the pulmonary vein does not enter the sinus venosus
along with the deoxygenated blood from the body. Instead,
the oxygenated blood remains separate and enters the
left side of the atrium. The atrium is partially divided
into two auricles, and the ventricle also has a partial
septum. Lungfishes show further signs of circulatory
developments in their venous system. As in land vertebrates,
there is a median posterior vena cava, and the posterior
cardinal veins are reduced.
The arterial system of fishes is also altered from
the basic plan. First there are the afferent (leading
to) and efferent (leading from) parts of the gill (branchial)
blood vessels. Each pair ofblood vessels looping up
between a pair of gills is called an arterial arch.
During the development of embryos, the arterial arches
become interrupted by capillaries in the gills. Thus,
each arch consists of a ventral afferent section that
brings blood to the gills from the heart and a dorsal
efferent section that collects blood from the gill capillaries
and carries it tothe dorsal aorta. The whole circulatory
system is a one-way arrangement, with the heart pumping
only deoxygenated blood from the body forward to the
gills to be oxygenated and redistributed to the body.
Although six gill slits appear in embryos, few adult
fishes retain all six. The first and most anterior gill
slit in the series becomes the spiracle, and the first
branchial arch is much modified; parts of it disappear
altogether. The second branchial arch is variable in
its presence in different fishes. In general, therefore,
adult fishes often have only four of the six original
arterial arches found in embryos. The external carotid
arteries also show modifications. Instead of arising
from the anterior part of the ventral aorta, they become
connected with the efferent portion of the second branchial
arch. This change ensures that, despite modifications
to the most anterior of the arterial arches, blood just
oxygenated in the gills will reach the head.
It may be that the prevalence of poorly oxygenated
water in certain habitats explains the evolution of
lungs and, hence, of land vertebrates. Fishes also have
evolved accessory structures for obtaining oxygen from
the air. These are often modified gill chambers, with
dense capillary networks. Even the intestine may be
involved, as in the loach Haplosternum.
Except for sharks and their relatives (elasmobranchs),
most fishes have a swim bladder, the structure from
which lungs may have evolved. Although its prime function
in fishes is to control buoyancy, the swim bladder may
also act as an oxygen reserve, for the gas in it often
contains a high concentration of oxygen derived from
the blood's own supply. Blood to the swim bladder usually
comes from the dorsal aorta. One African fish, Polypterus
, uses its swimbladder for respiration, and the veins
from it join the posterior cardinal veins close to the
heart. These swim bladder veins are almost where pulmonary
veins would be expected to be, if they were bringing
oxygenated blood from lungs straight to the heart.
The lungfishes have gone further in adapting their
circulatory systems to the presence of lungs, although
the different species do not breathe air to the same
extent. Some of their modifications foreshadow the changes
that have taken place in amphibians. The divided atrium
of the lungfish heart receives blood from the body on
the right side and from the lungson the left. The conus
is large and is divided by a complex system of valves
arranged in a spiral pattern and called the spiral valve.
The ventral aorta is also subdivided internally. The
result is that oxygenated blood from the left side of
the ventricle is directed into the ventral division
of the ventral aorta and passes to the anterior of the
arterial arches, while deoxygenated blood from the right
side of the ventricle is directed into the two most
posterior arterial arches and passes mainly to the lungs.
Four arterial arches are present even in the lungfish
species most dependent on breathing air (Lepidosiren),
where gills still exist. These are arches three to six
of the original series of six present in fish embryos.
Their arrangement is largely unaffected by the presence
of lungs, except that the gills may be reduced and the
arteries may pass straight through without intervening
capillaries. Arches five and six, however, join together
before entering the dorsal aorta and give rise to a
large pulmonary artery to the lungs. Thus, in lungfishes,
lungs and gills can be seen working side by side.
The circulatory systems of lungfishes are strikingly
similar to those of amphibians, and although lungfishes
do not seem to have been amphibian ancestors, they are
related to fishes that were. It is likely that several
groups of ancient fishes had lungs, partially divided
hearts, and ventral aortas, and from one of these groups
arose the land vertebrates.