system of lymphatic vessels, which absorb the colourless lymph from
the tissues and convey it to the blood. The lymphatics that absorb
from the gut and pour into the blood-stream the milky food-fluid
formed by digestion are distinguished by the special name of
"chyle-vessels." While the chyle is white on account of its high
proportion of fatty particles, the lymph proper is colourless. Both
chyle and lymph contain the colourless amoeboid cells (leucocytes,
Figure 1.12) that we also find distributed in the blood as colourless
blood-cells (or "white corpuscles"); but the blood also contains a
much larger quantity of red cells, and these give its characteristic
colour to the blood of the Craniotes (rhodocytes, Figure 2.358). The
distinction between lymph, chyle, and blood-vessels which is found in
all the Craniotes may be regarded as an outcome of division of labour
between various sections of our originally simple vascular system. In
the Gnathostomes the spleen makes its first appearance, an organ rich
in blood, the chief function of which is the extensive formation of
new colourless and red cells. It is not found in the Acrania and
Cyclostomes, or any of the Invertebrates. It has been transmitted from
the earliest fishes to all the Craniotes.

The heart also, the central organ of circulation in all the Craniotes,
shows an advance in structure in the Cyclostomes. The simple,
spindle-shaped heart-tube, found in the same form in the embryo of all
the Craniotes, is divided into two sections or chambers in the
Cyclostomes, and these are separated by a pair of valves. The hind
section, the auricle, receives the venous blood from the body and
passes it on to the anterior section, the ventricle. From this it is
driven through the trunk of the branchial artery (the foremost section
of the ventral vessel or principal vein) into the gills.