lways consists of a system of vessels with definite walls, which
rarely communicate with the coelom. It is in fact typically a closed
system. The larger trunks open into each other either directly by
cross branches, or a capillary system is formed. There are no lacunar
blood spaces with ill-defined or absent walls except for a sinus
surrounding the intestine, which is at least frequently present. The
principal trunks consist of a dorsal vessel lying above the gut, and a
ventral vessel below the gut but above the nervous cord. These two
vessels in the Oligochaeta are united in the anterior region of the
body by a smaller or greater number of branches which surround the
oesophagus and are, some of them at least, contractile and in that
case wider than the rest. The dorsal vessel also communicates with the
ventral vessel indirectly by the intestinal sinus, which gives off
branches to both the longitudinal trunks, and by tegementary vessels
and capillaries which supply the skin and the nephridia. In the
smaller and simpler forms the capillary networks are much reduced, but
the dorsal and ventral vessels are usually present. The former,
however, is frequently developed only in the anterior region of the
body where it emerges from the peri-intestinal blood sinus. On the
other hand, additional longitudinal trunks are sometimes developed,
the chief one of which is a supra-intestinal vessel lying below the
dorsal vessel and closely adherent to the walls of the oesophagus in
which region it appears. The capillaries sometimes (in many leeches
and Oligochaeta) extend into the epidermis itself. Usually they do not
extend outwards of the muscular layers of the body wall. The main
trunks of the vascular system often possess valves at the origin of
branches which regulate the direction of the blood flow. Among many
Oligochaeta the dorsal blood-vessel is partly or entirely a double
tube, which is a retention of a character shown by F. Vezhdovsky to
exist in the embryo of certain forms. The blood in the Chaetopoda
consists of a plasma in which float a few corpuscles. The plasma is
coloured red by haemoglobin: it is sometimes (in _Sabella_ and a few
other Polychaeta) green, which tint is due to another respiratory
pigment. The plasma may be pink (_Magelona_) or yellow (_Aphrodite_)
in which cases the colour is owing to another pigment. In _Aeolosoma_
it is usually colourl