ess. The vascular system is in the majority of
Chaetopods a closed system. It has been asserted (and denied) that the
cellular rod which is known as the "Heart-body" (_Herzkorper_), and is
to be found in the dorsal vessel of many Oligochaeta and Polychaeta,
is formed of cells which are continuous with the chloragogen cells,
thus implying the existence of apertures of communication with the
coelom. The statement has been often made and denied, but it now seems
to have been placed on a firm basis (E.S. Goodrich), that among the
Hirudinea the coelom, which is largely broken up into narrow tubes,
may be confluent with the tubes of the vascular system. This state of
affairs has no antecedent improbability about it, since in the
Vertebrata the coelom is unquestionably confluent with the haemal
system through the lymphatic vessels. Finally, there are certain
Polychaeta, _e g._ the _Capitellidae_, in which the vascular system
has vanished altogether, leaving a coelom containing
haemoglobin-impregnated corpuscles. It has been suggested (E. Ray
Lankester) that this condition has been arrived at through some such
intermediate stage as that offered by Polychaet _Magelona_. In this
worm the ventral blood-vessel is so swollen as to occupy nearly the
whole of the available coelom. Carry the process but a little farther
and the coelom disappears and its place is taken by a blood space or
haemocoel. It has been held that the condition shown in certain
leeches tend to prove that the coelom and haemocoel are primitively
one series of spaces which have been gradually differentiated. The
facts of development, however, prove their distinctness, though those
same facts do not speak clearly as to the true nature of the blood
system. One view of the origin of the latter (largely based upon
observations upon the development of _Polygordius_) sees in the blood
system a persistent blastocoel. F. Vezhdovsky has lately seen reasons
for regarding the blood system as originating entirely from the
hypoblast by the secretion of fluid, the blood, from particular
intestinal cells and the consequent formation of spaces through
pressure, which become lined with these cells.

_Nephridia and Coelomoducts_.--The name "Nephridium" was originally
given by Sir E. Ray Lankester to the members of a series of tubes,
proved in some cases to be excretory in nature, which exist typically
to the num