retains the character of a
series of paired chambers, showing the above relations to the exterior
and to the gonads. There are, however, further complications in some
forms. Especially are these to be seen in the more modified
Oligochaeta and in the much more modified Hirudinea. In the
Polychaeta, which are to be regarded as structurally simpler forms
than the two groups just referred to, there is but little subdivision
of the coelom of the segments, indeed a tendency in the reverse
direction, owing to the suppression of septa. Among the Oligochaeta
the dorsal vessel in _Dinodrilus_ and _Megascolides_ is enclosed in a
separate coelomic chamber which may or may not communicate with the
main coelomic cavity. To this pericardial coelom is frequently added a
gonocoel enclosing the gonads and the funnels of their ducts. This
condition is more fully dealt with below in the description of the
Oligochaeta. The division and, indeed, partial suppression of the
coelom culminates in the leeches, which in this, as in some other
respects, are the most modified of Annelids.

_Nervous System._--In all Chaetopods this system consists of cerebral
ganglia connected by a circumoesophageal commissure with a ventral
ganglionated cord. The plan of the central nervous system is therefore
that of the Arthropoda. Among the Archiannelida, in _Aeolosoma_ and
some Polychaetes, the whole central nervous system remains imbedded in
the epidermis. In others, it lies in the coelom, often surrounded by a
special and occasionally rather thick sheath. The cerebral ganglia
constitute an archicerebrum for the most part, there being no evidence
that, as in the Arthropoda, a movement forward of post-oral ganglia
has taken place. In the leeches, however, there seems to be the
commencement of the formation of a syncerebrum. In the latter, the
segmentally arranged ganglia are more sharply marked off from the
connectives than in other Chaetopods, where nerve cells exist along
the whole ventral chain, though more numerous in segmentally disposed
swellings.

_Vascular System._--In addition to the coelom, another system of
fluid-holding spaces lies between the body wall and the gut in the
Chaetopoda. This is the vascular or haemal system (formerly and
unnecessarily termed pseudhaemal). With a few exceptions among the
Polychaeta the vascular system is always present among the Chaetopoda,
and a