The Capitellidae, in which the vascular system has vanished altogether, leaving a coelom containing haemoglobin-impregnated corpuscles.
In this category are included (by Goodrich and Lankester) the gonad ducts of the Oligochaeta, certain funnels without any aperture to the exterior that have been detected in Nereis, &c., funnels with wide and short ducts attached to nephridia in other Polychaeta, gonad ducts in the Capitellidae, the gonad ducts of the leeches.
Frequently reduced in number of pairs; rarely (Capitellidae) more than one pair per segment.
In some genera the setae are in vertical rows, and in certain Capitellidae these rows so nearly meet that an arrangement occurs reminiscent of the continuous circle of setae in the perichaetous Oligochaeta.
Among the Capitellidae, which in several respects resemble the Oligochaeta, wide and short gonad ducts coexist in the same segments with nephridia, the latter being narrower and longer.
The parapodia, as in the Capitellidae, are hardly developed.
It is possible that this represents the syphon or supplementary intestine of Capitellidae, which has been shown to develop as a groving of the intestine ultimately cut off from it.