Similar larval nephridia occur in other Gastropoda.
The anterior nephridia, of which there are one to three pairs, contrast with the posterior series by their small funnels and large size, the posterior nephridia having a large funnel followed by a short tube.
C.s, cst, f, g f, n, n.c, n.p, nst, nu, 5, to be compared with similarly-placed cells in the nephridia of Amph.ioxus.
In Polynoe the nephridia are short tubes with a slightly folded funnel whose lumen is intercellular, and this intercellular lumen is characteristic of the Polychaetes as contrasted with leeches and Oligochaetes.
They agree with primitive nephridia in being of ectodermic origin, in consisting of perforated cells in linear series, and in having no communication with the coelom.
The body is composed of a large number of segments; the prostomium bears a pair of tentacles; the nervous system consists of a brain and longitudinal ventral nerve cords closely connected with the epidermis (without distinct ganglia), widely separated in Saccocirrus, closely approximated in Protodrilus, fused together in Polygordius; the coelom is well developed, the septa are distinct, and the dorsal and ventral longitudinal mesenteries are complete; the nephridia are simple, and open into the coelom.
It has been ascertained that the nephridia of Oligochaeta are preceded in the embryo by a pair of delicate and sinuous tubes, also found in the Hirudinea and Polychaeta, which are larval excretory organs.
The nephridia of the Polychaeta have been generally dealt with above in considering the nephridial system of the Chaetopoda as a whole.
In other forms genital ducts and nephridia coexist in the same segment.
There are so many differences of detail that no line can be drawn between the one-celled funnel of Aeolosoma and the extraordinarily large and folded funnel of the posterior nephridia in the Oligochaete Thamnodrilus.