In it the hairs are confined to the dorsal middle line and the creeping setae are hooked, of a finer structure than in Chaetosoma, and situated so far forward that the vagina opens amongst them.
I more comp complicated in the In all the figures the nephridial pores are indicated by dots and the setae by larger forms than in the strokes.
The external segments are usually definable by the setae; and if the setae are absent, as in the anterior segments a, Penial seta of Perichaeta ceylonica.
It is a remarkable and newly-ascertained fact that in regeneration (in Potamilla) the thorax is not replaced by the growth of uninjured thoracic segments; but that the anterior segments of the abdomen take on the same characters, the setae dropping out and being replaced in accordance with the plan of the setae in the thorax of uninjured worms. Among the Oligochaeta the sexually mature worm is distinguished from the immature worm by the clitellum and by the development of genital setae.
In these forms the bundles of setae are either capilliform or uncinate, and the dorsal setae of the thorax are like the ventral setae of the abdomen.
When feathered or provided with secondary barbs the setae will respond to movements or vibrations in the surrounding water, and have been supposed to have an auditory function.
Associated with these glands are frequently to be found bundles or pairs of long and variously modified setae which are termed penial setae,to distinguish them from other setae sometimes but not always associated with rather similarglandswhich are found anteriorly to these, and often in the immediate neighbourhood of the spermathecae; the latter are spoken of as genital setae.
The setae are organs of locomotion, though their large size and occasionally jagged edges in some of the Polychaeta suggest an aggressive function.
Paired setae of Oligochaetous pattern present in anterior segments.
There are no setae and apparently only two pairs of nephridia, of which the anterior pair open commonly by a common pore on the third segment after the head, whose segments have not been accurately enumerated.