The setae vary much in form and are often longer and stronger than in the Oligochaetes.
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 setae ar ° invariably formed each within an epidermic cell, and they are sheathed in involutions of the epidermis.
They are disposed in two groups on either side, corresponding in the Polychaeta to the parapodia; the two bundles are commonly reduced among the earthworms to two pairs of setae or even to a single seta.
On the other hand, in certain Polychaeta the bundles of setae are so extensive that they nearly form a complete circle surrounding the body; and in the Oligochaet genus Perichaeta (=Pheretima), and some allies, there is actually a complete circle of setae in each segment broken only by minute gaps, one dorsal, the other ventral.