XXI.—On a Rule of Proportion observed in the Setæ of certain Naididæ.

Abstract

The common forked setæ which constitute the ventral setal bundles in the Naididæ possess, like the simple setæ of many of the earthworms, a slight swelling on the shaft, the so-called nodulus. The setæ themselves are the principal organs of locomotion, being used as levers, which obtain a hold of the substratum by means of their hooked and forked distal ends; and besides being movable in an anteroposterior direction they are capable of some degree of protrusion and retraction. The position of the nodulus on the shaft is held to regulate the distance to which the seta can be protruded; though it can be retracted so that the nodulus is below the level of the epidermis, it cannot be protruded beyond that degree which brings the nodulus level with the surface.