The Naididæ are a family of small fresh-water Oligochæta, in which the usual mode of reproduction is the asexual, by fission; sexual forms are, in general, rare, and in the majority of species, and in some whole genera (e.g. Slavina), they have not so far been observed at all. Records of the genital organs are therefore of interest; and if they were available throughout the group it can hardly be doubted that we should be able to judge better of the affinities of genera and species, and consequently to improve our classification; since the diagnoses of species and genera, and the scheme of classification, depend at present to an unduly large extent on one single set of characters, the form and distribution of the setæ.

The study of sexual individuals of the genus Dero has resulted also in the discovery of the curious phenomenon of the atrophy of the alimentary canal at sexual maturity; this occurs also in Hæmonais (v. ant.); it has not hitherto been found elsewhere in the Oligochæta, though it is known to occur (apparently in a less extreme form) in a number of Polychætes.