FAM. 5. _Naidomorpha._[434]—This family contains eight or nine genera, perhaps more; they are all of them aquatic and of small size, and they multiply by fission as well as sexually. The most noticeable peculiarity of the family is the "cephalisation" which occurs in the head segments. In some genera, in _Pristina_ for example, there is no such cephalisation to be observed; but in others the dorsal bundles of chaetae commence a few segments farther back than the ventral, the segment where they commence being different and characteristic in the various genera. Thus in _Dero_ the first four segments are without dorsal chaetae, and in _Nais_ the first five are in this condition. There is thus a kind of "head" formed, whence the expression "cephalisation." _Dero_, _Nais_, and _Pristina_ are commonly to be met with in ponds, lakes, etc., in this country. _Bohemilla_ is rarer, and is to be distinguished by the remarkable serrated chaetae of the dorsal bundles. Of _Dero_ and _Nais_ there are a considerable number of different species; indeed it is usual perhaps to regard as distributable among three genera, _Nais_, _Stylaria_, and _Slavina_, the species which I am disposed to place in one genus, _Nais_. _Stylaria_ is defined on this view by its extremely long prostomium, which has given rise to both its popular and technical names. "Die gezungelte Naide" was the term applied by one of its earliest investigators, and the name _Stylaria proboscidea_ signifies the same peculiarity. But as the same inordinately long "proboscis" occurs in the South American _Pristina proboscidea_, belonging to a genus of which the other member does not possess so well developed a prostomium, it seems too variable a character upon which to differentiate a genus. _Chaetogaster_ and {378}_Amphichaeta_ have been placed by some systematists in a separate family. The first named contains four species which are fairly common. It is one of those worms in which the chaetae are not exactly related to the segmentation of other organs, which moreover sometimes show an independence in their segmentation; thus there are more nerve ganglia in the anterior segments of the body than there are septa. FAM. 6. _Tubificidae._—The worms belonging to this family are of small size, and are all inhabitants of fresh or salt water, or the margins of pools and the sea. They differ from the last family in that asexual reproduction never occurs, and that the reproductive organs are situated rather farther back in the body. The male pores are upon segment 11, and the oviduct-pores upon the following segment. This family differs from the Lumbriculidae in the fact that there are only a single pair of sperm-ducts.