The next family to the Aeolosomatidae in zoological position is clearly the Naididae. These are also small worms, but show in some respects a higher grade of organisation than their allies. While asexual generation is general, the reproductive organs are more commonly found than in _Aeolosoma_, though there are still many hiatus in our knowledge of the same in certain genera. Where they are known it has been found that the spermaries and ovaries are very far forward in the body, in the fifth and sixth segments respectively. The spermathecae are in segment six and the male ducts open into a terminal chamber, called 'atrium,' which is on the whole not unlike that of the Tubificidae. The blood in these worms is red as in the higher types, and thus differs from that of the genus _Aeolosoma_. The setae are rather varied, being in some cases long and slender, sometimes greatly exaggerated in length as in _Ripistes_; other setae are forked at the free end, and in _Paranais_ this is the only kind of setae met with. A marked feature of this family is that the dorsal bundles of setae do not always begin like the ventral setae upon the second segment of the body. Indeed in _Schmardaella_ there are no bundles of dorsal setae at all. The Indian genus _Branchiodrilus_ is remarkable for the fact that it has paired processes of the body which may be termed gills and which in some segments involve the dorsal setae. Another kind of gill is found in the genus _Dero_ (which has many species) and in the allied _Aulophorus_. These are placed round the vent, and are ciliated. Other genera are _Nais_, _Chaetogaster_, _Vejdovskyella_, _Amphichaeta_, _Stylaria_, _Macrochaetina_, _Pristina_, _Naidium_. Several genera, _Pristina_, _Nais_, _Dero_, are found in many parts of the world; but it is not possible at present to consider very seriously the facts of their geographical distribution. Next to the Naids a group of aquatic worms present themselves for consideration which are usually placed in three distinct families, which families are however rather hard to define. These three families are the _Tubificidae_, _Phreodrilidae_, and _Lumbriculidae_. The Phreodrilidae were at one time placed with the Tubificidae by Michaelsen. It is now perhaps the general opinion that they form a family of their own, at any rate since the discovery of two other genera _Phreodriloides_ and _Astacopsidrilus_, besides the original genus founded by myself, and named _Phreodrilus_ from the fact that the species was found in a deep well in New Zealand. The distribution of this family especially of the genus _Phreodrilus_ is extremely interesting. The genus _Phreodrilus_ is the only one genus of the aquatic Oligochaeta which has, like _Notiodrilus_, a circumpolar range, the pole being the south pole. It occurs in New Zealand, in Kerguelen, and, if we are to accept the opinions of Drs Michaelsen and Benham that my genus _Hesperodrilus_ is to be merged in _Phreodrilus_, in Patagonia also.