It is for example impossible to attempt any account of the fitness of some of these animals for their terrestrial life and of others for an aquatic life without treating of anatomy to some extent. I shall take one particular species as a type and indicate later the principal divergencies shown by other forms. According to the general opinion among those who have studied the Oligochaeta I take as a representative form a Megascolecid (this and the other families are dealt with _seriatim_ on p. 14 et seq.), as this group is presumed to be the oldest, and within that group a representative of the genus _Notiodrilus_ which is with some reason held to be the most primitive genus in the group. Finally I have no particular reason for selecting the species _Notiodrilus tamajusi_ except that there happens to be a longer and fuller description of it than of many. [Illustration: Fig. 1. _Notiodrilus tamajusi._ The worm shown from the ventral surface. About natural size. (After Eisen.)] This earthworm is a native of Guatemala and is some six inches in length with a diameter of perhaps a quarter of an inch. The front part of the body is thicker than posteriorly. The body will be seen to be divided into some 218 rings by circular furrows which run right round the body. These divisions are termed segments or somites. At the head the mouth is surrounded by the first of these, and on the dorsal surface of that segment is a projection like an incomplete segment which is known as the prostomium. From the XIIIth segment to the middle of the XXth the body has a different appearance, and this region is known as the clitellum. Each of the 218 segments of the body except the first, and possibly one or two at the hinder end, is furnished with eight minute projecting bristles, the setae; these are disposed in pairs and all lie upon the ventral aspect of the worm. The movement of these by special muscles aids in locomotion. [Illustration: Fig. 2. The same species; first two segments and prostomium shown from ventral surface, I prostomium, II first segment without any setae, III second segment with its four pairs of setae (the dorsalmost seta of each outer pair is not visible in this view).] [Illustration: Fig. 3. A section through the body of the same species showing the ventral position of the pairs of setae. (After Eisen.)]