196 W. BLAXLAND BENHAM. 
ment of the fork, but not in the direction of the palmate 
type. 
The sigmoid form itself is not necessarily the primitive form 
of cheeta; it is possible that a straight spine, such as we find 
in the Enchytreids, is an earlier form, which may have 
given rise to the capilliform shape by elongation, and to the 
sigmoid form by curvature, and hence to these other forms. 
Amongst other external features which are of interest may 
be mentioned the distinct annulation of the anterior seg- 
ments of the body (fig. 3). This is distinctly visible during 
life, and can be detected in longitudinal sections. Each seg- 
ment is marked out by three grooves into four rings, of which 
the third is the largest, and on this the chete are embedded. 
The most anterior segments show only two of these grooves, 
and on the clitellum they are present only ventrally. This 
feature is mentioned by Claparéde, and is of more frequent 
occurence in aquatic Oligocheta, as in earthworms, than is 
usually supposed.’ It is indistinctly marked in some Naids. 
Claparéde mentions the fact in Tubifex and Limnodrilus. 
It is present, in fact, in the majority of this family, and is 
carried to an extreme in Branchiobdella. It is present in 
Hemitubifex, though rendered less noticeable and even 
almost obscured by the dark cuticular papille. In Hetero- 
cheta, however, it is so distinct that it is almost the readiest 
means of distinguishing, under a low power, this worm from 
other forms with which it occurs, for the characteristic chzetz 
are not visible if the animal presents its ventral surface 
upwards. 
I am unable to say what may be the “ Langsfirchen” in 
the skin, mentioned by Claparéde. I see no such grooves. 
His figure may represent the bundles of longitudinal muscles 
which can be seen during movement of the worm, and will at 
times be thrown into wavy lines such as his drawing represents. 
The shape of the prostomium is exhibited in my figs. 3 
1 For example, Mr. W. Hatchett Jackson, in the 2nd edition of ‘ Forms of 
Animal Life,’ states on p. 593 that “‘annulation of the somites is very 
rare.” 
