ANATOMY AND HISTOLOGY OF PLEUROCH ATA MOSELEYIT. 501 
segment from the nerve cord are almost entirely derived from the 
central commissural masses, but some take their origin from other parts of the 
ganglion. 
Generative System. 
The genital apparatus of Plewrochwia is manifested externally by the 
clitellum, by the apertures of the spermathece, and by four other pairs of 
apertures, of which three lie in a hollow sucker-like structure just at the 
posterior end of the clitellum, and the remaining pair still within the clitellum, 
but opening more anteriorly in the 13th segment. 
The clitellum itself is a little difficult to map, but I have considered that 
all those segments form part of it upon which any glandular development is 
visible with the naked eye. Counting in this way, one specimen showed a 
clitellum consisting of seven, possibly eight segments, but in the last segment 
the glandular development was very slight, and the sete were as numerous as 
in any of the posterior rings not belonging to the clitellum. In the other 
specimen the clitellum was far more strongly developed, occupying apparently 
nine segments, in the first and last of which a complete series of sete were 
present. In the segments forming the clitellum, with the exceptions just 
mentioned, the development of setze is very slight, and entirely confined to a 
small tract on either side of the median ventral region, where, as in the rest 
of the body, no sete are developed. Among the Oligochwta some forms are 
provided with setz on the clitellum, and some are not; in the species of 
Pericheta described by Horst the clitellum is marked by an absolute lack 
of setee, but in Perichwta afinis described by Perrier, ‘ on distingue parfois 
nettement le circle des soies caracteristique des Pericheeta.” The clitellum 
commences after the 12th ring, and its segments can be counted either by the 
mesenteries or by the lines of setz which, as just described, exist on the 
clitellum of this animal; the last of the specially-thickened mesenteries forms 
its anterior boundary. Of the intimate structure an account has already been 
given, under the description of the body wall. 
At the posterior end of the clitellum in the median ventral line is a hollowed 
out area (Plate X XV. fig. 9), upon which no glandular development has taken 
place ; it is divided into four cavities, by two ridges running at right angles to 
each other ; the transverse ridge bears upon the end nearest to the clitellum, 
on either side, an aperture which is continuous with the duct of a solid white 
gland (Plate X XV. fig. 7), occupying the 18th segment. Each of the four cavities 
or hollows formed by the two ridges bears another aperture; the anterior 
pair open into the 17th segment, and the posterior into the 19th. These 
apertures were not directly visible from the interior, being apparently covered 
