488 MR. F. E. BEDDARD ON THE 
covering of the seta; in Plate XX VI. fig. 16, which represents an oblique section 
through the body wall in the neighbourhood of a seta, three of these bundles 
are seen in longitudinal section reaching from the cuticle, which surrounds 
the seta itself, to the base of the hypoderm layer; the others are cut trans- 
versely, and lie behind the seta. 
Clitellum.—The histological structure of this differentiated part of the 
integument will be best considered in this place in relation to the rest of the 
integument, while the description of the number of segments of which it is 
composed, and the positions of the apertures of the generative organs, will be 
deferred until the generative system is treated of. 
The clitellum has been carefully studied by CLaparEDE,* and more recently 
by Moussisovics,t in the common earthworm ; and since this is the only type in 
which, up till the present, the clitellum has been accurately described and 
figured, it will be as well to give briefly the results of these two observers, for 
the sake of comparing it with the clitellum of Pleurocheta, which is i some 
respects different. 
According to Mougstsovics, the clitellum of the earthworm differs from the 
rest of the integument (1) by the modification of the hypoderm cells, and (2) by 
the additional presence between the hypoderm and circular muscular coat of a 
glandular layer well supplied with blood-vessels. The hypoderm cells differ 
from those over the rest of the body by being much more granular; the 
glandular layer consists of several rows of flasked-shaped cells finely granular, 
and frequently possessing a. nucleus, imbedded in a network of connective 
tissue. These two layers differ chemically, as is shown by coloration with 
picro-carmine and logwood (see figs. 9, 11 of his memoir). The capillaries 
extend half way up the outer hypoderm layer. 
CLAPAREDE'’S account differs somewhat; he figures a hypoderm layer distinct 
from the subjacent glandular layer, which latter he subdivides into “obere ” 
and “untere Saiilenregion;” beneath this again a special vascular layer. 
Accordingly the “ hypoderm layer” of Mousisovics equals the hypoderm plus 
the “obere Saiilenregion” of CLaparEDE. Both observers agree that the 
glands are unicellular. MoJsisovics figures the capillaries extending half way 
up the hypoderm cells, leaving off just where, on CLAPAREDE’S interpretation, 
the hypoderm cells begin (compare figs. 9, 11 of Moustsovics, with plate xlvi. 
fig. 1 of CLAPAREDE’S memoir), and this makes one almost suspect that the 
earlier idea of CLAPAREDE may be after all the right one. 
In Pleurocheta the glandular layer of the clitellum appears to be distinct 
from the hypoderm layer which covers it; there is a band of fibrous tissue 
dividing the glandular layer from the hypodermic ; this band is readily seen 
* CLAPAREDE, loc. cit. 
+ Mosstsovics, “ Kleine beitriige zur Kenntniss der Anneliden,” Sitz. Acad. Wissensch., 1877. 
