ANATOMY AND HISTOLOGY OF PLEUROCHATA MOSELEYI. 489 
in sections stained with picro-carmine, as it shows a rose tint, while the gland- 
ular tissue is stained of an orange-yellow ; so far there is a similarity with the 
clitellum of Zumbricus, as understood by CLAPAREDE ; but in Plewrochwta, the 
capillaries, which are in this part of the integument larger, and far more 
numerous than over the rest of the body, are continued into the hypodermic 
layer, instead of stopping short at the septum, as they do in the earthworm. 
Unfortunately, none of the cells of the hypodermic layer were in a fit condition 
for examination ; as in the case of the rest of the integument, nothing was left 
to indicate the form of the cells, and their relations to the capillaries. The 
glandular layer is divided up into columns by trabecule of connective tissue, 
running down to join a stouter fibrous septum, which separates off the gland- 
ular layer from the circular muscular coat. This septum (Plate X XVI. fig. 7) 
is perforated here and there for the transmission of blood-vessels. Between the 
trabecule lie the glandular cells, which do not seem to have any definite 
arrangement ; they are of various sizes, occasionally provided with a nucleus at 
their lower end, and flask-shaped, the “neck” of the “flask” being produced 
upwards as the duct of the unicellular gland. The appearances displayed in 
fig. 17 do not mean that the glands are multicellular, they would seem rather 
to be the effect of reagents on a single cell. But though, as in Lumbricus, the 
glands of the clitellum are unicellular, there are here and there indications of the 
combination of the gland cells into veritable glands. Plate X XVI. fig. 3 shows 
this; here in a definite region the gland cells are massed together, forming a 
more or less oval-shaped body, while on either side the cells continue isolated 
from each other; but there is no common duct. Each cell is, as in the rest of 
the clitellum, as far as can be made out, provided with its own duct; this, 
however, is the first step in the formation of a specialised compound gland. 
These glands can be recognised on the clitellum with the naked eye as 
yellowish spots and patches. Plate X XVI. fig. 6 is a general view of a section 
through the clitellum; at a are the remains of the hypoderm cells, and 
the more or less distinct fibrous band separating the hypoderm from the 
subjacent glandular layer ; in this latter lie the cells 6 of various sizes, arranged 
in columns by the trabecule of connective tissue c. Beneath these are 
the two muscular layers. As in the rest of the body, the muscular fibres 
in the two coats are divided up into groups, by branching and anasto- 
mosing trabecule. The muscular fibres of the circular coat are occasionally 
found trespassing on the outer side of the septum, which separates the 
glandular from the muscular coat (see fig. 11). Seta are found on the 
clitellum, but their form is in nowise different from the sete found over 
the rest of the body. 
(4) Peritoneal Membrane.—This structure, which lines the peritoneal cavity, 
VOL. XXX. PART Il. 46 
