No. 3.] THE EMBRYOLOGY OF THE EARTHWORM. 407 
they are arranged in two layers which still further forward are 
seen to be the somatic and splanchnic layers, the coelom appear- 
ing between them. Posteriorly the cells of these layers have 
a pyramidal form and dove-tail together with great regularity. 
Further forward, as they separate, the narrow inner overlapping 
ends of the cells remain in contact, the coelomic cavities appear- 
ing as spaces between them, so that each cell forms part of a 
dissepiment. The number of cells rapidly increases, however, 
as the coelomic cavities enlarge, so that the dissepiments, which 
are at first in contact above and below, are separated by the 
somatic and splanchnic epithelium, and also become two-layered. 
The cavities of the two bands are at first wholly separate, 
since they make their appearance some distance behind the point 
of concrescence, as shown in Fig. 69. As the bands grow to- 
gether the corresponding cavities of the two sides are opposed to 
one another and finally fuse, at first incompletely on the ventral 
side where a ventral mesentery persists, afterwards completely 
on the dorsal side, so as completely to surround the archenteron. 
The most anterior pair of cavities (see Fig. 50) lie at the 
sides of the stomodzum, bounded in front by a mesoblastic 
wall which at first is horizontal, but subsequently becomes 
oblique and finally vertical. These cavities give rise to the coclom 
of the first post-oral somite, in which nephridia never develop. 
The following pair of cavities give rise to the second somite, 
which has no nephridia of its own, but receives the funnels of 
the following or third somite. The first pair of setigerous 
glands are developed in the second somite. 
The head-cavity appears in the middle line above the stomo- 
dzeum at about the same period as the anterior pair of trunk- 
cavities, directly between which it at first lies. Series of sec- 
tions in all planes show most clearly the unpaired character of 
the head-cavity from its first appearance onward. Its walls are 
bounded by a distinct layer of mesoblastic cells, and its cavity 
is traversed by a loose network of branching contractile cells. 
On the dorsal side these cells are continuous posteriorly with 
the migratory mesoblast of the trunk, and the head-cavity itself 
appears to be continuous with the lacunz between these cells. 
In later stages, however, as the trunk-mesoblast extends around 
the dorsal side of the archenteron, the head-cavity acquires a 
definite hinder limit, formed by the dorsal union of the first pair 
