398 THE ORIGIN OF VERTEBRATES 
splits into a dorsal and a ventral part. In the anterior segments of 
the body the dorsal part disappears (presumably its walls give origin 
to the mesoblast from which the dorsal body-muscles arise), while 
the ventral part remains and forms a nephroccele, giving origin to the 
excretory organs of the adult. According to von Kennel, the cavity 
becomes divided into three spaces, which for a time are in com- 
munication—a lateral (I.), a median (II.), and a dorso-median (III.). 
The dorso-median portion becomes partitioned off, and this, as well 
as the greater part of the lateral portion, which lies principally in 
the foot, is used up in providing elements for the formation of the 
body- and appendage-muscles respectively and the connective tissue. 
In Fig. 157 I reproduce von Kennel’s diagram of a section across 
a Peripatus embryo, in which I. represents the lateral appendicular 
part of the ccelom, II. the ventral somatic part, and III. the dorsal 
part which separates off from the ventral and lateral parts, and, as 
its walls give origin largely to the body-muscles, may be called 
the myoceele. The muscles of the appendages are formed from 
the ventral part of the original proccelom, just as I have argued 
is the case with the muscles of the splanchnic segmentation in 
vertebrates. 
Sedewick states that the ventral part of the ccelom extends 
into the base of each appendage, and there forms the end-sac of 
each nephric tubule, into which the nephric funnel opens, thus 
forming a coxal gland; this end-sac or vesicle in the appendage 
is called by him the internal vesicle (t.v.), because later another 
vesicle is formed from the ventral ccelom in the body itself, close 
against the nerve-cord on each side, which he calls the external 
vesicle (¢.v.). (Cf. Fig. 158, taken from Sedgwick.) This second 
vesicle is, according to him, formed later in the development from 
the nephric tubule of the internal vesicle, so that it discharges 
its contents to the exterior by the same opening as the original 
tubule. Of course, as he points out, the whole system of internal 
and external vesicles and nephric tubules are all simply derivatives 
of the original ventral part of thé ccelom or nephroccele. 
Here, then, in Peripatus, and presumably, therefore, in members 
of the Protostraca, we see that the original segmental organs of the 
annelid have become a series of nephric organs, which extended into 
the base of the appendages, and may therefore be called coxal glands ; 
also it is clear, from Sedgwick’s description, that if the appendages 
