^9$ THE ORIGIN OF VERTEBRATES 



v) 



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 myoccele. The muscles of the appendages are formed from 

 the ventral part of the original procoelom, just as I have argued 

 is the case with the muscles of the splanchnic segmentation in 

 vertebrates. 



Sedgwick 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 (i.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 (e.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 the 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 



