400 THE ORIGIN OF VERTEBRATES 
decides, it is true, in favour of the internal vesicle, and therefore 
considers the excretory organ to be appendicular, ¢.¢. a coxal gland, in 
these segments as well as in those more posterior. Still, the very 
discussion shows that in his opinion, at all events, the external 
vesicle might represent the end-sac of the tubule, in the absence of 
the internal or appendicular vesicle, 
Such an arrangement as Sedgwick describes in Peripatus is the 
very condition required to give rise to the pronephric and meso- 
nephric tubules, as deduced by me from the consideration of the 
vertebrate, and harmonizes and clears up the controversy about the 
mesonephros and pronephros in the most satisfactory manner. Both 
pronephros and mesonephros are seen to be derivatives of the original 
annelid segmental organs, not directly from an annelid, but by way 
of an arthropodan ancestor; the difference between the two is 
simply that the pronephric organs were coxal glands, and indi- 
cate, therefore, the presence of the original metasomatic appendages, 
while the mesonephric organs were homologous organs, formed in 
segments of later origin which had lost their appendages. For this 
reason the pronephros is said to be formed, in part at least, from 
a portion of the ccelom situated more ventrally than the purely 
somatic part which gives rise to the mesonephros. For this reason 
Sedgwick, Brauer, etc., can say that the mesonephros is strictly homo- 
dynamous with the pronephros; while equally Rickert, Semon, and van 
Wijhe can say it is not homodynamous, in so far that the two organs 
are not derived strictly from absolutely homologous parts of the ccelom. 
For this reason Semon can speak of the mesonephros as a dorsal 
derivative of the pronephros, just as Sedgwick says that the external 
or somatic vesicle of Peripatus is a derivative of the appendicular 
nephric organ. For this reason the pronephros, or rather a part of it, 
is always derived from the somatopleuric layer, for, as is clear from 
Miss Sheldon’s drawing, the part of the ccelom in Peripatus which 
dips into the appendage is derived from the somatopleuric layer 
alone. 
Such a ccelom as that of Peripatus, Fig. 157, would represent the 
origin of the vertebrate ccelom, and would therefore represent the 
proccelom of van Wijhe. In strict accordance with this, we see that it 
separates into a dorsal part, the walls of which give origin to the 
somatic muscles, or at all events to the great longitudinal dorsal 
“muscles of the animal, and a ventral part, which forms a nephroccele, 
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