430 THE ORIGIN OF VERTEBRATES 
genital ducts, may be classed as modified nephridial glands, and that 
therefore the thyroid gland of Ammoccetes, which, on the theory of 
this book, arose in connection with the opercular genital ducts of the 
paleeostracan ancestor, represents the coxal glands of this fused pair 
of appendages. Such a gland, although its function in connection 
with the genital organs had long disappeared, still, in virtue of its 
original excretory function, persisted, and even in the higher verte- 
brates, after it had lost all semblance of its former structure and 
become a ductless gland of an apparently rudimentary nature, still, 
by its excretory function, demonstrates its vital importance even to 
the highest vertebrate. 
By this simple explanation we see how these hitherto mysterious 
ductless glands, pituitary, thymus, tonsils, thyroid, are all accounted 
for, are all members of a common stock—coxal glands—which origi- 
nally, as in Peripatus, excreted at the base of the prosomatic and 
mesosomatic appendages, and are still retained because of the impor- 
tance of their excretory function, although ductless owing to the 
modification of their original appendages. 
Finally, there is yet another organ in the vertebrate which follows 
the same law of the conversion of an excretory organ into a lymphatic 
organ when its connection with the exterior is obliterated, and that is 
the vertebrate body-cavity itself. According to the scheme here put 
forth, the body-cavity of the vertebrate arose by the fusion of a 
ventral prolongation of the original nephroccele on each side; pro- 
longations which accompanied the formation of the new ventral mid- 
gut, and by their fusion formed originally a pair of cavities along the 
whole length of the abdomen, being separated from each other by the 
ventral mesentery of the gut. Subsequently, by the ventral fusion of 
these two cavities, the body-cavity of the adult vertebrate was formed. 
This is simply a statement of the known method of formation of 
the body-cavity in the embryo, and its phylogenetic explanation is 
that the body-cavity of the vertebrate must be looked upon as a 
ventral prolongation of the original ancestral body-cavity. Embryo- 
logy clearly teaches that the original body-cavity or somite was 
confined to the region of the notochord and central nervous system, 
and there, just as in Peripatus, was divisible into a dorsal part, giving 
origin to the myoccele, and a ventral part, forming the nephroccele. 
From this original nephrocele are formed the pronephric excretory 
organs, the mesonephric excretory organs, and the body-cavity. 
