43° THE ORIGIN OF VERTEBRATES 



genital ducts, may be classed as modified nephridial glands, and that 

 therefore the thyroid gland of Ammocoetes, which, on the theory of 

 this book, arose in connection with the opercular genital ducts of the 

 paheostracan 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 nephrocele 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 nephrocele. 

 From this original nephroccele are formed the pronephric excretory 

 organs, the mesonephric excretory organs, and the body-cavity. 



