4o8 



COMPARATIVE ANATOMY 

 FLATWORM. 



NEMERTEAN. 



ANNELID. 



VERTEBRATE EMBRYO. 



PRIMITIVE VERTEBRATE. 



Fig. 336. — Diagram illustrating the hypothetical evolution of the urogenital system 

 of vertebrates, beginning with the gonadic sacs of fiatworms. The diagram assumes 

 that the gonadic sacs of flatworms have become the coelomic pouches of chordate 

 embryos. The latter in turn unite to form the extended coelom of vertebrates. Coe- 

 lomoducts appear first in annelids and become in vertebrates the renal tubules. Verte- 

 brates "invent" a longitudinal (primitive) excretory duct. During phylogenesis the 

 region of proliferation of germ-cells becomes greatly restricted. 



