MORPHOLOGY OF ANIMALS 



embryos and adults of different classes of vertebrates is 

 shown in the accompanying table : 



Theories of Excretion. There are two conflicting theo- 

 ries as to the method of excretion, {a) The theory of 

 Heidenhain holds that the cells of the nephridial tubules 

 take urea out of the surrounding blood capillaries and pass 

 it into the tubules, while the fluid which comes through 

 the ciliated funnels or glomeruli merely flushes out the 

 tubules. Excretion on this theory consists in selection of 

 urea by the cells of the tubules, {b) The theory of Ludwig 

 maintains that fluids containing urea pass into the tubules 

 through the funnels or glomeruli and the cells of the 

 tubules then take up pure fluid from the tubules and pass 

 it back into the surrounding capillaries. The cells of the 

 tubule therefore merely concentrate the solution of urea 

 in the tubules. The latter theory is the more probable. 



Excretory Ducts as Sex Ducts. Finally, the nephridia 

 may carry off from the body-cavity not only coelomic 

 fluid, but also cells which are set free into this fluid; some 

 of these cells in the annelids may be loaded with urates 

 which are thus carried to the exterior (chlorogogue cells), 

 but the most important of the cells which thus escape from 

 the coelom are the sex cells, ova and spermatozoa. The 

 excretory ducts may be especially modified for carrying 

 off these sex cells, in which case they are known as gono- 

 ducts. Even among the vertebrates the oviducts and sper- 

 miducts are derived from the nephric system. In some 

 vertebrates the oviduct is split off in the embryo from the 

 segmental duct and opens into the body-cavity at its an- 

 terior end through a pronephric funnel (Fig. 21, C, /) ; its 



C 61 ] . 



