igoS.] EXCRETORY ORGANS OF METAZOA. 613 



almost entirely thus excluding the homology of the protonephridium 

 and metanephridium. 



It will be noted that in my considerations I have entirely ex- 

 cluded the argument from the side of the recapitulation theory, for 

 I have maintained (1906) that this theory is fundamentally errone- 

 ous. I have compared corresponding stages, adult or embryonic, 

 of the different groups, have stressed embryological resemblances, 

 but have not compared an adult stage of one organ with an em- 

 bryonic one of another. 



It might be expected that I should now enter upon the question 

 of the phylogenetic significance of the coelom, because this space 

 has so often a close anatomical connection with nephridia. But I 

 have nothing new to add to the discussion, and for a good repre- 

 sentation of it would refer to the treatments by E. Aleyer (1901) 

 and Lang (1903). I need only state that there are three main 

 theories in explanation of the origin of the coelom. The oldest was 

 founded by Sedgwick, and is to the effect that the coelom is an 

 enterocoelic diverticulum, referable to a gastral pocket of an 

 anthozoan. This has deservedly received little support. Xext came 

 the gonocoel theory, foreshadowed by Hatschek, elaborated partic- 

 ularly by Bergh and E. Meyer, and more recently supported by 

 Lang and Goodrich; it concludes that the coelomic sac of a higher 

 metazoan is the amplified derivative of the genital pouch (gonad) 

 of such a form as a Platode, therefore that the mesentoblast is 

 referable to germ cells The third view is the nephrocoel theory, 

 founded by Faussek (1901) and Ziegler (1898), that the coelom 

 was originally an excretory organ and that the germ cells have 

 associated themselves secondarily with it. Of these three theories 

 the gonocoel theory seems to me to receive the fullest support from 

 the facts of anatomy and embryology. 



LITERATURE LIST. 

 Ahting, K. 



1901. L'ntersuchungen iiber die Entwickehmg des Bojanusschen Organs und 

 des Herzens der Lamellibranchier. Jena. Zcit., 2^. 



Allen, E. J. 



1904. The Anatomy of Pcecilochaetus, Claparede. Otiart. Journ. Micr. Sci. 

 (2), 48. 



