346 ZOOLOGY. 



circulatory and excretory organs, and of a central nervous 

 system, all show this. With such a lowly Vertebrate as 

 Amphioxus, the earthworm shows some resemblances such 

 as the general metamerism, and the structure of the 

 nephridia ; but the differences are many and great. There 

 are no gill-slits, no notochord, the nerve-cord is ventral and 

 solid, and the anterior end of the central nervous system 

 encircles the mouth- region ; the circulation is different. 



10. Former Theory of the Nephridia. The older view 

 concerning nephridia and genital ducts was that they were 

 homologous structures, which could be recognised in at 

 least the majority of the phyla or sub-kingdoms of animals. 

 According to this view, largely due to the influence of 

 Prof. E. Eay Lankester, generative and excretory tubes 

 were all modifications of one type, one of whose primary 

 characteristics was that it had an opening into the coelom, 

 or a nephrostome. The excretory tubules of Anodon and 

 other mollusca, the pronephric and mesonephric tubules of 

 vertebrates, the excretory tubules of Amphioxus, and the 

 nephridia of the earthworm were all regarded as homolo- 

 gous structures, and this is the interpretation of them 

 which was given in earlier editions of this book and all 

 other zoological literature until recently. 



11. Recent Researches. This theory, however, has 

 been much modified by the results of recent investigations, 

 especially by those of Mr. GToodrich. Embryologists 

 had long ago found that the mode of development of 

 structures regarded as nephridia was different in different 

 cases the tubules in some arising from epiblast, in others 

 from mesoblast ; in others partly from one layer, partly 

 from the other. The most important factor, however, in 

 the development of the new theory was the discovery by 

 Mr. Goodrich that in several Polychaetes the nephridia did 

 not open into the coelom at all, but that their internal 

 extremities were blind, and that connected with these blind 

 extremities were remarkable cells having a structure similar 

 to that of the flame-cells of the Flat-worms, such as the 

 liver-fluke. These peculiar cells are connected with 



