THE REGION OF THE SPINAL CORD 395 



with the formation of the segmental duct, does constitute a true 

 morphological distinction between the two sets of organs. 



4. All the recent observers are in agreement that the vertebrate 

 excretory organs strongly indicate a derivation from, the segmental 

 organs of annelids. 



The very strongest support has been given to this last conclusion 

 by the recent discoveries of Boveri and Goodrich upon the excretory 

 organs of Amphioxus. According to Boveri, the nephric tubules of 

 Amphioxus open into the dorsal ccelom by one or more funnels. 

 Around each funnel are situated groups of peculiar cells, called by 

 him ' Fadenzellen,' each of which sends a long process across the 

 opening of the funnel. Goodrich has examined these ' Fadenzellen,' 

 and found that they are typical pipe- cells, or solenocytes, such as he 

 has described in the nephridial organs of various members of the 

 aunelid group Polychseta. Also, just as in the Polyclnrta, the ciliated 

 nephric tubule has no internal funnel-shaped opening into the ccelom, 

 but terminates in these groups of solenocytes. " Each solenocyte 

 consists of a cell-body and nucleus situated at the distal free 

 extremity of a delicate tube ; the proximal end of the tube pierces 

 the wall of the nephridial canal and opens into its lumen. A single 

 long flagellum arising from the cells works in the tube and projects 

 into the canal." 



The exceedingly close resemblance between the organs of 

 Amphioxus and those of Phyllodoce, as given in his paper, is most 

 striking, and, as he says, leads to the conclusion that the excretory 

 organs of Amphioxus are essentially identical with the nephridia of 

 certain polychsete worms. 



It is to me most interesting to find that the very group of 

 annelids, the Polychteta, which possess solenocytes so remarkably 

 resembling those of the excretory organs of Amphioxus, are the 

 highest and most developed of all the Annelida. I have argued 

 throughout that the law of evolution consists in the origination of 

 successive forms from the dominant group then alive, dominance 

 signifying the highest type of brain-power achieved up to that time. 

 The highest type among Annelida is found in the Chretopoda ; from 

 them, therefore, the original arthropod type must have sprung. 

 This original group of Arthropoda gave rise to the two groups of 

 Crustacea and Arachnida, in my opinion also to the Vertebrata, 

 and, as already mentioned, it is convenient to give it a generalized 



