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 
annelid group Polycheta. Also, just as in the Polycheta, the ciliated 
nephric tubule has no internal funnel-shaped opening into the celom, 
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 polychete worms. 
It is to me most interesting to find that the very group of 
annelids, the Polychzta, 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.Chetopoda; 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 
