39 6 THE ORIGIN OF VERTEBRATES 



name, the Protostraca, from which subsequently the Palasostraca 



arose. 



The similarity between the excretory organs of Amphioxus and 

 those of Phyllodoce suggests that the protostracan ancestor of the 

 vertebrates arose from the highest group of the Chaetopoda — the 

 Polychseta. The evidence which I have already given points, how- 

 ever, strongly to the conclusion that the vertebrate did not arise from 

 members of the Protostraca near to the polychtete stock, but rather 

 from members in which the arthropod characters had already become 

 well developed— members, therefore, which were nearer the Trilobita 

 than the Polychreta. Such early arthropods would very probably 

 have retained in part excretory organs of the same character as those 

 found in the original polychrete stock, and thus account for the 

 presence of solenocytes in the excretory organs of Amphioxus. 



In connection with such a possibility, I should like to draw 

 attention to the observations of Glaus and Spangenberg on the 

 excretory organs of Branchipus— that primitive phyllopod, which is 

 recognized as the nearest approach to the trilobites at present living. 

 According to Glaus, an excretory apparatus exists in the neighbour- 

 hood of each nerve-ganglion, and Spangenberg finds a perfectly 

 similar organ in the basal segment of each appendage — a system, 

 therefore, of excretory organs as segmentally arranged as those of 

 Peripatus. Claus considers that although these organs formed an 

 excretory system, it is not possible to compare them with the 

 annelid segmental organs, because he thought the cells in question 

 arose from ectoderm. Now, the striking point in the description of 

 the excretory cells in these organs, as described both by Claus and 

 Spangenberg, is that they closely resemble the pipe-cells or sole- 

 nocytes of Goodrich ; each cell possesses a long tube-like projection, 

 which opens on the surface. They appear distinctly to belong to the 

 category of flame-cells, and resemble solenocytes more than anything 

 else. According to Goodrich, the solenocy te is probably an ectodermal 

 cell, so that even if it prove to be the case, as Claus thought, that 

 these pipe-cells of Branchipus are ectodermal, they would still claim 

 to be derived from the segmental organs of annelids, especially of the 

 Polychaeta, being, to use Goodrich's nomenclature, true nephridial 

 organs, as opposed to ccelomostomes. 



These observations of Claus and Spangenberg suggest not only 

 that the primitive arthropod of the trilobite type possessed segmental 



