396 THE ORIGIN OF VERTEBRATES 
name, the Protostraca, from which subsequently the Palostraca 
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 Chetopoda—the 
Polycheta. 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 polychete stock, but rather 
from members in which the arthropod characters had already become 
well developed—members, therefore, which were nearer the Trilobita 
than the Polycheta. Such early arthropods would very probably 
have retained in part excretory organs of the same character as those 
found in the original polychete 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 Claus 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 Claus, 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 solenocyte 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 
Polycheta, 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 
