THE REGION OF THE SPINAL CORD 417 
in Ammoccetes which I imagine corresponds to 0, ¢ in Amphioxus, 
and therefore would represent the pleural fold, is the part ventral to 
the bend at & In both the animals this bend corresponds to the 
position of the notochord NC. 
The skeleton of Cephalaspis compares more directly with that of 
Ammoccetes than that of Amphioxus, for there is the same extra 
dorsal bend (Fig. 161, a, d) as in Ammoccetes; the lateral part of the 
skeleton again gives an angle a, 6,¢; the part from b to ¢ would 
therefore represent the pleural fold. I picture to myself the sequence 
of events somewhat as follows :— 
First, a protostracan ancestor, which, like Peripatus, possessed 
appendages on every segment into which ccelomic diverticula passed, 
forming a system of coxal glands; such glands, being derived from 
the segmental organs of the Chetopoda, discharged originally to the 
exterior by separate openings on each segment. It is, however, 
possible, and I think probable, that-a fusion of these separate ducts 
had already taken place in the protostracan stage, so that there was 
only one external opening for the whole of these metasomatic coxal 
glands, just as there is only one external opening for the correspond- 
ing prosomiatic coxal glands of Limulus. Then, by the ventral growth 
of pleural body-folds, such appendages became enclosed and useless, 
and the coxal glands of the post-branchial segments, with their 
segmental or pronephric duct, were all that remained as evidence of 
such appendages. This dwindling of the metasomatic appendages 
was accompanied by the getting-rid of free appendages generally, in 
the manner already set forth, with the result that a smooth fish-like 
body-surface was formed; then the necessity of increasing mobility 
brought about elongation by the addition of segments between those 
last formed and the cloacal region. Each of such new-formed 
segments was appendageless, so that its segmental organ was not a 
coxal gland, but entirely somatic in position, and formed, therefore, a 
mesonephric tubule, not a pronephric one. Such glands could no 
longer excrete to the exterior, owing to the enclosing shell of the 
pleural folds; but the pronephric duct was there, already formed, 
and so these nephric tubules opened into that, instead of, as in the 
case of the branchial slits, forcing their way through the pleural 
walls when the atrium became closed. 
