THE REGION OF THE SPINAL CORD 417 



in Ammoccetes which I imagine corresponds to b, c in Amphioxus, 



and therefore would represent the pleural fold, is the part ventral to 

 the bend at b. In both the animals this bend corresponds to the 

 position of the notochord NC. 



The skeleton of Cephalaspis compares more directly with that of 

 Ammocoetes 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, b, c ; the part from b to c 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 Chsetopoda, 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 prosomatic 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. 



2 E 



