THE REGION OF THE SPINAL CORD 43 1 



That the vertebrate body-cavity was originally a nephroccele is 

 generally accepted, and its excretory function is shown by the fact 

 that it communicates with the exterior in all the lower vertebrates, 

 either through abdominal pores or by way of nephridial funnels. 

 Bles has shown how largely these two methods of communicating 

 with the exterior mutually exclude each other. In the higher verte- 

 brates both channels become closed, except in the case of the 

 Fallopian tubes, and thus, so to speak, the body-cavity becomes a 

 ductless gland, still, however, with an excretory function, but now, 

 as in all other cases, forming a part of the lymphatic rather than of 

 the true excretory system. 



Summary. 



The consideration of the formation of the vertebrate cranial region, as set 

 forth in previous chapters, indicates that the ancestor of the vertebrates was 

 not an arachnid purely or a crustacean purely, but possessed partly crustacean 

 and partly araclinid characters. In order to express this conclusion, I have 

 used the term Protostraca, invented by Korschelt and Heider, to indicate a 

 primitive arthropod group, from which both arachnids and crustaceans may be 

 supposed to have arisen, and have therefore stated that the vertebrate did not 

 arise directly from the annelids, but from the Protostraca. Such an origin 

 signifies that the origin of the excretory organs of the vertebrate must not 

 be looked for in the segmental organs of the annelid, but rather in such 

 modified annelid org-ans as would naturally exist in a primitive arthropod 

 group. The nature of such organs may be inferred, owing to the fortunate 

 circumstance that so primitive an arthropod as Peripatus still exists, and we 

 may conclude that the protostracan ancestor possessed in every segment a pair 

 of appendages and a pair of ccelomic cavities, which extended into the base of 

 these appendages. The ventral portion of each of these ccelomic cavities 

 separated off from the dorsal and formed a nephrocele, giving- origin to a 

 segmental excretory organ, which, seeing- that its end-vesicle was in the base 

 of the appendage, and seeing also the nature of the known arachnid and 

 crustacean excretory organs, may fitly be termed a coxal gland. This, then, 

 is the working hypothesis to explain the difficulties connected with the origin 

 of the pronephros and mesonephros — that the original segmental organs were 

 coxal glands, and therefore indicated the presence of appendages. This 

 hypothesis leads to the following conclusions : — 



1. The coxal glands belonging- to the post-branchial appendag-es of the 

 invertebrate ancestor are represented by the pronephric tubules, and existed 

 over the whole metasomatic region. 



1. Such glands discharged into a common duct — the pronephric duct — 

 which opened into the cloacal region, either in the protostracan stage, when 

 the metasomatic appendages were still in existence, just as the coxal glands 

 of the prosomatic region in Limulus discharge into a common duct, or else the 

 pronephric duct was formed when the appendages were obliterated. 



