THE REGION OF THE SPINAL CORD 40 



5 



along the whole length of the body, and were supplanted by the 

 mesonephric tubules, which also belonged to the same segments. 



I also think that the paired appendages which have left the pro- 

 nephric tubules as signs of their past existence, existed originally, in 

 the invertebrate stage, on every segment of the body. But I do not 

 consider that such a statement is at all equivalent to saying that such 

 pairs of tubules must have existed upon every one of the segments 

 existing at the present clay ; for it seems to me that Eiickert is much 

 more likely to be right when he says that in Selachians the duct 

 clearly does grow back, and is not formed throughout in situ ,• so that 

 he gives a double explanation of the formation of the duct — a palin- 

 genetic anterior part formed by the fusion of the extremities of the 

 original excretory tubules, to which a posterior ccenogenetic lengthen- 

 ing has been added. 



It does not seem to me at all necessary that the immediate inver- 

 tebrate ancestor of the vertebrate should have possessed excretory 

 organs which opened out separately to the exterior on each segment ; 

 a fusion may already have taken place in the invertebrate stage, and 

 so a single duct have been acquired for a number of organs. Such a 

 suggestion has been made by Eiickert, because of the fact discovered 

 by Cunningham and E. Meyer, that the segmental organs of Lanice 

 conchilega are on each side connected together by a single strong 

 longitudinal canal. I would, however, go further than this and say, 

 that even although the nephric organs of the polychsete ancestor 

 opened out on every segment, and although the primitive arthropodan 

 ancestor derived from such polychaite possessed coxal glands which 

 opened out either on to or at the base of each appendage, similarly to 

 those of Peripatus, yet the immediate arthropodan ancestor, with its 

 palseostracan affinities, may already have possessed metasomatic coxal 

 glands, all of which opened into a single duct, with a single opening 

 to the exterior. 



Judging from Limulus, such was very probably the case, for 

 Patten and Hazen have shown (1) that the coxal glands of Limulus 

 are segmental organs belonging to the prosomatic segments ; (2) that 

 the organs belonging to the cheliceral and ectognathal segments 

 are not developed ; (3) that the four glands belonging to the endo- 

 gnaths become connected together by a stolon, which communicates 

 with a single nephric duct, opening to the exterior on the basal 

 segment of the 5th prosomatic appendage (the last endognath). At 



