THE REGION OF THE SPINAL CORD 397 



organs in every segment directly derived from those of a polyctuete 

 ancestor, but also that such organs were partly somatic and partly 

 appendicular in position. Such a suggestion is in strict accord with 

 the observations of Sedgwick on the excretory organs of the most primi- 

 tive arthropod known, viz. Peripatus, where also the excretory organs, 

 which are true segmental organs, are partly somatic and partly 

 appendicular. Further, the excretory organs of the Scorpion and 

 Limulus group are again partly somatic and partly appendicular, 

 receiving the name of coxal glands, because there is a ventral projec- 

 tion of the gland into the coxa of the corresponding appendage. 



Judging from all the evidence available, it is probable that when 

 the arthropod stock arose from the annelids, simultaneously with the 

 formation of appendages, the segmental somatic nephric organs of 

 the latter extended ventrally into the appendage, and thus formed 

 a segmental set of excretory organs, which were partly somatic, 

 partly appendicular in position, and might therefore be called coxal 

 glands. 



As already stated, all investigators of the origin of the vertebrate 

 excretory organs are unanimous in considering them to be derived 

 from segmental organs of the annelid type. I naturally agree with 

 them, but, in accordance with my theory, would substitute the words 

 " primitive arthropod " for the word " annelid," for all the evi- 

 dence I have accumulated in the preceding chapters points directly 

 to that conclusion. Further, the most primitive of the three sets 

 of vertebrate segmental organs — the pronephros, mesonephros, and 

 metanephros — is undoubtedly the pronephros ; consequently the 

 pronephric tubules are those which I consider to be more directly 

 derived from the coxal glands of the primitive arthropod ancestor. 

 Such a derivation appears to me to afford an explanation of the diffi- 

 culties connected with the origin of the pronephros and mesonephros 

 respectively, which is more satisfactory than that given by the direct 

 derivation from the annelid. 



The only living animal which we know of as at all approaching 

 the most primitive arthropod type is, as pointed out by Korschelt and 

 Heider, Peripatus ; and Peripatus, as is well known, possesses a true 

 ccelom and true ccelomic excretory organs in all the segments of the 

 body. Sedgwick shows that at first a true ccelom, as typical as that 

 of the annelids, is formed in each segment of the body, and that then 

 this ccelom (which represents in the vertebrate van Wijhe's pro-ccelom) 



