632 EXISTENCE OF A HEAD-KIDNEY 



III. 



General considerations. 



The excretory system of a typical Vertebrate consists of the 

 following parts: 



1. A head-kidney with the characters already described. 



2. A duct for the head-kidney the segmental duct. 



3. A posterior kidney (Wolffian body, permanent kidney, 

 &c. The nature and relation of these parts we leave out of con- 

 sideration, as they have no bearing upon our present investiga- 

 tions). The primitive duct for the Wolffian body is the segmental 

 duct. 



4. The segmental duct may become split into (a) a dorsal 

 or inner duct, which serves as ureter (in the widest sense of the 

 word); and (b) a ventral or outer duct, which has an opening 

 into the body-cavity, and serves as the generative duct for the 

 female, or for both sexes. 



These parts exhibit considerable variations both in their 

 structure and development, into some of which it is necessary 

 for us to enter. 



The head-kidney 1 attains to its highest development in the 

 Marsipobranchii (Myxine, Bdellostoma). It consists of a longi- 

 tudinal canal, from the ventral side of which numerous tubules 

 pass. These tubules, after considerable subdivision, open by a 

 large number of apertures into the pericardial cavity. From 

 the longitudinal canal a few dorsal diverticula, provided with 

 glomeruli, are given off. In the young the longitudinal canal is 

 continued into the segmental duct ; but this connection becomes 



1 I am inclined to give up the view I formerly expressed with reference to the 

 head-kidney and segmental duct, viz. "that they were to be regarded as the most 

 anterior segmental tube, the peritoneal opening of which had become divided, and 

 which had become prolonged backwards so as to serve as the duct for the posterior 

 segmental tubes," and provisionally to accept the Gegenbaur-Fiirbringer view which 

 has been fully worked out and ably argued for by Fiirbringer (loc. dt. p. 96). 

 According to this view the head-kidney and its duct are to be looked on as the pri- 

 mitive and unsegmented part of the excretory system, more or less similar to the 

 excretory system of many Trematodes and unsegmented Vermes. The segmental 

 tubes I regard as a truly segmental part of the excretory system acquired subse- 

 quently. F. M. B. 



