EXCRETORY ORGANS. 



565 



When the posterior region of the embryo becomes segmented, 

 paired excretory organs are formed in each of the posterior segments, 

 but the account of their development, as given by Hatschek, is so 

 remarkable that I do not think it can be definitely accepted with- 

 out further confirmation. 



From the point of junction of the two main branches of the 

 larval kidney there grows backwards (fig. 384 B), to the hind end 

 of the first segment, a very delicate tube, only indicated by its 

 ciliated lumen, its walls not being differentiated. Near the front 

 end of this tube a funnel, leading into the larval body cavity of 

 the head, is formed, and subsequently the posterior end of the tube 

 acquires an external opening, and the tube distinct walls. The com- 

 munication with the provisional excretory organ is then lost, and 

 thus the excretory tube of the first segment is established. 



The excretory tubes in the second and succeeding segments 

 are formed in the same way as in the first, i.e. by the continuation of 

 the lumen of the hind end of the excretory tube from the preceding 

 segment, and the subsecjuent separation of this part as a separate 

 tube. 



The tube may be continued with a sinuous course through 



J^ ^ -p^ -l^ 







J) 



Fig. 384. 



Diagram illustrating the development of the excretory system 

 OF PoLYGORDius. (After Hatschek.) 



several segments without a distinct wall. The external and internal 

 openings of the permanent excretory tubes are thus secondarily ac- 

 quired. The internal openings communicate with the permanent 

 body cavity. The development of the permanent excretory tubes is 

 diagram matically represented in fig. 384 C and D. 



The provisional excretory organ atrophies during larval life. 



If Hatschek's account of the flevelopment of the excretory system of 

 Polygordius is correct, it is clear tliat important secondary modilications 

 must have taken place in it, because his description implies that there 

 sprouts from the anterior excretory organ, while it has its own external open- 

 ing, a posterior duct, which does not communicate either with the exterior 



