EXCRETORY ORGANS. 



685 



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 without 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 communication 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 continu- 

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

 the preceding segment, and the subsequent separation of this 

 part as a separate tube. 



The tube may be continued with a sinuous course through 



A 



Y 

 Y 

 Y 



Y 



B CD 



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 acquired. The internal openings communicate with 

 the permanent body-cavity. The development of the perma- 



