EXCRETORY ORGANS. 687 



their inner extremities ; and there is at first but a single funnel 

 for each vesicle. The mode of increase of the funnels has not 

 been observed, nor has it been made out how the organs them- 

 selves become attached to the hind-gut. 



The provisional excretory organ of Echiurus is developed at 

 an early larval stage, and is functional during the whole of 

 larval life. It at first forms a ciliated tube on each side, placed 

 in front of that part of the larva which becomes the trunk of the 

 adult. It opens to the exterior by a fine pore on the ventral 

 side, immediately in front of one of the mesoblastic bands, and 

 appears to be formed of perforated cells. It terminates inter- 

 nally in a slight swelling, which represents the normal internal 

 ciliated funnel. The primitively simple excretory organ becomes 

 eventually highly complex by the formation of numerous 

 branches, each ending in a slightly swollen extremity. These 

 branches, in the later larval stages, actually form a network, and 

 the inner end of each main branch divides into a bunch of fine 

 tubes. The whole organ resembles in many respects the excre- 

 tory organ of the Platyelminthes. 



In the larva of Bonellia Spengel has described a pair of 

 provisional excretory tubes, opening near the anterior end of 

 the body, which are probably homologous with the provisional 

 excretory organs of Echiurus (vide Vol. II., fig. 162 C, se). 



Discophora. As in many of the types already spoken of, 

 permanent and provisional excretory organs may be present in 

 the Discophora. The former are usually segmentally arranged, 

 and resemble in many respects the excretory tubes of the 

 Chaetopoda. They may either be provided with a peritoneal 

 funnel (Nephelis, Clepsine) or have no internal opening 

 (Hirudo). 



Bourne 1 has shewn that the cells surrounding the main duct 

 in the medicinal Leech are perforated by a very remarkable 

 network of ductules, and the structure of these organs in the 

 Leech is so peculiar that it is permissible to state with due reserve 

 their homology with the excretory organs of the Chaetopoda. 



The excretory tubes of Clepsine are held by Whitman to be 

 developed in the mesoblast. 



1 "On the Structure of the Nephridia of the Medicinal Leech." Quart. J. of 

 Micr. Science, Vol. XX. 1880. 



