EXCRETORY ORGANS. 683 



been found by Hatschek and Joliet in the Entoproctous Polyzoa, 

 and are developed, according to Hatschek, by whom they were 

 first found in the larva, from the mesoblast. 



Brachiopoda. One or rarely two (Rhynchonella) pairs of 

 canals, with both peritoneal and external openings, are found in 

 the Brachiopoda. They undoubtedly serve as genital ducts, but 

 from their structure are clearly of the same nature as the 

 excretory organs of the Chaetopoda described below. Their 

 development has not been worked out. 



Chaetopoda. Two forms of excretory organ have been met 

 with in the Chaetopoda. The one form is universally or nearly 

 universally present in the adult, and typically consists of a pair 

 of coiled tubes repeated in every segment. Each tube has an 

 internal opening, placed as a rule in the segment in front of that 

 in which the greater part of the organ and the external opening 

 are situated. 



There are great variations in the structure of these organs, 

 which cannot be dealt with here. It may be noted however that 

 the internal opening may be absent, and that there may be 

 several internal openings for each organ (Polynoe). In the 

 Capitellidae moreover several pairs of excretory tubes have been 

 shewn by Eisig (No. 512) to be present in each of the posterior 

 segments. 



The second form of excretory organ has as yet only been 

 found in the larva of Polygordius, and will be more conveniently 

 dealt with in connection with the development of the excretory 

 system of this form. 



There is still considerable doubt as to the mode of formation 

 of the excretory tubes of the Chaetopoda. Kowalevsky (No. 277), 

 from his observations on the Oligochaeta, holds that they develop 

 as outgrowths of the epithelial layer covering the posterior side 

 of the dissepiments, and secondarily become connected with the 

 epidermis. 



Hatschek finds that in Criodrilus they arise from a continuous 

 linear thickening of the somatic mesoblast, immediately beneath 

 the epidermis, and dorsal to the ventral band of longitudinal 

 muscles. They break up into S-shaped cords, the anterior end 

 of each of which is situated in front of a dissepiment, and is 

 formed at first of a single large cell, while the posterior part is 



