564 CHACTOrODA. 



There is still considerable doubt as to the mode of formation of 

 the excretory tubes of the Chyetopoda. 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 epi- 

 dermis. 



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 continued into the seg- 

 ment behind. The cords are covered by a peritoneal lining, which 

 still envelopes them, when in the succeeding stage they are carried 

 into the body-cavity. They subsequently become hollow, and their 

 hinder ends acquire openings to the exterior. The formation of 

 their internal openings has not been followed. 



Kleinenberg is inclined to believe that the excretory tubes take 

 their origin from the epiblast, but states that he has not satisfactorily 

 worked out their development. 



The observations of Eisig (No. 512) on the Capitellidae support 

 Kowalevsky's view that the excretory tubes originate from the 

 lining of the peritoneal cavity. 



Hatschek (No. 514) has given a very interesting account of the 

 development of the excretory system in Polygordius. 



The excretor}^ system begins to be formed, while the larva is 

 still in the trochosphere stage (fig. 883, nph), and consists of a 



provisional excretory organ, which is placed 

 in front of the future segmented part of 

 the body, and occupies a position very 

 similar to that of the provisional excretory 

 organ found in some Mulluscan larvae (^ide 

 p. 562). 



Hatschek, with some show of reason, 



holds that the provisional excretory organs 



of Polygordius are homologous with those 



ail of the Mollusca. 



Fig. 383. Polygorwus In its earliest stage the provisional 



LARVA. (After Hatschek.) excretory organ of Polvgordius consists of a 



m. mouth ; ,g. supra-oeso- -^ ^f ^^^^\^ ciliated tubes, each with an 



phageal ganghon ; jyj/i. nephri- ^ . • r- '^ i n • -j. x j • xu 



dion; me.^. mesoblastic band; anterior tunnel-like opening situated in the 

 an. anns; ol. stomach. midst of the mesoblast Cells, and a poste- 



rior external opening. The latter is placed 

 immediately in front of what afterwards becomes the segmented 

 region of the embryo. While the larva is still unsegmented, a second 

 internal opening is formed for each tube (fig. 383, npK) and the two 

 openings so formed may eventually become divided into five (fig. 

 384 A), all communicating by a single pore with the exterior. 



