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 Capitellidæ 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 Chætopoda. Kowalevsky (No. [277]), from his observations on the Oligochæta, 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 continued into the segment 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 Capitellidæ 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.
Fig. 383. Polygordius larva. (After Hatschek.)
m. mouth; sg. supraœsophageal ganglion; nph. nephridion; me.p. mesoblastic band; an. anus; ol. stomach.