684 CHyETOPODA. 



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 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 excretory system begins to be formed, while the larva is 

 still in the trochospere stage (fig. 383, np/i), 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 excre- 

 tory organ found in some Molluscan 

 larvae (vide p. 68 1). 



Hatschek, with some shew of rea- 

 son, holds that the provisional excre- 

 tory organs of Polygordius are homo- 

 logous with those of the Mollusca. 



In its earliest stage the provisional 

 excretory organ of Polygordius con- 

 sists of a pair of simple ciliated tubes, FIG. 383. POLYGORDIUS 



, .,. c 11-1 LARVA. (After Hatschek.) 



each with an anterior tunnel-like open- 



m. mouth; sg. supra-ceso- 



ing situated in the midst of the meso- phageal ganglion; nph. nephri- 

 i i 11 , i dion ; me.p. mesoblastic band; 



blast cells, and a posterior external an . a nus ; ,/. stomach. 

 opening. The latter is placed imme- 

 diately 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, npJt] and the 

 two openings so formed may eventually become divided into 

 five (fig. 384 A), all communicating by a single pore with the 

 exterior. 



When the posterior region of the embryo becomes segmented, 



