i9o8] EXCRETORY ORGANS OF METAZOA. O/O 



stomes without genital funnels."* Allen (1904) has demonstrated 

 that in Poccilochcctns both kinds of organs occur, nephridia with 

 small nephridiostomes in the anterior somites, and nephromixia with 

 large funnels in the posterior. 



Thus the evidence is convincing that coelomoduct and nephridium 

 are two distinct organs, with originally separate origins and func- 

 tions, but that the two frequently unite to produce a compound 

 nephromixium. 



Mid-gut. — This is excretory in the Polynoidas ( Schimkewitsch, 

 1884), and so are the intestinal caeca in the Aphroditidae (Darboux, 

 1900). 



Chloragogue. — Schaeppi (1894) found the chloragogue of only 

 the peritoneum, nephridia and intrasinous connective tissue is ex- 

 cretory (contains guanin ). In Arenicola some of the vessels have 

 caeca whose walls possess chloragogue cells (Willem, 1899). For 

 the chloragogue of peritoneal origin (peritoneal glands) !Meyer 

 (1901) uses the term " phagocytic organs." 



Eisig (1887) has made the most thorough study of excretion in 

 the Polychaetes ; he determined that carmine is taken up by the mid- 

 gut, then by the peritoneum, and that the haemolymph is the vehicle 

 of its transport to the nephridia, blood vessels being absent in the 

 Capitellids ; it ultimately reaches also the setal glands and the skin : 

 the skin is not excretory though it becomes the seat of excretory 

 substances, and it is by the accumulation of such material that the 

 skin in necessitated to undergo moults. 



20. ECHIURIDA. 



Segmental Organs. — These serve mainly if not wholly as genital 

 ducts and in Bonellia the male lives within those of the female. 

 Boncllia has but a single one, while in Echiurus and Thalassema 

 there are from one to four pairs. Structurally (Greef, 1879, 

 Spengel, 1880) these are long tubes each with a nephrostome close 

 to a nephridiopore. Nothing seems to be known of their de- 

 velopment. 



* Hempelmann (1906) has since shown that in Polygordius the nephridio- 

 stomes are too small for the discharge of the germ cells, and that the latter 

 escape by rupture of the posterior end of the body. 



