lyos.j EXCRETORY ORGANS OF METAZOA. 573 



to the third somite and consists of two parts : ( i ) A mesenchymatous 

 portion, composed of two " Kopfchenzellen " beset with cihated 

 tubes, that later degenerates, and (2) a segmental portion, at least 

 in part ectoblastic in origin, that joins with the mesenchymatous 

 part. Woltereck finds this second pair to become the first pair of 

 definitive nephridia that differs from the others in the absence of 

 funnels. 



In Polygordiiis lactcus Woltereck (1902) found also two pairs 

 of larval nephridia: (i) Hauptnephridia, close to the epidermis of 

 the ventral hyposphere, beset proximally only with tube-cells; and 

 (2) Seitennephridia, lined >vith such cells along most of their 

 lengths. In the adult of this species also one of these pairs must 

 persist, since the foremost definitive nephridia lack nephrostomes 

 (Hempelmann, 1906). 



The present evidence is that the head kidneys are closed inter- 

 nally, and Aleyer accounts for this by the lack of a dissepiment in 

 front of them from which a nephrostome could form. But while 

 Meyer and Woltereck incline to an ectoblastic and mesenchymatous 

 origin, Lillie concludes a mesoblastic. There is no evidence that the 

 adult nephridia are division products of larval ones, but when there 

 is a second pair of larval nephridia it may persist in the adult. 



Provisory Nephridia. — Following on the larval nephridia and 

 before the adult one are formed there are in the Capitellids (and 

 so far as is known only here among the Polychaeta) provisory 

 nephridia, each of which participates in two segments (Eisig, 1887 J. 



Relation of Genital Ducts and Nephridia. — This question has 

 been so ably reviewed by Goodrich (1895, 1900), and his investi- 

 gations have contributed so much to its solution, that I need to 

 discuss it only briefly. Williams (1858) held that these organs 

 are homologous, and derived from a common " viscus." Then 

 Cosmovici (1880) concluded that the segmental organs of Annelids 

 are of two kinds: excretory organs (organs of Bojanus), and genital 

 ducts, and that the two may be separated or may be united. It is 

 the particular service of Eisig (1887) and Aleyer (1887 and later 

 papers) to have demonstrated by their anatomical and embryological 

 studies that the peritoneal funnels, the original genital ducts, are 

 evaginations caudad of dissepiments, and that they may or may not 



