^9o8.] EXCRETORY ORGANS OF METAZOA. 563 



his account only from the citation given by Lang (1903, p. 103). 

 " In Pontohdella the nephridia consist of very richly branched and 

 reticularly anastomosing tubes, among which one cannot distinguish 

 main trunks. The two nephridia of the same segment are many 

 times joined together, and the nephridia of the several segments 

 equally so. In Cystobranchus each nephridium has attained a com- 

 plete independence and connects neither with the other nephridia 

 of the same segment, nor with those of neighboring segments. It 

 consists then also only of a single, coarse, unbranched tube. The 

 remaining genera correspond in this relation more or less with 

 Pontobdella; one can, however, always distinguish particular 

 trunks. In Piscicola one part of the nephridium, that is much more 

 strongly developed than the remaining part, corresponds exactly 

 in position with the nephridium of Cystobranchus. Pontobdella 

 departs, finally, from all the other genera in this, that the nephridia 

 have inner openings." Nothing is yet known of the development 

 of these reticular organs. 



Genital Ducts. — These were considered by Nusbaum (1885) to 

 be modified nephridia. Burger first (1894) opposed this com- 

 parison, but later (1902) he maintained that the female genital 

 apparatus and the terminal portions at least of the vasa deferentia 

 are possibly homologous with nephridia in developing from gono- 

 blasts that are homodynamous with nephroblasts. 



Larval Nephridia. — In the Hirudinea three of the blastomeres 

 of the 4-cell stage give rise to a larval body that later perishes, 

 while the fourth blastomere alone produces the adult body 

 (Brandes, 1901). This larval body produces no nephridia. The 

 " Urnieren " arise from the germ band that develops within this 

 larval body, and they last only as long as the latter does. Bergh 

 (1884, 1901) has shown that there are three pairs of these in 

 Aulastoma and Hirudo and two pairs in Nephelis, all developing 

 from the germ band; and he and Sukatchofif (1900) demonstrated 

 that the inner ends are closed and the cavity intracellular. These 

 larval nephridia arise from cell rows of the germ band that are 

 generally considered mesoblastic, though this point is hardly finally 

 settled. 



E.vcretophores. — Excretory cells within the connective tissue 



