igoS.] EXCRETORY ORGANS OF METAZOA. 571 



(c) In the Terebelloids an impervious dissepiment separates the 

 anterior from the posterior thoracal cavity ; in the former there are 

 no germ ceUs, and the three pairs of nephridia have small funnels; 

 in the posterior space, which communicates with the abdominal 

 coelom, occur germ cells, and there the nephridia have large nephro- 

 stomes (peritoneal funnels) for the discharge of these cells. In 

 the Cirratulids, Serpulacea and Hermellids only the first pair of 

 nephridia are strictly excretory, and the others serve as genital 

 ducts CMeyer, 1887). 



(d) In Hermellids and Serpulacea the pair of thoracal nephridia 

 unite dorsally into an unpaired duct that opens near the anterior 

 end of the trunk (]\Ieyer, 1887). And in DyhozvsccUa the pair of 

 the " head " has a single medio-dorsal pore (Nusbaum, 1901). 



Development of the Definitive NepJiridia. — The nephrostome of 

 Polymnia (Meyer, 1887) arises as a fold of the peritoneum that 

 grows backward to join the loop; the latter developes independently, 

 simultaneously or a Kttle later, from retroperitoneal tissue (whether 

 mesectoblastic or mesentoblastic was not determined ) that is at first 

 solid and later acquires a cavity ; the distal excretory duct is prob- 

 ably ectoblastic. In Psygmobranchus (Meyer, 1888) there first 

 appears in the unsegmented larva a pair of large cells in the 

 blastocoel, apposed to the ectoblast and separated from the meso- 

 blast, these two cells become placed between the two layers of the 

 first dissepiment and give rise to the tubes, while there evaginates 

 to meet each of them a peritoneal funnel. ]\Ieyer holds that all tne 

 funnels of Terebelloids must have originally been parts of dissepi- 

 ments, and with the degeneration of the latter have either become 

 independent organs or else have become grafted upon nephridia. In 

 what is the most detailed account of any polych^etous nephridium, 

 Lillie (1905) finds for Arenicola that the nephridia arise seg- 

 mentally and independently, entirely from the somatic layer of the 

 mesoblast ; at first they are small tubes with intracellular cavities 

 and a minute opening into the ccelom ; " the anterior region of these 

 organs . . . together with a portion of the adjoining septum, con- 

 stitutes the primitive nephrostome, from which the adult nephro- 

 stome is directly derived." The terminal vesicle is also not ecto- 

 blastic, but " is formed as a differentiation of the most posterior 



