ijoS.] EXCRETORY ORGANS OF METAZOA. 557 



a nephridiopore on the ventral surface of the body, and a ciliated 

 nephrostome placed at the inner end of the tube or else near the 

 external opening. But in Sternaspis Goodrich (1897) found no 

 nephridiopores, and in an immature individual of Phascolosoma 

 proki Sluiter (1882&) found no nephrostomes. In all cases these 

 serve as genital ducts. Metalnikoff (1900), in the most detailed 

 memoir, concludes that the nephrostome cannot serve excretion but 

 acts merely to swallow the germ cells, while excretion must be 

 accomplished by osmosis through the wall of the organ that is 

 lined by cells resembling the chloragogue of annelids. Goodrich 

 holds these are not true nephridia, but " peritoneal funnels peculiarly 

 modified." 



The embryological data are conflicting. In Pliascolosouia 

 Gerould (1906) found no excretory organs in the trochophore, and 

 in the " larva " (that succeeds the trochophore) the definitive 

 nephridia arise as solid ectoblastic ingrowths (" a pair of ingrowths, 

 probably of ectoderm"), to which are added funnels of mesoblastic 

 origin. In Sipunculns Hatschek (1883) described a pair of 

 " Nierenzellen " in the mesoblast of the embryo; each of these 

 divides into four cells which acquire an intracellular cavity, then 

 one end of each cell cord becomes attached to the ectoblast while 

 the other opens into the coelom. Gerould's account is the much 

 more detailed and thorough, and renders it probable that both 

 ectoblast and mesoblast enter into these nephridia. The trocho- 

 phore lacks nephridia. 



15. Priapulida. 

 For these animals we have only the brief description of Schauins- 

 land (1886), unaccompanied by figures. From each side of the 

 posterior end there is said to invaginate a pair of ectoblastic tubes. 

 Then a series of short excretory tubules grow out from the walls 

 of these ; the " Endorgane " are multicellular, closed from the body 

 cavity, each cell with a long flagellum. Still later other folds 

 evaginate from the walls of the main ducts, and their cells become 

 the reproductive elements. According to this description this 

 would be a unique ectoblastic organ, not unlike that of the Plathel- 

 minthes, that proliferates germ cells. 



