igoS] EXCRETORY ORGANS OF METAZOA. 561 



Ascidia) ; (3) sacs, less numerous, on both sides of the body below 

 the mantle, with walls of cubical cells {Cynthia, Microcosmus) ; 

 and (4) renal organs, a single voluminous sac on the right side of 

 the body with epithelial wall {Molgula). Todaro (1902a, h) de- 

 scribed them for the Salpidas as hollow vesicles in the number of 

 three pairs, to which waste products are carried by the blood cor- 

 puscles. 



Thus in the majority of Tunicates they are vesicles without 

 ducts placed in the mesenchyme. Van Beneden and Julin (1886) 

 found them to be derived from mesenchyme, and concluded that 

 this embryonic tissue is a modification of what was ancestrally 

 enterocoelic mesoblast; Conklin (1905), however, has shown that 

 all the mesoblast is peristomial, consequently the tissue from which 

 these organs develop may be mesectoblast. 



The genital ducts are outgrowth of the gonads, therefore prob- 

 ably have no relation to nephridia. 



16. DiNOPHILEA. 



Korschelt (1882) described for Dinophilus apatris, and Weldon 

 (1887) ^o^ ^- 9W^^> ^ nephridial system of the platyhelminthan 

 type, though both of them saw clearly only the flame cells. Subse- 

 quent observations have demonstrated that there are metamerically 

 arranged, separated nephridia. Thus Schimkewitsch (1895) found 

 in D. vorticoides four pairs of these in the male and five pairs in the 

 female; Harmer (1889) and Shearer (1906) for D. tceniatus, 

 Nelson (1907) for D. conklini, and E. Meyer (1887) for D. gyro- 

 ciliatus discovered five pairs. These are ciliated tubes each with its 

 own nephridiopore, closed internally, and (according to Shearer) 

 beset with solenocytes. In D. conklini the first pair is much more 

 complex than the others and consists of a considerable number of 

 cells; each of those of D. gyrociliatiis is described by Meyer as con- 

 sisting of only two cells. 



Schimkewitsch considered the genital ducts of the male to be a 

 fifth pair, and the corresponding ducts of the female to be a sixth 

 pair of nephridia, and Harmer regarded the seminal vesicles as 

 segmental organs. This is, however, little more than a supposition, 

 since the genital ducts are quite different in structure from the 



