328 



EMBRYOLOGY 



the median line of the entire ventral surface (Robin). The 

 larvae of the Gnathobdellidfe also possess provisional excretory 

 organs which are comparable to those of the Oligochaeta, even 

 though in number and form they are different. In Nephelis 

 tliere are two (Fig. 156 ?*% and un2), in Hirudo three, and 

 in Aulastoma four pairs of provisional kidneys. In the last 

 form they are found lying on the ventral surface of the larva, 

 on either side of the germ bands from which, according to 

 Bekgh, they take their origin as cell-growths, composed at 

 first of one, then of several rows of cells (Fig. 155). Sub- 

 sequently they separate from the germ bands, and then con- 

 sist of structures somewhat annular in shape and composed 

 of two rows of cells (Fig. 156 un^). These two cell-rows 

 subsequently differentiate in such a way that they consist 

 of two adjacent canals ; one of them 

 becomes the stouter, chief canal, and 

 the other one winds several times 

 about it (Fig. 156 unj) . At the turn- 

 ing point the two are continuous 

 with each other, and therefore really 

 constitute only one canal. A cilia- 

 tion has not been observed in the 

 canals. Not only is there in Ne- 

 phelis the ring-like canal, which is 

 wound about itself, but this is pro- 

 longed into a duct, which to a cer- 

 tain extent constitutes the efferent 

 duct of the organ, and has been 

 compared to such by Bergh, in the 

 sense that the two primitive kidneys 

 would correspond to the two arms 

 of the primitive kidney of Polygor- 

 dius, and that the duct would lead to the point of union of 

 the two. As has been stated, there are in Hirudo three and 

 in Aulastoma four pairs of primitive kidneys, and Leuckart 

 even describes in the medicinal leech their opening to the 

 exterior. Bergh, however, could not confirm this. 



The primitive kidneys of the Hirudinea are said by 

 Bergh to have nothing to do with the permanent excretory 



-«<«.. 



un 





/23' 



Fig. 155.— Origin of the 

 primitive kidneys (uiii to un^) 

 from the germ bands (Riaiip/- 

 fceim) of Aulastoma gulo (after 

 Bkbgh). m, mouth ; pz, pole 

 cells terminating the cell-rows 

 of the germ bands. 



