828 



EMBRYOLOGY 



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

 larvse of the Gnathohdellidpe also possess provisional excretory 

 organs which are comparable to those of the Oligochseta, even 

 though in number and form they are different. In Nephelis 

 there are two (Fig. 156 mto^ and tm^), in Hirudo three, and 

 in AuJastoma 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 

 Bergh, 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 un^ . 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- 

 2:>heUs the ring-like canal, which is 

 Avound 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 



Fig. 155.— Origrin of the 

 primitive kidneys (itiij to ini,) 

 from the germ bands (Rur)i)i/- 

 fceim) of Aulastoma <}ulo (after 

 Bebgh). «I, mouth ; pi, pole 

 cells terminating the cell-rows 

 of the germ bands. 



