THE KIDNEY. 



75 



As the kidney develops further, its tube becomes coiled (Figs. 21 

 A p. 44, and 31). Three sections can then be made out in it : a short 

 ciliated section, a long glandular section and an efferent section. 

 The latter, which is not ciliated in the embryo, shows ciliation at a 

 later stage when the efferent duct of the genital organs opens into 

 it near its end, and it thus serves to transmit the genital cells. 



The three sections of the embryonic kidney are the same as those 

 that can be distinguished in the adult organ, but the latter is further 

 modified in so far as the middle section is more coiled. This gives 

 rise to the renal sac and to the more complicated portion, the renal 



i. v. p. ai. 



ma. 



".. 



. \ 



_-._\ vnt. 





hsm. /-- 



s 

 m.T. 



W. 





x 



-— -^. of 



£. 



P9- 



Fig. 31.— Embryo of Cydas cornea (combined from figures by E. Ziegler). a, anus ; 

 at, auricle; by, byssal thread and gland; eg, eereln-al ganglion; <l. intestine; f\ 

 foot; ij, genital rudiment; km, posterior retractor of the foot; lis, a, posterior ad- 

 ductor; / . gill ; /. liver ; m, mouth ; ma, stomach ; mr, edge of the mantle ; n and 

 '„. kidney ; ot, otocyst ; p, pericardium ; pg, pedal ganglion : s, shell ; v, ventricle ; 



'■'/, visceral ganglion 



anterior retractor of the foot : vsm, anterior a.lductor. 



coil of the Unionidae ; the middle segment is that portion of the kidney 

 where secretion takes place. In the Unionidae, and in other Lamelli- 

 branchs, the middle section is not so much coiled as in .'</,/„>•, but 

 the area of the secretory epithelium is increased by the formation 

 of internal folds. In the primitive forms (Nttcula, Solenomya), the 

 kidney retains the form of a slightly coiled tube, the inner wall 

 of which shows no great increase of surface. 



When the body grows longer, as in the Unionidae, the gills also lengthen 

 and the organ of Bojanus takes up a somewhat different position. Its original 

 position between the pericardium and the posterior adductor which is illus- 



