350 COMPARATIVE ANATOMY 



thus also serves as an excretoiy duct (Figs. 219 and 272, a). 

 The segmental arrangement of the tubules in the adult corre- 

 sponds to that of the branchial apparatus, and not to that of the 

 myotomes. No nephridia are present posteriorly to the pharynx, 

 and it is possible that the excretory system of Amphioxus may be 

 to a certain extent comparable to an early stage of the pronephros 

 of the Craniata. 



In Cyclostomes the pronephros persists beyond the larval 

 period, and for some time at any rate, functions as the sole excre- 

 tory organ : it possesses three or four nephrostomes. In Petro- 

 myzon it is soon replaced by a mesonephros, and the pronephros 

 then becomes rudimentary : between the two a fat-body is situated. 

 In Myxine it is uncertain whether the whole kidney, or only its 

 anterior part, I'epresents the pronephros. The kidney does not 

 come into relation with the generative organs, and its duct, which 

 opens on either side into the uriuogenital sinus, probably represents 

 the unaltered pronephric duct. 



In the Teleostei the pronephros has, in the majority of cases.^ 

 only a temporary significance, and the mesonephros constitutes 

 the excretory organ of the adult : it consists of a natrow 

 band varying in size and diameter in different regions, situated 

 on the dorsal side of the body-cavity, between the vertebral 

 column and the air-bladder. Secondary fusions between the 

 organ of either side often occur. The urinary duct in both groups 

 probably represents the pronephric duct, and may lie more or less 

 freely, or be embedded in the substance of the kidney. Posteriorly 

 the two ducts usually fuse together and become expanded to form 

 a kind of urinary bladder (compare Figs. 286 and 287) which has 

 nothing to do with' the allantoic bladder of Amphibia and Am- 

 niota. The bladder usually opens behind the anus — either inde- 

 pendently or together with the genital ducts — by a simple pore, or 

 on the summit of a urinogenital papilla. 



Thus a differentiation of the pronephric (or primary mesonephric) 



duct into a Wolffian and a Miillerian duct is not known to occur in 



Teleostei, nor does the mesonephros come into connection with 



the gonads; in Elasmobranchii, in which the pronephros is 



more rudimentary, this differentiation takes place (p. 346), and 



at the same time a distinction between an anterior and a 



posterior section of the mesonephros may be observed (compare 



Figs. 278, 289, and 290). In the male, the former (j)arorehis or 



e]pididymis) comes into connection with the testis by means 



of small ducts, the vasa effercntia, and its tubules open directly 



into the Wolffian duct, which thus functions as a vas deferens 



only ; while the latter, which persists as the permanent kidney, 



empties its secretion by means of separate urinary ducts into 



the base of the Wolffian duct. In the female the Wolffian 



' It is said to pei-sist in Fii'i-a-ijrr, Loiihins, Dacti/hjiterux, Orthagori«cus mola, 

 Mora iiieiliterranea, and tlie Macriiriihf. 



