452 COMPARATIVE ANATOMY 



I. SPECIAL PART. 

 URINARY ORGANS. 



In Amphioxus a series (90 or more) of independent segmental 

 tubules are present on either side in the reduced section of the 

 ccelome situated on the dorsal side of the pharynx, in close rela- 

 tion to the branchial blood-vessels. 



Each tubule is partly glandular and partly ciliated, and gives 

 off several knobs provided with peculiar club-shaped cells project- 

 ing into the ccelome and corresponding to closed nephrostomes : 

 its other end opens into the atrial or peribranchial chamber 

 (p. 352) by a single aperture. 1 The segmental arrangement of 

 the tubules in the adult corresponds to that of the branchial 

 apparatus, and not to that of the myotomes. No tubules are 

 present posteriorly to the pharynx. 



Cyclostomes. In the Petromyzontidae, the excretory system 

 resembles that of the Amphibia much more closely than in the 

 Myxinoidei. In the former, rudiments of at least thirteen prone- 

 phric tubules gradually appear, but only the five most anterior of 

 these become functional. In Ammocoete larvse about 10 cm. in 

 length, the pronephros reaches its highest stage of development, 

 and by this stage a mesonephros has also arisen from the coelomic 

 epithelium, so that for some time both organs are functional 

 (Fig. 341). As the mesonephros slowly develops further, from 

 before backwards, the pronephros becomes gradually reduced, and 

 eventually also a reduction takes place of about two-fifths of 

 the entire mesonephros also. 



Observations on the excretory organs of Myxinoids are still 

 incomplete, and it is uncertain whether both pronephros and 

 mesonephros are developed, or whether only one pair of these 

 organs is represented. 



In none of the Cyclostomes does the kidney come into relation 

 with the generative organs, and its duct, which opens into the 

 urinogenital sinus, probably in all cases represents the unaltered 

 pronephric duct. 



Elasmobranchs. In these Fishes, the transitory pronephros 

 has a rudimentary character, and is very variable : it usually only 

 extends over 3-5 body-segments (Raia, Torpedo) more rarely over 

 7-8 segments of the embryo, and apparently has no excretory 

 function. As already mentioned (p. 446) a differentiation of the 

 primary mesonephric duct into Wolffian (secondary mesonephric) 



1 It is very doubtful whether the suggestion is justified that these epi- 

 branchial tubules correspond to some extent to the pronephros of the Craniata, 

 and the peribranchial chamber to its duct. 



