90 Evolution and Distribution of Fishes 



vessels (55: loi). It should also be observed that Hu- 

 brecht and Oudemans, almost simultaneously with each 

 other, (^9 :5i ;50 rsuppl.i ), showed that direct connection 

 may exist between the two systems. 



Shortly stated then, the evolutionary history of the 

 excretory system, from nemerteans to higher fishes, might 

 be tentatively sketched as follows: In metanemerteans and 

 heteronemerteans a set of fine paired tubules became grad- 

 ually separated off from the main longitudinal vessels that 

 hitherto functioned in common with the tubules, as circu- 

 latory and excretory tubes. While the main vessels then 

 became more and more modified into a blood-vascular sys- 

 tem, formed migrant corpuscles, and inside these gradually 

 elaborated haemoglobin, the finer metamerically-arranged 

 tubes became purely excretory structures. These at first 

 seem to have been simple coiled tubules that closely sur- 

 rounded the blood vessels, in a few anterior ones, but later 

 in twenty to possibly forty or fifty longitudinal pairs. Each 

 tubule-coil opened externally by a minute lateral pore, as 

 still seen in species of the above groups; internally they 

 became increasingly elongated, closely coiled on themselves, 

 and by their blind ends maintained intimate contact with 

 the blood-vessels, by slightly enlarged swellings provided 

 with ciliate or "flame" cells. Such a system then, might 

 be called an archinephros. In transition to cyclostomes and 

 higher fishes the paired tubules of each side probably fused 

 during embryonic life-stages to form a longitudinal "arch- 

 inephric or segmental duct" that drew off the excretory 

 products and that opened up by paired pores into a urinary 

 sinus As a result, closure of the two rows of external pores 

 seems to have occurred, except for two posterior terminal 

 ones that we would correlate with the pair of abdominal 

 pores present in most fishes. In embryos of cyclostomes 

 the two ducts, along with twenty, condensing to six, and 

 even in adults to two, anterior pairs of coiled tubules can 

 be traced, and these together form the pronephros or 

 head kidney that is centred around the head-region. This 

 pronephros is more or less retained in some teleosts also. 



A set of later-formed and more posterior tubules, that 

 similarly opened into the segmental ducts, and that may be 



