332 Mr Kerr, The Genito-urinary Organs 



fused posteriorly across the middle line. In Protopterus, however, 

 there passes off to each side only a single vas efferens which 

 arises from the extreme posterior end of the fused testes. 



The presence of a testicular network in Dipnoi is of great 

 morphological interest, adding as it does another to the great 

 groups of gnathostomatous vertebrates in which such an arrange- 

 ment occurs. These groups are the Selachians, the Dipnoans, the 

 Amphibians, the Reptiles, the Birds, and the Mammals. In fact, 

 the only groups in which it does not characteristically occur are 

 those of the Crossopterygii, and the Teleostomi. But in the 

 latter group we have only to look at those forms which the 

 common consensus of Zoologists regards as most primitive — the 

 Ganoids — and we here find a testicular network of the most 

 typical kind (Acipenser, Lepidosteus, Amia). There only remain 

 then the higher Teleostomes, viz. the Teleostean fishes, and the 

 Crossopterygians, in which the network is absent. We should, 

 I think, require very strong evidence before we could believe 

 that an arrangement characteristic of a single one of the groups 

 of Gnathostomata, and of the admittedly more highly specialized 

 members of a second group, was the primitive one amongst 

 gnathostomatous vertebrates, rather than another arrangement 

 which is characteristic of all the remaining groups. Evidence of 

 the required weight is, I believe, completely wanting. Against 

 such a view we have, in the first place, the balance of morpho- 

 logical probability. We know that it is one of the most fundamental 

 characteristics of the coelomata that two main functions — that 

 of nitrogenous excretion and that of reproduction — are carried 

 on by the cells lining the coelom, their products passing into that 

 cavity. Surely it is probable that these two products should 

 primitively find their way to the exterior, by the same ancient 

 channels of communication of that cavity with the exterior, — the 

 nephridial tubes. 



Secondly, we have the fact that the conditions in Lepidosiren 

 and Protopterus when taken with certain facts in regard to other 

 groups, furnish a very simple explanation of how the arrangements 

 in Crossopterygians and Teleosts may have been derived from 

 those common to the other groups of Gnathostomes. 



If we regard the different forms of anurous Amphibians 1 we 

 find that typically there is an extensive testicular network, con- 

 necting testis and kidney over a considerable part of their total 

 length. In the male Bombinator the anterior transverse canals of 

 the network have become enlarged, and their course to the kidney 

 duct has become direct. In the male Alytes the same happens, 

 but here the (usually two) enlarged canals are the sole represent- 

 atives of the network. 



1 v. Gadow, Cambridge Natural History, vol. vm. p. 51. 



