134 THE HUMAN SPECIES 



wall (Aplysia, Doris) and discharging itself into the stomach. 1 

 In all the vertebrates the pancreas originates in an expansion 

 of the intestinal wall, occurring behind the rudimentary liver. 

 In man also there first exist two rudiments, a dorsal and a 

 ventral. In the domestic mammals the dorsal rudiment 

 becomes eventually the duct of Santorini, the ventral the duct 

 of Wirsung ; in man, with rare exceptions, the latter duct alone 

 persists. 



In man, as also in the fishes, birds and mammals, small 

 aciniform glands, assumed to be mucous glands, are found in 

 the wall of the Wirsung's duct. 



VII. Urogenital System. 



In the foregoing chapters we have seen that, in spite of 

 man's position at the summit of the organic scale, he stands, 

 nevertheless, in close connection with the more lowly organisms, 

 being able to present few specific peculiarities in his structure. 

 His urogenital system is no exception in this respect. Here, 

 as before, we must go back to the lower forms of the animal 

 organism in order to fully understand how the human organs 

 have acquired the form we know them to possess. In the very 

 lowest organisms, the Protozoa, with their extreme simplicity 

 of structure, an organ of excretion is hardly to be expected ; in 

 the Ccelenterata the excretory functions are performed by the 

 general gastro-vesicular apparatus, and only a few possess 

 organs which may be regarded as rudimentary kidneys. Such 

 are the Actinia possessing mesenteric filaments, which have 

 been ascertained to contain guanin, and, among the Siphono- 

 phora, the Porpita with their curious spongy organ situated 

 beneath the discoid air cavity. 



It is clear that in the lower marine animals, whose organism 

 freely communicates with the surrounding water, an excretory 

 organ of the nature of a kidney could not be developed to any 

 great extent. Among the Echinodermata there are only a 

 few types in which special organs of excretion are found ; such, 

 apparently, are the interradial tubes with glandular walls as 

 possessed by the Asteroidea, with and without an anal orifice, 



1 C. Gegenbauer, Vergl. Anat., p. 533. 



