136 THE HUMAN SPECIES 



other members of the Arthropoda (Arachnida, Myriapoda 

 and Insecta) and formerly thought to be bile ducts are now 

 ascertained to be urinary organs, for they undoubtedly excrete 

 uric acid, as is seen, for instance, in considerable quantity, in 

 the red or yellowish moisture of the Lepidoptera, on their 

 leaving the chrysalis. 



Typical organs of excretion are the looped canals of the 

 molluscs ; they commence externally and after a course of 

 varied length open into the body-cavity; the chemical com- 

 position of their excreta confirms the view that they act as 

 kidneys. 



In the Brachiopoda and Otocardia the kidneys are repre- 

 sented by one or two pairs of canals ; in the Pteropoda and 

 Heteropoda a kidney of a spongy nature is found, while of 

 still more markedly spongiform construction are the renal 

 bodies which occupy the two final, branchial branches of the 

 vena cava'as far as the branchial heart. 1 



Proceeding from the Invertebrates to the Vertebrates we 

 find the organs of excretion bearing an increasingly close re- 

 semblance to those of man. In the Myxinoidea, belonging to 

 the Cyclostoma, the simplest of conditions still prevail, it is true, 

 but in the Bdellostomata the caecal transverse canaliculi of the 

 ureter enclose a vascular convoluted body, or glomerulus, and 

 the kidneys of the Petromyzontidae - are still more highly convo- 

 luted, though as regards their uriniferous tubules they are very 

 similar to the above. Peculiar to all members of the verte- 

 brate kingdom is the early development of an excretory organ 

 which, however, retains its original function of excretion only 

 in the lower vertebrates, being superseded in the higher by a 

 new organ, whereby the first either remains with a new func- 

 tion, or disappears from the organism. 



In the fishes the two symmetrical, bilateral, lobular Wolffian 

 bodies of glandular character situated at the lower part of 

 the vertebral column originate from the posterior parts of 

 the primordial renal ducts on either side of the vertebral 

 column. The Wolffian bodies combine to form a single duct 

 which opens either into a cloaca or separately behind the 

 anal aperture. Thus an expansion in the form of a urinary 



1 C. Gegenbauer, Vergl. Anat., p. 355. 2 Ibid., p. 861. 



