EXCRETING ORGANS. 625 



FIRST SECTION. 



Internal Organs of Excretion. 



The most complicated and the most distinct of all emunc- 

 tory apparatus developed in animals, are the urinary organs, 

 which early appear in the animal scale and in the embryo, 

 and exert the most immediate and extensive influence over 

 the condition of the vital fluids, and the entire economy. 

 Developed in the vertebrated tribes, from the cloacal end of 

 the alimentary canal, like the lungs from the buccal extre- 

 mity, the urinary, like the pulmonary organs, excrete largely 

 the aqueous constituents with other materials of the blood, 

 and both these complex secerning organs remove their hete- 

 rogeneous products by the terminal openings of the diges- 

 tive tube. As carbonic acid is the most characteristic in- 

 gredient of the pulmonary excretion, so are uric acid, or urea, 

 and lithic acid, both abounding with nitrogen, the chief 

 urinary products ; and as the structure of no gland is yet 

 indicative of its function, the lowest and most ambiguous 

 forms of the urinary organs are determined rather by their 

 general analogies of form and connections, and the chemical 

 nature of their products, than by any peculiarity of internal 

 structure. The urinary organs thus corresponding in func- 

 tion, and complimentary, to the respiratory, commonly how- 

 ever exhibit less tendency to ramification and vesicular 

 dilatation in their ultimate tubuli, than the lungs and most 

 other complex glands. 



The means of respiration possessed by the simplest 

 forms of animals, whether internal or external, may serve 

 likewise as general emunctories for the urinary and other 

 excretions, without the necessity of special organs for each 

 of these products. As all parts of the body in the radiated 

 classes of animals, the cutaneous, mucous, and serous 

 surfaces, are constantly bathed by ciliary currents of the sur- 

 rounding liquid element, the excretions may be removed di- 

 rectly from every point of the system without distinct organs 

 for their elimination. The tubular, ramified, internal respira- 

 tory organs of holothuria (Fig. 114. A.), developed like renal 

 glands from the cloacal end of the alimentary A canal, and ex- 

 tending like the kidneys of vertebrata along the interior of the 



PART VII. S S 



