THE UROPOIETIC SYSTEM. 61 



The Uropoietic System. — Uropoietic organs, distinct from 

 the alimentary canal, are probably represented by the water- 

 vascular system and segmental organs of the worms. The 

 " organs of Bojanus " of Mollusks are sacs or tubes opening, 

 on the one side, on the exterior of the body, and, on the 

 other, into some part of the blood-vascular system. So far, 

 as Gegenbaur has shown, they resemble the segmental organs 

 of Annelids. In the majority of the Mollusca, some part of 

 the wall of the organ of Bojanus is in close relation with the 

 venous system near the heart, and the nitrogenous waste of 

 the body is here eliminated from the venous blood. In the 

 Vertebrata, the renal apparatus is constructed en the same 

 principle. If for simplicity's sake we reduce a mammalian 

 kidney to a ureter with a single uriniferous tubule, it cor- 

 responds with an organ of Bojanus, so far as it contains a 

 cavhVy communicating with the exterior at one end, and hav- 

 ing a vascular plexus — the Malpighian body — in intimate 

 contact with the opposite end. In the adult mammal there is 

 no direct communication between the urinary duct and the 

 blood-vascular system. But, inasmuch as recent researches 

 have proved that the ureter is formed by subdivision of the 

 Wolffian duct, and that the Wolffian duct is primitively a di- 

 verticulum of the peritoneal cavity, and remains for a longer 

 or shorter time (permanently, in some of the lower Verte- 

 brata, as Myxine) in communication therewith ; and since it 

 has further been shown that the peritoneal cavity communi- 

 cates directly with the lymphatics, and therefore indirectly 

 with the veins ; it follows that the vertebrate kidney is an 

 extreme modification of an organ, the primitive type of which 

 is to be found in the organ of Bojanus of the Mollusk, and in 

 the segmental organ of the Annelid ; and, to go still lower, 

 in the water- vascular system of the Turbellarian. And this, in 

 its lowest form, is so similar to the more complex conditions 

 of the contractile vacuole of a Protozoon, that it is hardly 

 straining analogy too far to regard the latter as the primary 

 form of uropoietic as well as of internal respiratory apparatus. 



The JYervous System. — In its essential nature, a nerve is 

 a definite tract of living substance, through which the molec- 

 ular changes which occur in any one part of the organism 

 are conveyed to and affect some other part. Thus, if, in the 

 simple protoplasmic bod} 7 of a Protozoon, a stimulus applied 

 to one part of the body w T ere more readily transmitted to 

 some other part, along a particular tract of the protoplasm, 



