TIIH UROPOIETIC SYSTEM. (j| 



The Urnitnt.-t;,' System. Uropoietic organs, distinct 

 the alimentary canal, are probably represented |v tin- u 

 vascular system and segmental organs of the worms. The 

 "organs of Bojanus" of Mnllusks are sacs or tubes opc-i, 

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

 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 .}/>,//>/*, ^^ some par 

 the wall of the organ of Bojanus is in close 

 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 . n the same 

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

 kidney to a ureter with a single uriniferous tulnil 

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

 cavity 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 Wolflian duct is primitivek 

 verticulum of the peritoneal cavity, and remains for a longer 

 or shorter time (permanently, in some of the low. 

 brata, as Myxine] in communication therewith ; and MUCC it 

 has further been shown that the peritoneal cavity communi- 

 cates directly with the lymphatics, and therefore indin 

 with the veins; it follows that the vertebrate ki in. \ 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 thi*, in 

 its lowest form, is so similar to the more complex com lit 

 of the contractile vacuole of a Protozotin, 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 Nervous System. In its essential nature, a nerve is 

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

 ular chinges which occur in anyone part of the or^ani-m 

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

 simple protoplasmic body of a Proto/OMii. a stimulus applied 

 to one part of the body were more readil it ted to 



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



