Morphology of Organs. 155 



between the coelom on the one hand and the renal veins on the 

 other. The funnels are coelomic, and a short tnbe leads into, 

 the renal veins. This, however, is not precisely comparable 

 to the communication between coelom and exterior in the 

 typical fashion. 



Finally, the generative products are local proliferations of 

 the lining membrane of the coelom ; thus the gonads and the 

 mouths of the nephridia and the gonad ducts are the only 

 organs which can be said to lie actually inside the coelom. It 

 is this reason which leads to the inference that the cavity of 

 the generative organ in Astaciis is coelom. These characters 

 do not apply to the haemocoel. 



Excretory Organs. 



It seems probable that, throughout the entire series of 

 coelomate animals, the preponderance of those organs which 

 function as the eliminators of nitrogenous waste products are 

 homologous organs. We shall take these organs first, and 

 then discuss briefly certain excretory organs which are not so 

 plainly derivatives of the same common ground-plan. 



In the earthworm, it will be remembered, there are a series 

 of paired nephridia ; these are, apart from details, tubes of a 

 glandular character, opening on the one hand into the coelom 

 by a ciliated funnel, and on the other hand by a muscular sac 

 on to the exterior of the body. There is thus, by their means, 

 a communication established between the coelom and the out- 

 side world. The anodon is not a segmented animal. It 

 possesses a single pair of excretory organs, the organs of 

 Bojanus. These are essentially tubes communicating on the 

 one hand with the coelom (pericardium), and on the other 

 with the exterior. As is the nephridium of Lumbricus, the 

 organs of Bojanus of the mollusc are glandular tubes with a 

 non-glandular terminal point. It is the prevalent opinion that 

 they entirely correspond to a single pair of nephridia of the 

 earthworm. So, too, with the single " kidney " of ffelix. 



In the frog the same style of excretory organ can still be 



