248 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



to the exterior. In the same manner we may sup- 

 pose that in the Sponges, which certainly do get rid 

 of nitrogenous waste, the currents of water that pass 

 through the canals of the body wall carry away with 

 them waste nitrogenous formations ; but here, as in 

 the Protozoa, experimental evidence is still wanting. 

 We are hardly better off for information as concerns 

 the Ccelenterata or the Echiiiodermata. Of the 

 former class, indeed, the mesenterial filaments of 

 Actiniae, and a whitish layer on the lower side of the 

 umbrella of Porpita, have been stated to contain 

 guanin, which is a waste nitrogenous product ; and 

 the same compound has been said to be found in the 

 rectal caeca of the starfish, and in the Cuvierian 

 organs of certain holothurians ; as to the last, how- 

 ever, it is doubtful whether the true Cuvierian organs 

 were really examined, and, as to their function, the 

 great balance of evidence is in favour of their being 

 rather offensive than excretory organs. 



In the Verifies we have the advantage of being 

 able to detect organs which, by their position, rela- 

 tions, and homologies, afford considerable support to 

 the view that they have a renal function. It will be 

 most convenient to first examine the so-called seg- 

 mental or kidney -like organs (iiephridia) of so well 

 developed a form as the earthworm. 



In all but the first segment of the body we find 

 on the ventral surface and on either side of the middle 

 line, a convoluted tube, which opens by a funnel- 

 shaped orifice into the body cavity, penetrates 

 the membranous wall which separates one segment 

 from the next succeeding, and in the latter opens to 

 the exterior by a small pore. The ciliated funnel- 

 shaped opening, and the thinner-walled portion of 

 the coiled duct, may be looked upon as the receiving 

 portion of the organ ; the true excretory activity is, 

 no doubt, limited to the part where the walls are 



