MUSEUM OF COMPARATIVE ZOOLOGY. 315 



the Vertebrate renal organs are those belonging to the bilateral cladus ; 

 and among these I shall consider only those forms which are usually- 

 included in the rather heterogeneous class Vermes. This restriction is 

 justified by the circumstance that the only similarities of structure 

 which are to be found between the excretory system of Vertebrates and 

 those of Atollusks and Arthropods recur with greater force in the case of 

 several groups of Vermes. 



In comparing the kidneys of Vertebrates and those of Worms, I shall 

 distinguish three types of structure in the latter group: (1) the water- 

 vascular system of Plathelminthes, (2) the excretory system of Xemer- 

 tines, and (3) the nephridia of Annelids. The various organs which 

 serve as excretory and genital passages in Rotifers, Nematodes, Echi- 

 urids, and Sipunculids are either referable to one of these types, or are 

 valueless for the purpose in hand. 



In endeavoring to find what points of similarity exist between the 

 excretory system of Plathelminthes and that of Vertebrates, I have been 

 unable to formulate any more definite statement than that both consist 

 of longitudinal internal canals, which bear numerous lateral branches, 

 and which open directly or indirectly to the exterior. On the contrary, 

 the two sets of organs appear to me to perform the function of excretion 

 by anatomical devices which are diametrically opposed. In Vertebrates, 

 the excretions are either (primitively) poured into the ccelom and con- 

 veyed thence by a simple series of conduits, or excreted from the blood 

 in the course of the tubuli uriniferi of the kidneys. In Plathelminthes, 

 oil the other hand, the excretory tubes ramify throughout the entire 

 body parenchyme, and, so to speak, seek out the waste products of 

 metabolism at the seat of their formation. It is a contrast such as 

 exists between lungs and tracheae, and appears to me of fundamental 

 importance. According to Fraipont ('80) and Francotte ('81 and '83, 

 pp. 734, 735), it is true, there is a communication between the excretory 

 tubules of Plathelminthes and certain interior canalicular spaces, which 

 they interpret as a rudimentary coelom. The evidence in favor of the 

 latter interpretation is certainly far from complete, but could not, if 

 true, overthrow the fundamental contrast which I have just emphasized. 

 Furthermore, I am not aware that any subsequent writers have con- 

 firmed this account of the termination of the excretory capillaries ; while 

 Pintner ('80, p. 302), von Graff ('S2 a , pp. 106 et seq., '82 b , p. 80), Lang 

 ('81, p. 208, '84, p. 167), Iijima ('84, p. 400), Zschokke ('87, p. 1G5), 

 and Bohmig ('90, p. 243) have all asserted that the terminal sacs are 

 entirely closed. The conclusion seems warranted that no direct evi- 



