URINARY ORGANS. 307 



direct, or else open first into a longitudinal collecting duct (t^from 

 which fine canals pass to the urinary tubules. Thus the seminal 

 fluid passes through the nephridia as well as through the Wolffian 

 duct, which serves as a urinogenital duct (Fig. 240, A, ///, ). 



In Urodela and Anura of both sexes the Wolffian duct opens 

 separately on either side into the cloaca, receiving first, in Urodeles, 

 a number of ducts from the posterior part of the kidney. 1 



The urinary bladder, which is usually bilobed, opens on the 

 ventral wall of the cloaca opposite to the urinogenital apertures. 

 The morphological signification of the bladder has already been 

 explained in the chapters on the alimentary canal and vascular 

 system (pp. 231 and 273). 



Slight indications of a segmental arrangement are found only 

 in the anterior sexual portion of the kidney of Urodeles ; in the 

 posterior part, and in the entire kidney of Anura, all traces of 

 segmentation have disappeared. In both cases, however, the 

 nephrostomes remain throughout life in great numbers on the 

 ventral surface of the kidney, which is covered over by the 

 peritoneum. 



The nephrostomes are connected with the urinary tubules in larval Anura, 

 but later on they become separated from them, and open into the renal-portal 

 vein. In consequence of this change of function, for such it must be con- 

 sidered, the body-cavity of adult Anura serves as a closed lymph -sinus, 

 as in the Amniota ; the peritoneal fluid, which in the larva was carried to the 

 exterior and lost, is in the adult poured into the general circulation, like the. 

 rest of the lymph. 



Reptiles and Birds. In these, as in all other Amniota, the 

 mesonephros, so far as it is retained beyond the embryonic period, 

 is entirely separate from the functional excretory apparatus ; this 

 consists of a metanephros. entirely wanting in nephro- 

 stomes (compare p. 298). 



The metanephros never extends so far along the body-cavity as 

 does the mesonephros ; as a rule it has the form of a small compact 

 or tabulated organ, usually lying within the posterior half of the 

 body-cavity, or even entirely confined to the pelvic region ; it has 

 the latter position, for instance, in most Reptiles (Fig. 241, N') and 

 all Birds (Fig. 242, N~). The posterior end of the kidney, which is 

 generally narrower than the rest, may even extend under the root 

 of the tail, as in Lacerta, in which region there is a fusion of the 

 organ of either side. 



Thus, according to the position of the kidneys, the ureters 

 either do not" extend any distance along the body-cavity, or they 

 may have a longer or shorter free course. The latter is the case, 

 for instance, in Crocodiles, and more especially in Birds (Fig. 

 242, Ur) : in the last-named the kidneys lie closely embedded 



1 In Anura the Wolffian ducts pass some distance independently along the body- 

 cavity, in correspondence with the position of the kidneys ; in the male each is often 

 provided with an enlargement, the vesicula semiualis. 



x 2 



