xm URIXOGEXITAL ORGANS 601 



(pp. 146 and 340) situated along the dorsal region of the 

 coelome ; but owing to the fact that they do not all arise 

 at the same time and that their development is modified 

 by neighbouring structures, there is much variation as 

 regards these tubules and their relations. They may in 

 general be classified into three groups, constituting what 

 are known respectively as the fore-kidney or pronephros 

 (Fig. 162, A, p. nph}, the mid-kidney or mesonephros 

 (ms. nph}, and the hind-kidney or metanephros (nit. nph}. 



The pronephros appears at an early stage of develop- 

 ment, and very rarely remains functional throughout life, 

 though in some cases (e.g. tadpole) it serves as an 

 excretory organ in the larva : in many cases it is merely 

 represented by vestiges (e.g. dogfish, bird, rabbit). The 

 pronephric tubules open at one end into the coelome 

 by ciliated nephrostomes, and at the other into a longi- 

 tudinal pronephric duct (sg. d), which communicates with 

 the cloaca posteriorly. They are generally from two to 

 four in number, are not strictly segmental, and are con- 

 fined to the anterior end of the coelome : opposite their 

 nephrostomes a single large glomerulus (see p. 146) is 

 present on either side of the body. 



The mesonephros appears rather later than the pro- 

 nephros, and extends along the greater part of the 

 coelome. Its tubules are segmentally arranged, and thus 

 furnish another example of metamerism in the Verte- 

 brate body ; each tubule is provided with a separate 

 glomerulus enclosed within a Malpighian capsule (me). 

 The nephrostomes of the mesonephros may lose their 

 connection with the tubules and come to open into certain 

 veins (e.g. frog, Fig. 47 and p. 98), or may disappear at a 

 later stage to a greater or less extent (dogfish) or 

 entirely (e.g. bird, rabbit) : at their other ends the 

 tubules communicate with the pronephric duct, which 

 may now be spoken of as the mesonephric duct (Fig. 



PRACT. ZOOL. p p 



