XIII 



PHYLUM CHORDATA 



117 



Urinogenital Organs. In all Crania ta there is so close a 

 connection between the organs of renal excretion and those of 

 reproduction that the two systems are conveniently considered 

 together as the urinogenital organs. 



Speaking generally, the excretory organ consists of three parts, 

 all paired and situated along the dorsal wall of the coelome ; the 

 fore-kidney or pronephros (Fig. 791, A, p. nph.), the mid-kidney or 

 mesonephros (ms. nph.), and the hind-kidmy or metanephros (int. nph.}. 

 Each of these is provided with a duct, the pro- (pn. d.), meso- 

 (msn. d.), or meta-nephric (nit. n. d.) duct, which opens into the 

 cloaca. The gonads (gon.) lie in the coelome suspended to its dorsal 

 wall by a fold of peritoneum : in some cases their products are 

 discharged into the ccelome and make their exit by genital 



Fie. 7t>0. A, part of a urinary tubule with blood-vessels, ai, artery ; yt, Malpighian capsule con- 

 taining glomerulua ; . veinlet returning blood from capillary network (to the right) to vein 

 ri ; ra, afferent vessel of glomerulus ; re, efferent vessel. B, longitudinal, and C, transverse 

 sections of urinary tubules, a, secreting part of tubules ; 6, conducting part of tubules ; 

 r. capillaries ; n. nuclei. (From Foster and Shore's Physiology.) 



pores, but more usually the pronephric duct in the female assumes 

 the functions of an oviduct and the mesonephric duct in the male 

 those of a spermiduct (cf. p. 120). The pronephros is almost always 

 functionless in the adult, and usually disappears altogether. The 

 mesonephros is generally the functional kidney in the lower 

 Craniata, in which, as a rule, no metanephros is developed, and 

 the mesonephric duct, in addition to carrying the seminal fluid of 

 the male, acts as a ureter. In the higher forms the mesonephros 

 atrophies, and the metanephros is the functional kidney, the 

 metanephric duct becoming the ureter. 



The kidney meso- or meta-nephros of the adult is a massive 

 gland of a deep red colour made up of convoluted urinary tubules 

 (Fig. 790), separated from one another by connective-tissue con- 



