XIII 



IMIVLUM CHORD AT A 



ll-i 



Urinogenital Organs. — In all Cianiata 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 thive parts, 

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

 fore-Udiien ov pronrphros (Fig. 791, A, ^;. n])/i.), the mid-kidney or 

 viesonc2)hros{ms. njjh.), and the hind-kiduii/ or mctancpliros{mt. nph.). 

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

 (msn. d.), or liieta-nephrir (int. n. d.) dud, which opens into the 

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

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

 discharoed into the ccelome and make their exit by genital 



B 



C 



Fir:. 700. — A, part of a urinary tubule with blood-vessels, ai, artery ; <il, Maliii;=chiaii capsule con- 

 taining glomerulus ; c. veinlet returning blood from capillary network (to the right! to vein 

 i-i ; i-a, afferent vessel of glomerulus ; (v, efferent vessel. B, longitudinal, ami C, transvei-se 

 sections of urinary tubules. «, secreting part of tubules ; 'i, conducting jjart of tubules ; 

 c. capillaries ; n. nuclei. (From Foster and Shore's Pkysiology.) 



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

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

 those of a spermiduct fcf. p. 120). The pronephros is almost alwaj-s 

 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 kidneif—meso- or meta-nephros — of the adult is a massive 

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

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



