XIII 



PHYLUM CHORDATA 113 



each gives off a blind pouch which dilates at the end and forms a 

 Malpighian capsule (m. c.), and a branch from the aorta entering it 

 gives rise to a glomerulus. 



In some forms the archinephric duct now becomes divided by a 

 longitudinal partition into two tubes : one retains its connection 

 with the mesonephros and is known as the mesonephric or Wolffidn 

 duct (C, ms.n.d.): the other has no connection with the nephridia, but 

 opens into the coelome in the region of the vanishing pronephros : 

 it is the pronephric or Mullerian duct (p. n. d.). In some Craniata 

 the Mullerian appears quite independently of the Wolffian duct : 

 the latter is then simply the segmental duct after the union with 

 it of the mesonephric tubules. 



In the higher Vertebrata, from Reptiles to Mammals, a diverti- 

 culum (D, E, mt. n. d.) is given off from the posterior end of the 

 Wolffian duct, which grows forwards and becomes connected with 

 the hindmost nephridia. In this way is formed a metanephros 

 (mt. nph.), which becomes the permanent kidney, and a metane- 

 phric duct (mt. n. d.), which becomes the ureter. The Wolffian 

 body ceases to discharge a renal function, and becomes a purely 

 vestigial organ. 



In many Fishes there is a dilatation of the ureter, the urinary 

 bladder, which serves as a receptacle for the urine. In the higher 

 Craniata the ventral wall of the cloaca sends off a pouch, the 

 allantoic bladder (al. U.\ which serves the same purpose although 

 morphologically an entirely different structure. 



The gonads (gon.) are developed as ridges growing from the 

 dorsal wall of the coelome, and covered by ccelomic epithelium, 

 from the cells of which, as in so many of the lower animals, the 

 ova and sperms are derived. The test is consists of crypts or 

 tubules, lined with epithelium, and usually discharging their pro- 

 ducts, through delicate vasa efferentia (D, v. c.), into the Wolffian 

 duct, but in some groups into the coelome. The sperms are 

 always motile. The ovary is formed of a basis of connective 

 tissue or stroma, covered by epithelium, certain of the cells of 

 which become enlarged to form ova. In the majority of cases the 

 ova are discharged from the surface of the ovary into the open 

 ends of the Mullerian ducts (E, p. n. d.), which thus function simply 

 as oviducts, having no connection in the adult with the urinary 

 system. In some groups the ova, like the sperms, are shed into 

 the ccelome and escape by the genital pores, and in many teleo- 

 stean or bony Fishes, the ovary is a hollow organ, as in Arthro- 

 poda, discharging its ova into an internal cavity, whence they are 

 carried off by a duct continuous with the gonad. 



A few Craniata are normally hermaphrodite, but the vast 

 majority are dioecious, hermaphroditism occurring, however, occa- 

 sionally, as an abnormality. 



In close connection with the urinogenital organs are found 



VOL. II I 



