GONADS 



45 



hand into the pericardial coelom, and on the other into discontinuous 

 remains of a longitudinal duct lying in a venous sinus. There is no 

 communication with the kidney duct. Degenerate as this organ is, 

 it is better developed in the Cyclostomes than in any known adult 

 Craniate, excepting perhaps some aberrant Teleostei (p. 364). 



The permanent functional kidney, or mesonephros, is repre- 

 sented by a single longitudinal duct on each side, into which open 

 a number of tubules leading from closed renal capsules with 

 glomeruli (Fig. 32). In all cases the tubules have lost their primi- 

 tive openings into the abdominal coelom ; in Petromyzon they do 



Tit 



Fio. 28. 



PetroDU/zon marimts, L. Left-side view of the trunk region near the base of the dorsal fin ; 

 the sl-in and muscles have been partially removed, a, anus ; ao, dorsal aorta ; c.r, cartilage 

 rays supporting fin ; d.f, dorsal fin ; go, left genital aperture into urinogenital sinus ; i, intestine ; 

 t mesonephros. and kd, its duct ; my, niyotome ; n.a, neural arch ; nt, notochord ; p.c, posterior 

 cardinal vein ; r.m, radial muscles of tin ; ug.o, urinogenital opening on papilla. 



not appear even during development (Wheeler [486]). While in the 

 Lampreys the elongated kidney consists of crowded coiled tubules 

 much more numerous than the segments of the body they occupy, 

 in the Myxinoidea the tubules preserve their original metameric 

 order (Miiller [306]). In this character the Myxinoids appear to be 

 more primitive than any other known Craniate (p. 82, where the 

 morphology of the excretory organs is dealt with). 



The right and left kidney ducts of the adult end behind in a 

 common urinary sinus, which opens by a median papilla, placed in 

 a narrow cloacal depression at the front end of which is the anus 

 (Figs. 19, 28). 



