34 



MORPHOLOGICAL TYPES 



the mesonephros. Such a primitive kidney approaches very 

 closely to the hypothetical Archinephros, with its archinephric 

 duct. It is important to note that in the Cyclostomes there is 

 but one kidney-duct on each side. The kidneys and their 

 ducts hang down in the ccelom as the so-called nephric fold. 



Genital System. — The gonads are situated in a ridge hang- 

 ing down from the roof of the coelom. Originally paired, the 

 gonad is single and median in Petromyzon. In both sexes the 

 genital products are shed freely into the splanchnoccel. In 

 front of the place where the two mesonephric ducts join, each 



a u<3 



FlG. i 8. — Petromyzon : view of a dissection from the left side of the anus (a) 

 and urinogenital (ug) aperture. 



df, dorsal fin ; i, intestine ; kd, kidney duct ; n, notochord ; nc, nerve- 

 cord ; pv, perivisceral coelomic cavity. The arrow is passed through the 

 genital pore from the coelom. 



duct has a small pore opening into the splanchnoccel, and it is 

 through these pores that the genital products escape to the 

 exterior ; they have no special ducts. 



Ccelom and Mesoderm. — The eventual division of the 

 coelom by the transverse septum into pericardial and peri- 

 visceral cavities has already been mentioned. The myoccels 

 are completely obliterated, and the only other portions of 

 coelomic cavity are the kidney- tubules and ducts. The first 

 three somites are drawn off into the service of the eyeballs, 

 and the fourth forms the first proper trunk-muscle. The 



