STRUCTURE OF LAMPREY 



583 



hag. The red blood cells are biconcave, circular, nucleated 

 discs. 



The segmental or pronephric ducts persist as ureters, 

 and are connected with lateral mesonephric tubules forming 

 a kidney more complicated than that of the hag. The 

 pronephros, which is functional in the larva, entirely dis- 

 appears. The ureters unite terminally in a urinogenital sinus 

 (not present in the hag), into which there open two genital 

 pores from the body cavity. The sinus opens, like the 

 anus, into an integumentary cloacal chamber. 



Reproductive system. — The sexes are separate, but ova 



h 



Fig. 328 



-Longitudinal vertical section of anterior end 

 of larval lamprey. 



in., Mouth ; tli., thyroid ; g.p., one of the gill-pouches ; v.ao., 

 ventral aorta ; h., heart ; N., notochord ; S.C., spinal cord ; 

 E., auditory vesicle ; cb., cerebellum ; p-b., pineal body ; 

 c.h., cerebral hemispheres ; olf., olfactory involution. 



sometimes occur in the testes. The reproductive organ 

 is elongated, unpaired, and moored by a median dorsal 

 mesentery. There are no genital ducts. The ova and 

 spermatozoa are liberated into the body cavity, and pass 

 by two genital pores (true abdominal pores) into the urino- 

 genital sinus, and thence to the exterior. In the male 

 there is an ejaculatory structure, or so-called " penis." 

 There are many more males than females. 



Development of P. planeri. — The ripe ovum has a considerable 

 quantity of yolk, but segmentation is total though slightly unequal. A 

 blastosphere is succeeded by a gastrula. The blastopore persists as 

 the anus of the animal, and there is no neurenteric canal. 



The formation of the central nervous system is peculiar, for the sides 



