STRUCTURE OF THE LAMPREY. 407 



in the intestine which may be compared with the spiral 

 valve of Elasmobranchs. 



Respiratory System. — Seven gill-pouches with plaited walls 

 open directly to the exterior on each side, and communicate 

 indirectly with the gullet as already described. 



When the lamprey is sucking a victim, and perhaps at other 

 times, water passes in as well as out by the external openings 

 of the gill-pouches. In the larva there is an eighth most 

 anterior cleft which does not open to the surface. It 

 corresponds to the spiracle of Elasmobranchs. 



The Vascular System is essentially the same as in the hag. 

 The red blood-cells are biconcave, circular, nucleated discs. 



Excretory System. — There are two elongated kidneys 

 (mesonephros), each with a wide ureter. The ureters open 

 terminally into a urinogenital sinus, the external aperture of 

 which lies behind the anus and in the same depression. 



Reproductive System. — -The sexes are separate. The re- 

 productive 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 

 seem to pass by two abdominal pores into the urinogenital 

 sinus, and thence to the exterior. 



Development of P. planeri. — In the ripe ovum, which has a considerable 

 quantity of yolk, the nuclear substance of the germinal vesicle is expanded 

 like a cup at the "animal pole," forming the so-called "pole-plasma." 

 Outside of this is a clear cupola which several spermatozoa may enter, 

 though only one really penetrates into the egg. After a spermatozoon 

 has begun to make its way inwards, two polar bodies are formed. The 

 elements of the sperm-nucleus combine as usual in an intimate manner 

 with those of the reduced nucleus of the ovum, and a segmentation- 

 nucleus is formed about three hours after fertilisation. 



Segmentation is total, but slightly unequal owing to the yolk ; a 

 blastosphere results which is invaginated into a gastrula. The blastopore 

 or mouth of the gastrula persists as the anus of the animal, and there is 

 never a neurenteric canal. 



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

 of the epiblastic infolding remain in contact instead of forming an open 

 medullary canal. 



In the head region, where the gut is not surrounded by yolk-cells, the 

 mesoblast is formed from hollow folds in "enterocoelic" fashion ; but in 

 the trunk region the cushions of hypoblastic yolk-cells change gradually 

 into mesoblast, and acquire a coelome-cavity in " schizocoelic " fashion. 

 Thus the two main ways in which a body-cavity arises, (a) from coelome- 

 pouches of the archenteron, {b) from a splitting of the solid mesoblast 

 rudiments, are here combined. 



