472 CYCLOSTOMATA. 



Vascular System. 



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 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 seem 

 to pass by two pores into the urinogenital 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, — In the ripe ovum, which has a consider- 

 able quantity of yollc, 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 inti- 

 mate manner with those of the reduced nucleus of the ovum, and a seg- 

 mentation 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 blasto- 

 pore or mouth of the gastrula 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 

 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 " enterocrelic " fashion ; but in 

 the trunk region the cushions of hypoblastic yolk cells change gradually 

 into mesoblast, and acquire a ccelome cavity in " schizoecelic " fashion. 

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

 pouches of the archeiiteron, {b) from a splitting of solid mesoblast rudi- 

 ments, are here combined. 



The time between fertilisation and the hatching of the larva, or 

 .Immoccetes, varies with the temperature, being seventeen days in North 

 Germany, but only eight at Naples. 



