REPRODUCTIVE SYSTEM 525 



Respiratory, vascular, and excretory systems. Seven 

 gill-pouches with plaited walls open directly to the exterior 

 on each side, and communicate indirectly with the gullet. 

 Water enters the pouches partly via the mouth, partly 

 by the external apertures (spiracula), and the movements 

 of the branchial basket and of the tongue-piston aid greatly 

 in the process. In the larva there is an eighth most 

 anterior pouch which does not open to the surface. It 

 corresponds to the spiracle of Elasmobranchs. With each 

 of the seven open pouches in the larva four thymus 

 rudiments are associated. 



The vascular system is essentially the same as in the 

 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 urogenital 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 

 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 uro- 

 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 

 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 " enteroccelic " fashion ; 

 but in the trunk region the cushions of hypoblastic yolk-cells change 

 gradually into mesoblast, and acquire a ccelom cavity in " schizoccelic " 



