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 v/a 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 liberatéd 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 acquirea coelom cavity in ‘‘ schizoccelic’” 
