156 



AN INTRODUCTION TO ZOOLOGY 



ous system, consisting of a bilobed mass of tissue just beneath the 

 eye spots, called the brain (Fig. 77, en) and two lateral longitu- 



dinal nerve cords (In) connected 

 by transverse nerves. From the 

 brain, nerves pass to various parts 

 of the anterior end of the body, 

 imparting to this region a highly 

 sensitive nature. 



REPRODUCTIVE SYSTEM. Re- 

 production is by fission or by the 

 sexual method. Each individual 

 possesses both male and female 

 organs, i.e. is hermaphroditic. 

 The male organs may be located 

 easily in Figure 77; they consist 

 of numerous spherical testes (/.) 

 connected by small tubes called 

 vasa deferentia (vd.); the vas 



FIG. 78. Flame cell of Planaria. deferens from each side of the 



bod joins ^ ^^ Qr ^ ( $ } 





 a muscluar organ which enters 



the genital cloaca. A seminal 

 vesicle lies at the base of the penis, also a number of unicellular 

 prostate glands. Spermatozoa originate in the testes, and pass, 

 by way of the vasa deferentia, into the seminal vesicle, where 

 they remain until needed for fertilization. The female repro- 

 ductive organs comprise two ovaries (g), two long oviducts (od.) 

 with many yolk glands (v) entering them, a vagina ( $ ) , which 

 opens into the genital cloaca, and the uterus, which is also con- 

 nected with this cavity. The eggs originate in the ovary, pass 

 down the oviduct, collecting yolk from the yolk glands on the 

 way, and finally reach the uterus. Here fertilization occurs, and 

 cocoons are formed, each containing from four to more than twenty 

 eggs, surrounded by several hundred yolk cells. The develop- 

 ment of the egg is illustrated and explained in Figure 79. 



c, cilia ; e, opening into excre- 

 tory tubule. (From Lankes- 

 ter's Treatise ) 



