GENERAL ZOOLOGY 



Yolk 

 glands 



Pharynx 



Fig. 11.6. Reproductive organs of a planarian; 

 ventral view, diagrammatic. Parts of the female 

 system are shown on the left, parts of the male sys- 

 tem on the right. The copulatory bursa, which con- 

 nects with the genital atrium, has been omitted for 

 clarity. 



brain and cords nerves extend to all parts of the body, particularly to the 

 anterior end. Neurons are not easily recognizable in planarians, but they 

 can be demonstrated in certain regions. It is assumed that the relationships 

 of sensory, motor, and perhaps adjustor cells are comparable with those in 

 other invertebrates that have central nervous systems. Planarians evidently 

 have a well-organized nervous mechanism as the basis for their complex 

 sensory and muscular reactions. At the surface of the body, in addition to 

 the eyes, occur a variety of sensory receptor cells specialized for the reception 

 of chemical and tactile stimuli. The structure of the eye in many planarians 

 indicates that light from a definite direction may be an effective stimulus, 

 as well as mere illumination; however, there seems to be no mechanism for 

 image formation. Planarians respond to many different stimuli with a rather 

 high degree of coordination. 



The Reproductive System, Reproduction, and Development. The plan- 

 arian reproductive system is complex and highly specialized. The worms 

 are monoecious, each individual having both male and female systems (Fig. 

 11.6). The male system includes numerous testes, arranged in paired longitu- 

 dinal rows. Each testis is connected by a ductus eflferens to the longitudinal 

 ductus deferens on its own side of the body. The two ductus deferentes enlarge 

 posteriorly to form spermiducal vesicles in which spermatozoa are stored in 

 advance of copulation. The spermiducal vesicles unite in the midline of the 

 body, posterior to the pharynx sheath, and form the tubular cavity traversing 

 the penis, or male copulatory organ. The penis lies in a cavity, the male 

 antrum, which is just anterior to, and connects with, a second cavity, the 



320 



