38 NERVOUS SYSTEM AND SENSE ORGANS 



ganglia the proboscis in this way acts as a reflex organism. The 

 freed proboscis is able to carry out the three usual coordinated 

 muscular movements when the muscles are intact. The free pro- 

 boscis cannot determine food from other substances. The central 

 nervous system is necessary for this. 



The eyes of turbellarians have been extensively studied by 

 Hesse, '96. In tricladids they consist of visual cells and pigment 

 or acsessory cells. These last inclose the enlarged ends of the 

 visual cells, the rhabdomes. The number of visual cells or retinulse 

 as well as the accessory or pigment cells differs greatly. Kepner 

 and Taliaferro, '16, found the retinulse to consist of three regions; 

 a lateral nucleus bearing region closely applied to the brain with 

 a nerve fiber extending into it, a middle region lens shaped, homo- 

 geneous and highly refractive, and the true rhabdome in the pig- 

 ment cup. Kepner and Foshee, '17, compare the three regions 

 of the retinula with the rods and cones of vertebrates. The parts 

 show a close analogy if not homology with the myoid, ellipsoid and 

 rhabdome. The retinulse of both flatworms and vertebrates are 

 also of the inverted type. Taliaferro, 1920, has an important paper 

 on the reactions of Planaria to light. The species considered was 

 negative to light and turned itself accurately to horizontal rays. 

 In some cases the reactions were direct, they turned away at once 

 without preliminary movements. Specimens with both eyes re- 

 moved do not react exactly as normal individuals, but they do 

 move in general away from light. The rate of locomotion in these 

 is not appreciably affected, but the removal of the anterior end 

 greatly retards the rate of locomotion. Specimens with one eye 

 removed orient themselves accurately to light when illuminated 

 on the normal side, but do not when stimulated in this way on 

 the blind side. 



According to Taliaferro, light must strike a given rhabdome 

 parallel with its longitudinal axis in order to cause stimulation. 

 'Thus, the position of the longitudinal axis of the rhabdome re- 

 sults in a localization of photic stimulation." It is possible, accord- 

 ing to this investigator, to explain the localization of photic stim- 

 ulation in one of two ways. First, the refractive central region of 

 the retinula acts as a sort of lens to concentrate the light on the 

 sensitive rhabdome. Second, by assuming a certain structure of 

 the rhabdome coupled with a shading action of the pigment-cup. 

 Hesse, '97, ascribes the localization of the stimulus entirely to the 

 pigment-cup. 



Trematoda. In monogenetic forms such as Tristonnnu Lang, 

 1881, or Epidella, Heath, 1902, the brain consists of a rather short, 

 semicircular band near the dorsal surface just in front of the 

 pharynx. From it six longitudinal nerves arise, four ventral and 

 two dorsal. These extend the length of the body and end in the iios- 

 terior sucker. Many small nerves spring from the brain and the six 



