36 NERVOUS SYSTEM AND SENSE ORGANS 



The brain consists of a rather broad flat mass of nerve fibers 

 and cells occupying quite a large part of the forward portion of 

 the head end. Many nerves run out to the surface of the body and 

 two chief longitudinal strands run the length of the body. Usuallv 

 a number of commissures connects the two parts of the brain as well 

 as the two longitudinal strands. The number of these is somewhat 

 variable in the diff"erent species and also in members of the same 

 species. In some forms at least, terminal fibers connect peripheral 

 branches at the margin of the body. Fig. 9, A, D, E. Usually two 

 eyes are found connected with the brain by short nerves, but in some 

 cases at least, such as in Sorocelis, as described in Seidl, 1911, there 

 are neurone eyes scattered over the anterior region of the forward 

 end. 



Lateral extensions of the head end are often especially sensitive 

 and provided with abundant nerve cells. The eyes, simple or com- 

 plex have been well described and figured by Hesse, 1896. A sen- 

 sory cell or cells with expanded ends terminate in a pigment cup 

 which aids in centering the light on the protoplasmic ends of the 

 sense cells. Fig. 9 F-H. 



Very little has been done in analyzing the motor and sensory 

 components of the brain and nerves. Branches to the eyes and to 

 the surface of the body, especially the forward end of the body, are 

 undoubtedly sensory in nature. The brain has been divided by 

 some into an anterior and superior sensory region and a posterior 

 and inferior motor portion. Soine of the chief w-orks on this group 

 are by Chichkoff , '92 ; lijoma, '84 ; Lang, '81 ; Woodworth, '91 ; 

 Wheeler, '94; Veidovskv, '95; Hesse, '97; Micoletzky, 1907; Weiss, 

 1910; Seidl, 1911. 



Rina Monti, 1896, has studied the nerve terminations in the 

 skin of fresh-water planarians. 



The Polycladida are usually considered as having a more com- 

 plex nervous s.vstem than the tricladids, but it is more concentrated. 

 As a rule there is a number of simple eyes scattered over the for- 

 ward end of the body such as shown by De Quatrefages, 1844, 

 although in Plauocera Lang, '82, shows rather concentrated eye 

 areas. In Leptoplana, the eye spots are scattered about in the 

 region of the nervous system, as shown by Schmidt as early as 

 1862. 



Although locomotion in planarian worms may in part be by the 

 surface cilia, the chief activities seem to be by means of muscles 

 of the body under the control of the nervous system. Weak chemical 

 or tactile stimuli cause them to react positively. The resting worm 

 responds less readily than the moving one. Some forms with much 

 more hierhly organized eyes react less well than others with simnler 

 eye spots. As a rule strength of light is less important in reactions 

 than the number of sensory elements in the eye, or the former 



