BRAIN PHYSIOLOGY OF WORMS 



349 



FIG. 99 



brain (Fig. 99) at the anterior end of a large (about 3 cm. 

 long) Thysanozoon is cut off. The very small piece lying 

 anteriorly then continues to creep or swim while the 

 comparatively enormously large body executes no further 

 progressive movements. The spontaneity of progressive 

 movement in Thysanozoon is therefore 

 a function of the brain. 



Both pieces of a divided Thysano- 

 zoon continue to live and regenerate 

 the missing portions. Only the oral 

 piece regenerates more rapidly than the 

 aboral, which has to form a head. I 

 have not studied whether the latter 

 forms a new brain. I kept such pieces 

 alive for four months. The spontaneity 

 of the aboral piece never returned, while 

 the spontaneity of the oral piece persisted. 



2. The beheaded frog will remain upon its back when 

 the cut lies behind the medulla oblongata; when the cut 

 lies in front of the medulla the frog will not remain upon 

 its back. We assume in this case that the geotropic func- 

 tions of the ear compel the frog to resume the normal 

 orientation. It is probable that the tactile stimuli also act 

 in such a way that they compel the frog to bring the soles 

 f its feet in contact with the surface of solid bodies, or to 

 allow the weight of its body to press upon the nerve endings 

 in these portions of the skin. 



I designated the fact that an animal is compelled to orient 

 its body in a definite way toward the surface of solid bodies 

 as stereotropism. Geotropism cannot be demonstrated in 

 Thysanozoon as the animal assumes any orientation toward 

 the center of gravity for a long time. Stereotropism is, 

 however, present as the animal is compelled to bring its 

 ventral surface in contact with solid bodies, or to allow its 



