EXPERIMENTS ON WORMS 



73 



Fig. 20), contains the brain of the animal. This 

 consists of two connected ganglia, from which a series 

 of nerves, containing single ganglion-cells, go out ; 

 among the latter, the 



two large longitudinal 

 nerves running length- 

 wise throughout the 

 animal (it, Fig. 20) are 

 conspicuous. In the 

 periphery a plexus is 

 formed ( i ). The central 

 nervous system consists 

 of the double ganglion 

 in the forward end. 

 Like all Planarians, 

 Thysanozoon crawls on 

 the side of the aquarium 

 or on the surface film 

 of the water. It differs 

 from the fresh - water 

 Planarians in being able 

 to perform, in addition, genuine swimming move- 

 ments. With the sides of its body it makes vibra- 

 tions similar to those made by the wings of a but- 

 terfly. If while a Thysanozoon is gliding about on 

 the surface of the water it be divided transversely 

 with a pair of scissors, the posterior or aboral half 

 (b, Fig. 21) at once falls to the bottom, while the 

 oral piece (a, Fig. 21) containing the brain creeps 

 on undisturbed. If the division be made with a 



FIG. 21. THYSANOZOON DIVIDED 

 TRANSVERSELY. 



The anterior piece a, containing the brain, shows 

 spontaneity ; the posterior piece 6, none. 



