348 STUDIES IN GENEKAL PHYSIOLOGY 



its possession of two tentacles (Fig. 98). From the posterior 

 side of the brain arise the two large nerves (/?, Fig. 98), 

 which traverse the entire length of the animal. A number 

 of other nerves also run to the brain. The nerves contain 

 separate ganglion cells, a characteristic which, as is well 

 known, is found also in certain peripheral nerves in the 

 higher animals. The nerves form a plexus at the periphery. 1 



The central nervous system of this animal therefore con- 

 sists mainly of the ganglion situated at the anterior extremity 

 of the animal. As is the case with all Planarians, Thysano- 

 zoon creeps over the walls of the aquarium or along the 

 surface of the water. It differs in its movements from the 

 fresh-water Planarians only in so far as it is able to execute 

 actual swimming movements. In swimming the animal 

 executes pendulum-like movements with the lateral portions 

 of its body as does a butterfly with its wings. 



If a Thysanozoon is cut across transversely into two 

 halves, while the animal is moving at the surface of the 

 water, the posterior aboral half at once falls to the bottom 

 like a dead mass, while the oral piece which contains the 

 brain continues to move along quietly. If the cut is made 

 rapidly and with a sharp pair of scissors, the behavior of 

 the oral piece gives no indications of such reactions as 

 accompany the sensation of pain in higher animals. If the 

 animal is divided with a sharp knife while it is creeping over 

 a glass plate, we notice the same phenomenon: the oral 

 piece continues its motion undisturbed while the progressive 

 movements of the posterior piece cease at once. It happens 

 occasionally that the transverse division of a Thysanozoon 

 causes the oral piece to execute more lively and rapid move- 

 ments. That the progressive movements of this animal are 

 indeed a function of the brain is shown particularly well 

 when only the exceedingly small piece which contains the 



1 Mittheilungen aus der zoologischen Station zu Neajel, Vol. I. 



