BRAIN PHYSIOLOGY OF WORMS 



frequently however one observes that the posterior piece 

 apparently begins its swimming motions spontaneously, and 

 pushes before it the anterior piece which contracts and 

 ruffles itself. 



Ill 



1. Our observations therefore show that when a worm is 

 cut through transversely that piece which contains the brain 

 retains to a greater degree, generally speaking, the biological 

 or psychic character of the species than the brainless piece, 

 even when the latter far exceeds the anterior piece in mass. 

 The difference which the oral and aboral pieces show in this 

 regard is different in different species of worms. In Thysano- 

 zoon this difference is marked, also in leeches and inCerebratu- 

 lus, while in Lumbricus and especially in Planaria torva 1 it is 

 less. It is however questionable whether this difference is 

 chiefly determined by the brain. For we do not know how 

 far the specific irritability of the individual peripheral 

 elements of the oral pole has to do with it. 



2. The latter thought may go too far for many readers. 

 But it seems to me that we are too much inclined to seek the 

 "irritable structure" which determines the reaction of an 

 animal exclusively in the central nervous system, while fre- 

 quently a more careful analysis of the phenomena by no 

 means compels such a conclusion. The Ascidians are the 

 simplest reflex animals. The central nervous system is 

 reduced to a single ganglion which receives sensory fibers 

 from the surface and sends motor fibers to the muscles. If 

 the skin of the animal is touched, the muscles contract, and 

 the oral and aboral openings of the animal close. Under 

 these circumstances the stimulus passes from the touch i-d 

 spot to the ganglion and from here to the muscles. The 

 ganglion can be readily extirpated in transparent forms such 



1 In t hi.- fur in 1 >anli MTI ha- n ((; 1 1 ly fun ml that t In- -u-ra !]<'! longitudinal nerves 

 resemble more closely tbe oral ^'an^lim! in their hi-tnlo^'iiM] -I rncturc. [ 1903J 



