Brain Puysiotocy or Worms 363 
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. 
III 
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 in Cerebratu- 
lus, while in Lumbricus and especially in Planaria torva’ 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 touched 
spot to the ganglion and from here to the muscles. The 
ganglion can be readily extirpated in transparent forms such 
1In this form Bardeen has recently found that the so-called longitudinal nerves 
resemble more closely the oral ganglion in their histological structure. [1903] 
Digitized by Microsoft® 
