ORGANIZATION AND GROWTH 219 
the tip of one of the tubes is touched, this only may contract. 
It is generally believed that when this occurs the external 
stimulus which brings about a change in the sensory nerve 
endings is conducted to the central organ, here to be trans- 
formed into a motor impulse which causes the muscles to 
contract. These ideas harmonize with the facts. But it is 
also generally believed that without the central nervous sys- 
tem the reflexes are no longer possible. This idea, as the 
following experiments will show, is not true in Ciona intes- 
tinalis. When the ganglion has been removed from a Ciona, 
it at first remains fully contracted. After some time, how- 
ever, in favorable cases, as early as the next day the animal 
again relaxes. If a drop of water is allowed to fall into the 
basin of water, the entire Ciona contracts rapidly, just as an 
animal whose central nervous system is intact. 
If, therefore, a normal Ciona and one without the central 
nervous system are kept in the same vessel, both have the 
same reflex irritability qualitatively. It is nevertheless pos- 
sible to differentiate clearly between the reaction of the nor- 
mal and the brainless Ascidian. In the latter the thresh- 
old of stimulation is much higher than in the former, To 
determine this I utilized a procedure which I have employed 
in a series of comparative experiments on the irritability of 
the lower animals, and which might be used with advantage 
in human beings. I allow a drop of water to fall from a 
pipette upon the organ to be stimulated, which lies a certain 
distance (varying according to circumstances) beneath the 
surface of the water. If the same pipette is always used, 
the weight of the drop is nearly always the same, so that the 
height which the drop must fall just to bring about a reac- 
tion is a convenient measure of the threshold of stimulation. 
In what follows I shall give the height of the fall of a drop 
of water which just sufficed to bring about a contraction of 
the entire Ciona. The normal and the brainless animals 
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