On INSTINCT AND WILL IN ANIMALS 111 
animals which are forced to bring their bodies in contact 
with solid objects on all sides as far as possible, there 
are others which show exactly the opposite form of irrita- 
bility and immediately draw themselves away from a solid 
body with which they chance to have come in contact. To these 
belong the Nauplii of Balanus perforatus, the tiny Mysidex 
of the Bay of Naples, the gills of Spirographis Spallan- 
zanii, etc. That that form of irritability which I have 
called “stereotropism” plays a prominent rdle in life- 
phenomena, however, follows from the fact that the entrance 
of the spermatozoon into the egg (as shown by the investi- 
gations of Dewitz’) is governed by this form of irritability, 
and that the migration of leucocytes is likewise determined 
largely by contact-irritability. I have, moreover, inciden- 
tally found, in my investigations on the influence of external 
stimuli upon the form of the body, that stereotropism influ- 
ences not only the shape, but also the size and velocity, of 
the growth of certain organs. These investigations were 
made upon Hydroids. I succeeded in producing stercotropic 
curvatures (away from solid bodies) in certain organs with 
the same certainty that I produced heliotropic curvatures. 
Certain organs, when not in contact with solid bodies, 
attain, within the same period of time and under otherwise 
similar conditions, only one-tenth the length which they attain 
when in contact on one side with a solid body. It is for 
these reasons that I have made no mistake and performed no 
useless task in calling attention to the importance of this 
contact-irritability in the animal kingdom, to which I have 
found it necessary to give a special name. 
3. I have thus far given only examples in which a single 
source of stimulation determines the “voluntary” movements 
of animals. Butin a large number of cases the movements of 
animals are not dependent upon one cause of stimulation 
1Dewitz, Pfliigers Archiv, Vol. XXXVII. See also MAssartT, Bulletin de 
l Académie royale de Belgique (Bruxelles, 1888). 
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