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 Mysidese 

 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 role 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 1 ) 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 stereotropic 

 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. But in a large number of cases the movements of 

 animals are not dependent upon one cause of stimulation 



i DEWITZ, Pflilgers Archiv, Vol. XXXVII. See also MASSAET, Bulletin de 

 V Academic royale de Belgique (Bruxelles, 1888). 



