vii WILL IN INFUSORIA 319 



movements are entirely spontaneous, and the variety of 

 these is due to their power of moving all or different sets of 

 cilia more slowly or more quickly, or keeping them at rest. 

 The hypotrichous Infusoria, 1 e.g. Euplotes Charon, at one 

 moment shoot through the water with their cilia in rapid 

 motion, at another run about on the bottom on algae or other 

 objects, using -their cilia as legs, and moving like Isopods. 

 The behaviour of such Infusoria towards one another, accord- 

 ing to some observers (Engelmann), looks as if they were 

 running after one another in play. 



Yet in these Infusoria, endowed with mental faculties, we 

 can discover no nerves and no brain. It seems to me prob- 

 able, from comparison with the relations of the cells of multi- 

 cellular animals, that their nuclei, while at the same time 

 connected with reproduction, or one of them, also acts as the 

 central organ of the nervous system. This idea is supported 

 by the fact recently demonstrated, that when an Infusorian is 

 cut into two only one part grows again to a complete animal, 

 namely the part which contains the nucleus, or at least a 

 piece of it. 2 The same holds good for snails in regard to the 

 brain. If from a garden-slug (Limax agrestis) the head with 

 the brain is cut off, it no longer grows again into a complete 

 animal, as it does after mutilations in which the brain has 

 not been removed. 3 A. Gruber, on the contrary, believes that 



1 Those provided with cilia on the under surface only. 



2 A. Gruber, Beit. aur. Kenntn. d. PhysioL u. Biologie der Protozoen. 

 Berichte d. naturf. Oes. zu Freiburg i.B. 1886 ; M. Nussbaum, Sitzungsber. d. 

 niederrhein. Oes. f. Nat. u. Heilkunde in Bonn, 1884, p. 259, seq. Fr. Schmitz 

 and J. v. Hansteiu found the same was true of the division of plant-cells. In 

 this connection, in my opinion, the general importance of the nucleus as the organ 

 of life comes into consideration. Cf. the following. 



3 The reproduction of the head of snails was shown to occur by Spallanzaui 

 as early as the year 1764. The Rev. Schaeffer was astonished to observe that 

 the snails in his garden whose heads he had cut off with the shears in order to 

 destroy them again became perfect (Schteffer, Versuche mit Schnecken, 1768-70). 

 According to Tarenne (Cochlioperie, recueil d' experiences sur les Ilttices terrestres, 

 1808), the brain and the buccal nerve mass also grow again. But it seems certain 

 that the retention of the cesophageal nerve-ring is indispensable to the continua- 



