354 W. von BUDDENBROC'K 



first case by the sum of both forces which work in the same 

 direction, in the second case by the force of one side alone. 

 The criterion is that there is no opposed action of the two sides. 

 This case is exemplified in all tropisms which occur in nature 

 and it evidently cannot be reconciled with Loeb's theory. Some 

 examples will now be mentioned. 



The first concerns the diaheliotropic movements of marine 

 Crustacea, the " light-dorsal ' reflex whicif has already been 

 described. Two years ago I showed that illumination from one 

 side always produces a sidewise paddling of the swimmerets of 

 both sides with the same tendency, a clearly coordinated and pur- 

 poseful action which it is impossible to explain, or even to obtain, 

 according to the tropism theory. Such an interaction of the 

 musculature of the appendages of both sides may be observed 

 in the entire animal kingdom in almost all movements involving 

 a facing about, ^ntil recently, however, very little interest 

 has been shown in these matters, which are of great theoretical 

 importance. 



The second case, in which one side of the body moves while 

 the other remains perfectly still, is naturally rarer. The best 

 known example is the behavior of the Mysidae in response to 

 light stimuli, studied by Bauer 1 -. These peculiar animals are 

 positively heliotropic after having been exposed to light, but are 

 negatively heliotropic after shading; in the first case therefore, 

 darkening, in the second case, illumination, is the stimulus that 

 makes the animal try to escape. The escape takes place through 

 the fact that the legs turned away from the stimulus have their 

 movements inhibited, while those turned toward it keep on 

 paddling, so that necessarily a turning away from the place of 

 the stimulus results. 



So much for the facts. If we try to find their meaning, it 

 is clear, first, that the result is not unfavorable for Loeb, since 

 de facto only those legs react which are connected with the stim- 

 ulated eye, and therefore the movement can be regarded as not 

 coordinated. On the other hand, it must be admitted that the 

 movement, like that of the row boat, can just as well be regarded 

 as a purposeful and coordinated one, since there is no hindering 

 of one side by the other. We find ourselves to some extent on 



12 Bauer, V. Ueber die reflectorische Regulierung der Schwimmbewegungen bei 

 der Mysideen, etc. Zeitschr. J. allg. Physiol., Bd. 8, 1908. 



