350 W. von BUDDENBROCK 



entirely contradictory to the theory, although all the necessary 

 conditions of the theory are present. Beginning with the 

 diaheliotropic movements of Crustacea, it should be noted that 

 many species, marine as well as fresh water forms, always swim 

 with their backs turned toward the light (the " light-dorsal 

 reflex "). They move, therefore, at right angles to the rays of 

 light, a true diatropism. By arranging the lights so that the 

 aquarium can be lighted either from above or from below, a 

 sudden illumination from beneath causes the animal, which has 

 been swimming dorsal side up, to turn over on its back, so that 

 the dorsal surface, now down, is still turned toward the light. 

 This, however, need not contradict the tropism theory, for the 

 elongated Crustacea always turn over on their backs by turning 

 around the longitudinal axis which lies in the plane of sym- 

 metry. Now, if we assume for the sake of simplicity that the 

 original position 10 is somewhat oblique, the theoretical scheme 

 may be applied as follows: Unlike illumination of the two 

 symmetrical body halves results in a turning of the organism 

 until it reaches the final position, symmetrical with the rays 

 of energy. The short-bodied compressed Crustacea, however, 

 behave in a very different manner on account of their structure, 

 turning over on their backs by turning a somersault. The 

 amphipod Hyperia always does this, the larvae of Squilla very 

 often, and these are only two examples of a very frequent occur- 

 rence. Such a somersault utterly disregards all the precon- 

 ceived rules of the tropism theory in that there is no turning 

 of the body around an axis in the plane of symmetry. There 

 is instead, a turning around a horizontal cross axis perpendicular 

 to the plane of symmetry, through which no plane can be passed 

 dividing the body into two symmetrical halves, as the theory 

 requires. It is therefore utterly impossible to bring this move- 

 ment into harmony with the theory, and yet it is a genuine 

 tropism. 



Exactly the same consideration applies to the diageotropic 



movements of those Crustacea which swim ventral side up, 



manifesting the maintenance of equilibrium instead of the 



' light-dorsal " reflex. The crustacean Palaemon may be chosen 



10 The reversed symmetrical position, in this case with the plane of symmetry 

 in the direction of the light rays, but with back turned away from the light, pre- 

 sents great difficulties for an explanation by the tropism theory (see p. 352). 



