Orientation 207 



In the ocean abyss by contrast, many fish possess well-developed 

 eyes and the bottom-living organisms produce abundant luminescence 

 (Sverdrup et al., 1942). 



In caves no luminescence is produced by the aquatic inhabitants 

 and only one terrestrial form— the New Zealand glowworm {Arach- 

 nocampa liniiinosa)—\s known to be luminescent. The fauna of caves 

 that are completely cut off from daylight consists typically of species 

 that are blind or have degenerate eyes. Cave animals usually have 

 little pigment and contrast strongly with the jet black or dark red 

 deep-sea animals. The heavy pigmentation of the latter may perhaps 

 be explained as a protective adaptation serving to reduce the reflec- 

 tion of luminescent light from their surfaces and hence the chance 

 of detection. Since cave waters are completely dark, the whitish 

 coloration of the inhabitants would be of no disadvantage on this 

 score. 



Orientation 



Light often plays a significant role in orienting the growth or 

 locomotion of plants and animals. Since orientation to light is often 

 associated with reactions to other factors in the environment, such as 

 gravity, we shall first consider the subject in general terms. Orienta- 

 tion is brought about either by the differential growth or movement 

 of parts of the organism or by a change in the direction of locomotion 

 of the whole organism. The question of why plants and animals grow 

 or move in the directions they do has occupied the attention of in- 

 vestigators for a long time. Many of the theories and terms used have 

 been in conflict, and for a more extended discussion reference should 

 be made to Fraenkel and Gunn (1940) and to Griffin (1953). 



The term tropism is best used for orientation by growth or turgor 

 movements as exhibited by sessile forms. These forms are usually 

 plants, but essentially the same type of orientation is exhibited by 

 many sessile animals such as the hydroids. If the orientation is to 

 gravity, the term geotropism is used. If it is to light, the growth 

 movement is referred to as phototropism, and other prefixes are used 

 for other orienting factors. On the other hand, the orientation of 

 the locomotion of motile organisms is best referred to as a taxis, 

 although it is sometimes also called a tropism. Here the forms in- 

 volved are usually animals, although motile plants such as the green 

 flagellates and motile plant gametes or zoospores are included. Ap- 

 plying suitable prefixes, we obtain the terms geotaxis, phototaxis, etc., 

 for orientation by the corresponding factors. Orientation in the direc- 

 tion of an orienting force is referred to as a positive tropism or 



