1 62 COMPARATIVE PHYSIOLOGY 



movement of organisms is dependent on such a finely balanced 

 interplay of phototropic, geotropic, chemotropic, and thigmo- 

 tropic reflexes that only careful analysis can evaluate their 

 respective influences. Thus the normal swimming move- 

 ments of Mysids are in ordinary circumstances unaffected 

 by removal of either the eyes alone or the statocysts alone. 

 If, however, both statocysts are extirpated the mysid swims 

 on its back when illuminated from below. Deprived of 

 both eyes and statocysts, the animals can effect efficient 

 orientation only when in contact with a surface (through touch- 

 receptors in the appendages). Other factors which underlie 

 seemingly inconsequent modes of behaviour are seen in the 

 effect of physico-chemical conditions on the sign of a tropism. 

 It was shown many years ago by Loeb that the larvae of Poly- 

 gordius, and various small Crustacea, which are in normal 

 life negatively phototropic, can be made either indifferent to 

 Hght or definitely positive in their phototropic response when 

 certain salts and acids are added to the medium. Similar 

 phenomena have been described by Minkiewicz, Moore, 

 Ewald, and others, who have also succeeded in producing a 

 reversal of sign in the normal tropism of various animals by 

 chemical means. An interesting example is recorded by Kanda 

 (19 14), who studied the effect of electrolytes on the behaviour 

 of Arenicola larvae with reference to gravity and hght. In 

 normal circumstances the larvae of Arenicola are positively 

 phototropic, swimming towards the Hght by ciHary and 

 muscular movements. When placed in darkness they swim 

 upwards, i.e. they are negatively geotropic. When excess 

 of K or Na ions is present {i.e. on addition of a certain amount 

 of isotonic potassium or sodium chloride solution to the sea 

 water) the larvae cease to swim towards the source of illumina- 

 tion when placed in the light ; they tend to swim in the opposite 

 direction, becoming negatively phototropic, though feebly 

 so. The geotropic response of the larvae in darkness is not 

 affected by such treatment. When, however, there is an 

 excess of Ca, or Mg ions the larvae swim do\vn wards in dark- 

 ness ; their behaviour towards gravitational attraction is 

 reversed, they have become positively geotropic. The reaction 



