396 F. AV. GAMBLE AND FREDERICK KEEBLE. 



auother by the viscid excretion with wliicli their surfaces are 

 covered. Whilst thus, tliey refuse altog'ether to react when 

 the water in which they he is agitated. Green himps of 

 cohering thousands are often broken off from such masses 

 and carried seaward by a water current running down the 

 beach (cf. Pearl, 1903). 



The power of reaction to gravity resides in the anterior 

 end of the animal. If Convoluta is divided by a trans- 

 verse section into a "head" end and a "tail" end, the head 

 end rises; the tail end, although capable of active swimming 

 movements, remains at the bottom in still water. The tail end 

 falls when placed at the surface of the water ; the head end 

 remains there unless the water is agitated, when their reaction 

 to gravity reverses its sign and they fall. 



Animals subjected to prolonged darkness, or dim light, 

 exhibit negatively geotropic movements in a constantly in- 

 creasing measure. To such an extent may the reaction to 

 the stimulus become developed, that animals contained in a 

 funnel lined with hlter-paper and kept half filled with water 

 may crawl up the paper beyond the surface of the water and 

 be ultimately dried up at the top. Since Convoluta possess 

 a great power of resisting desiccation, the animals to die thus 

 must have maintained themselves for days in this unwonted 

 position. This is only one of several instances we mention 

 which show that these tropisms are hard taskmasters. The 

 lethargic condition in particular is interesting, and suggests 

 the interpretation that the modifications of habit which are 

 favourable to the photosyuthetic function of the green cells 

 are apt to be embarrassing and even dangerous to the animal 

 itself. 



2. Thermotropism. 



We deal next with the reaction of Convoluta to change 

 of temperature — a reaction which is less marked than that 

 due to light or gravity, and has not been noticed by Haber- 

 landt or other observers. If Convolutas, uniformly illuminated, 

 are placed in a trough containing water, the animals spread 



