60 PLANT-ANIMALS [OH. 



light intensity. It exposes itself on the beach to the 

 bright light of the midday sun with nothing between 

 it and desiccation but a constant, filmy stream of 

 salt, drainage-water. In such situations, it retains its 

 powers of activity unimpaired for hours. If, however, 

 it is placed in a vessel with some sand and water, taken 

 to the laboratory and kept in darkness, it passes after 

 some days into a lethargic condition. In this state 

 of dark-rigor C. roscoffensis remains on the surface of 

 the sand and may even fail to respond by downward 

 migration when subjected to the stimulus of vibration. 

 So also, after prolonged exposure to high light 

 intensity, a similar lethargic condition a light-rigor- 

 comes over the plant-animals. Even in their natural 

 positions on the beach, after long hours of exposure 

 to the sun's glare, colonies of C. roscoffensis may be 

 observed in which all the members appear to be 

 overcome by light-rigor. They lie roped together 

 by the slimy excretion of their skin, inert, floating 

 in water-puddles. At times, chunks of a colony in this 

 state may be detached by running water, and small 

 green masses, each of many thousand individuals, are 

 borne seaward by the drainage stream. In this 

 lethargic state of light-rigor, which both young and 

 old animals exhibit, C. roscoffensis is difficult to 

 manipulate ; for example, attempts to transfer them 

 from one vessel to another by means of a pipette 

 result generally in damage to the animals. To this 



