22 PLANT-ANIMALS [CH. 



creasing load of water and C. paradoxa, clinging 

 painfully to the floating, swaying weed, finds itself 

 exposed to a light intensity none too high for its 

 requirements. 



Unlike C. paradoxa which, as we see, migrates 

 periodically, its Sittings coinciding with the phases 

 of the moon, C. roscoflensis, having selected its station 

 on the beach, maintains it in spite of time and tide. 

 Small wonder therefore that the latter organism 

 has learned to respond so swiftly to vibrations 

 that it sinks below the sand at the approach of 

 heavy feet. How sure and swift are the uprisings 

 and downlyings of C. roscofiensis may be learned by 

 standing at the water's edge near by the situations 

 known to be occupied by C. roscoffensis colonies. 

 Scarcely has the tide run off them when a faint green 

 discolouration of the sand marks the contours of each 

 colony, and before the water has receded more than 

 a few yards the dark greenness of the patch indicates 

 that all the animals have risen to the surface. Or if, 

 when the sea is smooth, we watch the incoming tide 

 making its way with gentlest approach toward the 

 patches, we see the animals inert and lying massed 

 together, bound into scum-like lumps by the muci- 

 laginous excretion of their bodies. Thev lie motion- 



*/ 



less, oblivious of the lapping waves a yard or so away. 

 Tli en, as the latest wave washes over the patch, 

 lethargy gives place to action and, in an instant, 



