26 PLANT- ANIMALS [CH. 



the rocks and, now, still clinging to the weed, left 

 stranded, prone upon the ooze beneath the glare of 

 an August sun. 



With C. roscoffensis, also, egg-laying is a periodic 

 phenomenon, though, in this species, the times at 

 which it occurs coincide not with the beginnings of 

 neap tides but with the onset of the springs. A 

 colony of C. roscoffensis is indeed a well-drilled army. 

 Not only do all its members take cover as one unit at 

 a given signal, not only do the individuals keep their 

 ranks when the order comes to climb to the surface 

 once again, but they are born together, grow up to- 

 gether, mature at the same time and lay their eggs 

 simultaneously. As a consequence, it is easy to obtain 

 large numbers of egg-capsules, though only at definite 

 tidal periods. To secure them, all that is necessary 

 is to visit a fertile colony at low water during one of 

 the earlier spring tides, tap with the foot and thus 

 drive C. roscoffensis below the surface, scoop up a 

 little sand, shake it with sea-water in a glass tube, 

 and isolate the slow-sinking, transparent capsules. 

 It is still easier, however, to rear them in the labora- 

 tory. By collecting a cupful of sand and C. roscoffen- 

 sis just before the onset of a series of spring tides, 

 bringing the cup into the laboratory, adding a little 

 sea-water, leaving it till the plant-animals have col- 

 lected on the surface, scooping them off with a 

 watch-glass and putting them with sea-water in a 



