THE BIONOMICS OP CONVOLUTA ROSCOFFENSIS. 407 



iu a pipette, but if the nipple is pressed even gently the 

 water is expelled, and Convoluta is left sticking to the side 

 of the glass tube. The pipette must be held vertically and 

 as still as possible. The slight involuntary shaking of the 

 hand suffices to render it negatively geotropic. The animal 

 is allowed to swim down till it reaches the drop of water at 

 the tip of the pipette. The gentlest pressure of the nipple 

 is then sufficient to transfer it to the required vessel. 



No attempt should be made to transfer young or old 

 Convoluta whilst they are in the lethargic stage (see p. 396). 

 To separate young from old Convoluta the animals are 

 allowed to take up their light position in an unequally illu- 

 minated dish containing several inches of water. The dish 

 is then tilted slightly on the side where the Convolutas are 

 congregated. The old descend more rapidly ; and the young, 

 less active, and readily visible against a dark ground, may be 

 secured with a pipette. 



Section V. — Ecology. 



1. Position of the Colonies of Convoluta. 



The most striking feature of the distribution of Con- 

 voluta is its occurrence in vast colonies. We now describe 

 the appearances presented by these colonies at Tregastel, 

 Cotes du Nord, Brittany. The colonies occupy a definite 

 and usually narrow zone high up the beach just below the 

 high-water mark of the neap tides. Usually each colony 

 consists of a number of separate well-defined patches, the 

 appearance of which can be understood from PI. 30, figs. 1, 2. 

 The shapes and sizes of the patches and the sites they 

 occupy a*emain, broadly speaking, uniform for months together, 

 though subject, as we shall see, to secular periodic changes. 

 Immediately after the receding tide has uncovered the 

 '^Convoluta zone" of the beach, the colonies rise up to 

 the surface of the sand. Each constituent patch takes up a 

 certain position, which may be shifted landwards, but more 



