52 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



them. The sand-grains in the field of view are always 

 peppered over with a number of specimens of the 

 Aryiphidinium (figs. 23, 24, 25), and if individuals be 

 watched they are seen after swimming round to come 

 back to rest on a sand-grain and remain there for a time 

 before starting olf on another excursion. If they are 

 thus constantly associated with sand-grains or other solid 

 particles, and never swim more tlian a microscopic 

 distance from such a resting-place, that may account for 

 the fact that we have never found them in our plankton 

 gatherings. 



Fig. 25. Sketch from living preparation, to show some AinjjJiidhiia resting 

 on the sand-grains and others swimming about (low power). 



" Ampliidinium operculatuni is also, however, 

 positively heliotropic, congregating in quantity on the 

 lighter side of the dish in the laboratory, and shifting in 

 bulk from the sand at the darker part of a tank to the 

 end nearer the window. This property accounts for the 

 invariable occurrence of the discoloured sand on the 

 surface only and never in the deeper layers. 



" Tlie published figures of this species are not very 

 good, so a view of both dorsal and ventral surfaces, as 

 seen under a high magnification, is given here (fig. 26). 

 There certainly seems to be a slighi btil dcfiiiilc cuticle 

 covering the greater ])ari of the surface, althougli this 



