428 THE FORMS OF CELLS [ch. 



Heliozoa (such as Actinosphaerium), the organism is apt to be 

 differentiated into layers, so constituting sphere within sphere, 

 whose inter-surfaces become the seat of adsorption, and the locus 

 of skeletal secretion. One layer at least is close-packed with 

 vacuoles, forming an "alveolar meshwork," with the configura- 

 tions of which we shall attempt in another chapter to correlate 

 certain characteristic types of skeleton. In Actinosphaerium the 

 radial filaments pass through the outer layer, and seem to rest on 

 but do not penetrate the layer below; this must happen if the 

 surface-energy between the one plasma-layer and the other be less 

 than that between the filament and the water around*. 



A very curious conformation is that of the vibratile "collar," 

 found in Codosiga and the other "Choanoflagellates," and which we 

 also meet with in the "collar-cells" which line the interior cavities 

 of a sponge. Such collar-cells are always very minute, and the 

 collar is constituted of a very dehcate film which shews an undu- 

 latory or ripphng motion. It is a surface of revolution, and as it 

 maintains itself in equihbrium (though a somewhat unstable and 

 fluctuating one) it must be, under the restraining circumstances of 

 its case, a surface of minimal area. But it is not so easy to see 

 what these special circumstances are, and it is obvious that the 

 collar, if left to itself, must shrink or shrivel towards its base and 

 become confluent with the general surface of the cell ; for it has no 

 longitudinal supports and no strengthening ring at its periphery. 

 But in all these collar-cells, there stands within the annulus of the 

 collar a large and powerful cilium ot flagellum, in constant move- 

 ment; and by the action of this flagellum, and doubtless in part 

 also by the intrinsic vibrations of the collar itself, there is set up a 

 constant steady current in the surrounding water, whose direction 

 would seem to be such that it passes up the outside of the collar, 

 down its inner side, and out in the middle in the direction of the 

 flagellum; and there is a distinct eddy, in which foreign particles 

 tend to be caught, around the peripheral margin of the collar f. 



* Cf. N. K. Koltzoff, AnaL Anzeiger, xli, p. 190, 1912. 



t The very minute size of Codosiga, whose collar and flagellum measure about 

 30-40 /i,, and of all such collar-cells, make the apparently complex current-system 

 all the harder to comprehend. Cf. G. Lepage, Notes on C. botrytis, Q.J. M.S. 

 LXix, pp. 471-508, 1925. 



