302 G. p. BIDDER 



Keally, far from express-train speed, no part of the most 

 rapidly moving flagellum ever attains the rapidity of motion of 

 a snail. We forget, as we look through a microscope magnifying 

 1,000 diameters, that though distance is magnified, time is not 

 magnified, and therefore any velocity is y^Vo what it appears 

 to be. I have estimated the vibrations of the flagella in healthy 

 and lively collar-cells at twenty to the second.-^ The flagellum 

 is 30 ju. long ; therefore if it vibrates through an angle of 60°, 

 its tip travels 30 ij. each half-vibration and 60 ij. each complete 

 vibration, making twenty vibrations 1 -2 mm. per second ; 

 which is 14 ft. an hour. This is the speed of only the extreme 

 tip of the flagellum, the base being motionless ; so that some 

 7 ft. an hour is the mean speed of the middle point of this 

 invisibly rapid flagellum. 



I confess that I have often adduced this calculation to 

 show why there are no flagella in the cloaca of Leucandra, 

 where the mean velocity is 1^ centimetres a second, or twenty 

 times the speed of the flagellum. But, really, viscous flow at 

 a mean velocity of 1 cm. means an axial velocity of 2 cm. 

 diminishing gradually to zero on the walls. The bareness of 

 the cloaca we must attribute to the fact that the sponge 

 flagella do not, and cannot, act like oars, an action which 

 requires direction of movement and nervous co-ordination. 

 All observers agree that the movements of sponge-flagella are 

 neither co-ordinated, synchronous, nor in parallel planes.^ 

 A collar-cell flagellate surface is comparable mechanically to 

 a seine-net with a number of fishes fixed by their gills in the 

 meshes. Their movements cannot establish a current along the 

 face of the net : that would involve their tails all striking 

 strongly in the same direction and weakly in its opposite. 

 But if the net be fixed, the uncoordinated movement of their 

 tails will draw water through the net from the side on which 

 are their noses. Sponge collar-cells are similarly capable of 



^ ' Quart. Journ. Micr. 8ci.', vol. 38, p. 17. 



^ Permanent sections of Oscar ella show the pmacocytal flagella of 

 the afferent canals looking as if they work as oars, and far enough apart to 

 do so without collision; say, one to the area of 30 collar-cells. 



