Chap, iv.] INTRACELLULAR DIGESTION. 107 



along the course of the canals which traverse the body 

 of a sponge we find a single layer of cells, each of which 

 is provided with a long whip-like process (flagellum), 

 and has the free edge of its protoplasm converted into 

 a collar-like fringe. By the action of the flagellum 

 currents are set up around the cell, and directed to the 

 space surrounded by the collar ; these currents of 

 water bear with them minute food-particles, which 

 thus make their way into the substance of the cell. 

 Such flagellate cells recall the Flagellate Infusoria, 

 among the Protozoa. Finally, in the case of the lower 

 worms, we have the evidence which is afforded by 

 Mesostomum ehrenbergii, a Turbellarian which lives 

 on the small Annelid Nais. In observations on intra- 

 cellular digestion no method is more fruitful in its 



O 



results than that which consists in feeding an animal 

 with some finely-divided colouring matter such as 

 carmine ; Mesostomum, however, has been found to 

 reject this substance, and the ingenious expedient had 

 to be resorted to of first feeding the Nais with carmine, 

 and then inducing the turbellarian to eat the annelid. 

 This experiment, which was completely successful, 

 afforded certain evidence as to the persistence of the 

 intracellular mode of digestion in this animal, for a 

 large quantity of the coloured material was found in its 

 digestive cells. 



The phenomenon of intracellular digestion has 

 been now seen to be very widely distributed among the 

 lower Metazoa, and observations are continually being 

 made in confirmation of the facts here described. 

 With a single exception, no observer has as yet seen 

 any combination of this primitive method of taking in 

 food with the more complex one of the presence of a 

 set of cells which secrete a special gastric juice ; we 

 may expect, however, to find that the sharp distinction 

 between the lower and the higher methods will be 

 bridged over by other forms than the fresh-water 



