148 



DIGESTION IN THE LOWEE ANIMALS 



The amoeba, for example, camiot digest either starch or 

 fats, while other forms have no difficulty in doing so. 

 This is accomited for on the supposition that the amceba 

 can produce no starch or fat dissolving enzymes, while 

 the other forms do produce them. 



Digestion in multicellular animals. — As an animal in- 

 creases in number of cells it becomes manifestly more 

 difficult for the food to reach the cells. In such animals 

 we should expect to find various adaptations to remedy 

 this difficulty. 



Hydra and sponges. In the little animal kno'mi as the 

 hydra, anfl in sponges, the cells which form the animal are 

 arranged in layers about a central cavity. (See Fig. 50 ) 



Fig. 50 — Longitudinal section of a Hydra ; 6, bud wlilch will form a younghydra ; 

 &a, base by wbicli it is att;u'hed when not creeping ; m, moutb ; ow, ovary with 

 an egg ; sp, spcrniary. 



The cells lining this cavity are provided with hairlike 

 projections, or cilia, which are in constant motion, and 

 by their motion create a current which draws the water 



