GENERAL ANIMAL FUNCTIONS. 75 



organs such as we have found in the crayfish. In the 

 crayfish these organs are all made up of many special 

 cells, each one of which is potentially equal to the whole 

 animal of the lower types. All the work is done by this 

 living protoplasm. If food is captured it is by the proto- 

 plasm. In case there is a cell wall about the protoplasm, 

 the food must be absorbed through it in a fluid form or 

 be taken through an opening in the cell wall. There is 

 no stomach : food is merely digested in, and used directly 

 by, the protoplasm. 



The protoplasm has the power of taking up oxygen 

 from the water and of giving off its waste materials into 

 the water, through the cell wall. 



This living stuff is sensitive in some degree to chemical 

 conditions of the water and of foods, to light, to gravity, 

 to contacts, to heat, and the like though there are no 

 eyes, nor ears, nor taste organs. There may, however, 

 be special projections of the protoplasm through the cell 

 wall, and these may be more highly sensitive than the 

 general surface (Figs. 7, j; or 8, 5). 



When the protoplasm is aroused by some outside stim- 

 ulus the most noticeable way in which the animal responds 

 is by moving. Motion may take place by the general 

 contraction of the whole mass of protoplasm, or special 

 strands of it may be especially contractile. Often there 

 are projections of the protoplasm beyond the cell wall 

 (cilia) which are highly contractile, and these by their 

 activity enable the cell to swim (Fig. 7, 6-9}. 



In such an organism as we have been describing we 

 have the simplest living, active, food-using, sensitive, 

 self-perpetuating machine known to us. We may infer 

 that such an animal has its functions reduced to their 

 lowest terms. 



