THE SIMPLEST MANY-CELLED ANIMALS 53 



may be called. The digestive fluid secreted by the 

 gland-cells acts upon it so that it becomes broken into 

 small parts. These particles are seized by the projecting 

 pseudopods of the sub-amceboid cells and taken into the 

 body-protoplasm of these cells. The cells of the outer 

 layer of the body do not take food directly, but receive 

 nourishment only by means of and through the cells of 

 the inner layer. The body-cavity of Hydra is a very 

 simple special organ of digestion. 



In the outer layer of cells there are some specially 

 large cells whose inner ends are extended as narrow 

 pointed prolongations directed at right angles with the 

 rest of the cell. These processes are very contractile and 

 are called muscle-processes. Each one is simply a 

 specially contractile continuation of the protoplasm of the 

 cell-body. There are also in this layer some small cells 

 very irregular in shape and provided with unusually large 

 nuclei. These cells are more irritable or sensitive than 

 the others and are called nerve-cells. We have thus in 

 Hydra the beginnings of muscular organs and of nerve- 

 organs. But how simple and unformed compared with 

 the muscular and nervous systems of the toad and crayfish ! 

 There is no circulatory system, nor are there any special 

 organs of respiration. 



But Hydra is far in advance of Amoeba or Paramoccium. 

 Its body is composed of thousands of distinct cells. Some 

 of these cells devote themselves especially to food-taking, 

 some especially to the digestion of food; some are 

 specially contractile, and on them the movements of the 

 body depend, while others are specially irritable or sensi- 

 tive, and on them the body depends for knowledge of the 

 contact of prey or enemies. In the cnidobla^t cells, those 

 with the stinging threads, there is a very wide departure 

 from the simple primitive type of cells. There is in 

 Hydra a manifest differentiation of the cells into various 



