32 CCELENTERATES. 



ments are varied and interesting. They are ceaseless, 

 too, as may be seen by watching its continually changing 

 form. Though all its individual cells have the power 

 of motion, some cells, as those in the tentacles, move 

 more freely than others. The lasso cells are capable 

 of a very sudden and remarkable and highly specialized 

 act, the thro wing-out of their paralyzing filament. 



IV. Sensation. No nervous system is developed in the 

 hydra; but a few nerve cells, developed between the bases 

 of the ectoderm cells, are known to exist. There is a 

 marked advance in the powers of sensation of this animal 

 over those previously studied, seen in greater reaction 

 from disturbances, in greater skill in securing food and 

 more choice in selecting it, in preference for light, etc. 



Tissue and Organ. The cells of the ectoderm in 

 hydra, being much alike in form and in the work they 

 have to do, constitute a primitive tissue. The cells of the 

 endoderm constitute another such tissue. A tissue may 

 be defined as an aggregation of similar cells, together with 

 whatever intercellular substance may be developed from 

 the cells. 



When any part of an animal is set apart for a particular 

 work, that part is called an organ, and the work it has to 

 do is called its function. Thus the tentacles of the hydra 

 are organs, and their function is reaching out after food. 



Differentiation becomes very evident in the hydra. 

 The thread cells of the tentacles are very unlike the 

 digestive cells of the endoderm in the digestive cavity. 

 In the work they have to do, there is quite as great dif- 

 ference as in their form. The thread cells are given a 

 specific work to do, the capturing of food. Other cells 

 do the digesting, and can do it better for being relieved 

 of the work of capturing the food. This is what is meant 

 by a physiological division of labor. This sort of spe- 



