Protective 



muscular, 



cells 



Muscular cell 



Bud 



M 



gestive 

 ceU 



i Endoderm-^ 



Sensitive 

 cell 



Digestive 

 cell 



Ectoderm 



Endoderm 



& General Biological Supply House, Inc. 



SPECIALIZATION IN HYDRA 



In each of the two layers of cells that make up the hollow bag and tentacles of 

 Hydra, there are sensitive "nerve" cells and also especially contractile "muscular" 

 cells. In the outer layer there are also "protective" stinging cells, which dart out fine 

 hollow needles and on acrid fluid when disturbed. Some inner-layer cells secrete 

 digestive fluids 



go on in all protoplasm, but in gland cells there is mass production of par- 

 ticular kinds of compounds. All protoplasm contracts more or less, but 

 muscle cells contract more energetically and more extensively than most 

 other protoplasm. All protoplasm is irritable, or sensitive to disturbances, 

 but nerve cells are especially sensitive. 



There are, of course, no special tissues or special kinds of cells in the 

 protozoa. But where the cells, as they are formed by the division of the 

 mother-cell, cling together instead of drifting apart, division of labor takes 

 place. Thus, in the poriferans and in the coelenterates (see Appendix A), 

 we can see two or more different kinds of cells. 



The simplest organism having distinct nerve cells is the hydra (see il- 

 lustration above). When one of these is disturbed, it does not contract, as 

 does the ameba, but it transmits the disturbance, or stimulus, to all parts of 

 itself and on to other cells. It specializes in sensitiveness and in transmit- 

 ting. Its many branches touch many different cells; its structure suggests a 

 reaching out in all directions. When delicate nerve-endings at the surface 



274 



