DIVISION OF LABOUR 



quently met with in 

 fresh water aquaria, 

 and one of the com- 

 monest forms of sea- 

 weed, only too abun- 

 dant on the bottoms 

 of ships and known 

 to sailors as "grass." 

 The animal goes by 

 the name of Hydra, 

 and the plant by that 

 of Enteromorpha. It 

 is unnecessary for us 

 to study the struc- 

 ture of either of these 

 organisms in detail it 



will be sufficient for -, 



... FIG. 10. Hydra : m. mouth ; tent, ten- 



our purpose if we re- tacle . ca / ? body cavity; end ^ inner 



cognise that both are layer of body wall; ect, outer lay-r 

 of body wall. (x20.) 



composed not of one cell but of many. 



In the case of Hydra, the body- Hydra, 

 wall consists of two distinct layers 

 (Fig. 10). The cells of these two 

 layers differ from each other in 

 form and contents, although not very 

 markedly ; those of the outer layer 

 are smaller, more regular, and ob- 

 viously perform different functions 

 from those of the inner layer. They 

 are primarily protective, for they 

 contain stinging appliances, of the 

 Hydra. effect of w } lic h most pe ople who have 

 Transverse sec- . , , , . i .. i /> i 



tion of body indulged in sea-bathing or fishing 



wall. ( x 100 ) have had experience from contact 



