24 THE BEGINNINGS OF LIFE. 



Certain it is that this weapon is of great use to the hydra, as you 

 will realise if you watch one of them feeding. A water flea or 

 other small creature comes in contact with one of these machine 

 guns. The spring uncoils with such force that the cavity and 

 the thread are turned inside out. Then you see that the water 

 flea, before so alert and lively, is completely paralysed, and if the 

 hydra is feeling in need of a meal the captured prey is brought 

 within reach of the tentacles and gradually transferred to the 

 digestive cavity. The interior layer of cells comprises cells of 

 two different varieties. Like the outer cells they are nucleated, 

 but, unlike them, some of them are seen to possess one or more 

 filaments or fiagella, and others are constantly varying their free 

 ends by the protrusion and drawing in of pseudopodia or 

 processes ; they are, as it were, a series of amoebae fixed in their 

 places. The inner cells are a rank of flagellate monads and of 

 amoebae marshalled into line and working for a common end — 

 the nutrition of the cell aggregate. 



Ordinarily the hydra reproduces by budding ; a small 

 excresence makes its appearance on the exterior. It is formed by 

 a pushing out of the two layers of cells. It becomes lobed at the 

 outer end, and gradually develops into a young hydra. This 

 capacity for reproducing by budding brings about a curious result 

 in the hydra. You remember in the Greek mythology which 

 you studied at school reading the myth of the hydra or many- 

 headed serpent which Hercules destroyed. It was a nasty 

 customer to deal with, because when Hercules cut off any one of 

 its heads two new ones grew in their place. Our hydra realises 

 something akin to that, for you may make mincemeat of it, and 

 each piece will develop into a perfect hydra. I have referred to 

 the volvox and hydra only so far as to show the advance in 

 differentiation, or division of labour. A lecture of many hours 

 duration might be devoted to the life histories of either of them. 

 The hydra is of the greatest interest to us, for it represents the 

 permanent form of what is known in embryology as the gastrula 

 stage in development — a stage through which the majority of the 

 higher organisms pass early in their prenatal history, a stage in 

 which they consist of a purse-like form composed of but two 

 layers of cells, the inner performing nutritive functions, and the 

 outer the functions of sensation and protection, just as in the 

 hydra. From the hydra we proceed to the hydroid polypes, or 



