CNIDARIANS. 479 



then, a long footless stocking, sewn up at each end. By thrusting one- half of this 

 stocking into the other half, there would be obtained a long bag with a double 

 wall. Suppose this bag fixed by its blind end to the ground, while the open 

 mouth-end stood up in the air, to catch anything that fell into it, and then suppose 

 that, close round the mouth, the double wall grew out into arms or tentacles, which 

 could catch anything passing and draw it into the mouth, then we should have a 

 structure somewhat resembling the fundamental form of the Ccelenterata. But it 

 must further be supposed that the two woollen walls of the stocking are replaced 

 by two layers of living cells, so that the outer one forms the skin, which is armed 

 with the stinging-cells, while the cells of the inner layer are hungry creatures 

 waiting to digest anything digestible which comes down into the bag. This is still 

 not enough, as the whole animal must be able to move its tentacles, and to stretch 

 or contract its body ; so that between the layers there is a special gelatinous layer 

 in which run muscle and nerve fibres. Further, in order that the tentacles, when 

 they seize a passing animal, may have no trouble with it, but may be able to bring 

 it to the mouth as easily as possible, they are thickly covered with batteries of 

 stinging-cells. But how, it may be asked, can we get the beautiful bell-shaped 

 jelly-fish from such a creature ? The imaginary animal just described was fixed 

 to the bottom of the sea, or to weeds and stones under water, and here it would 

 grow. But there is a law of life that, after a certain size has been reached, further 

 growth does not add to the animal's stature, but takes the form of buds, which 

 may either be cast off as eggs to hatch and develop elsewhere, or may remain 

 attached to and branching out from the parent animal. Both these processes take 

 place in the simple Cnidarians. Some branch and rebranch to form beautiful 

 trees, or stocks, made up of living animals. Now if all these animals were to drop 

 eggs which fell to the ground to grow up around the parent stock, so fast would 

 they grow that they would soon be killing one another through overcrowding. 

 Hence it has come to pass that in many forms only a certain number of the 

 animals forming a stock produce eggs, and these are able to break away and swim 

 off with their load of eggs, to drop them far away. In this way, swimming-bells 

 have been produced, originally only as carriers for scattering eggs broadcast, just 

 as many trees have arrangements for scattering seeds as far as possible from the 

 parent stem. From this beginning, all the race of jelly-fish appear to have sprung. 

 The free-swimming life offered new fields for catching food. Myriads of small 

 creatures swim near the surface of the water ; the Cnidarian fixed to the bottom 

 of the sea may stretch its arms in vain for these, while the free-swimming bell 

 can go amongst them and follow them along the surface currents, feeding as it 

 goes. Hence, while the eggs of many jelly-fish when dropped develop first into 

 fixed tree-like stocks, which, when grown, let loose another swarm of jelly-fish, 

 the eggs of others, as if to save time as it were, and impatient of the fixed tree-like 

 stage, hatch out at once as young jelly-fish, which rise at one bound to all the free- 

 swimming privileges of their immediate parents. 



The former process is termed alternation of generations ; the egg producing a 

 stock, which is one generation, the stock producing a jelly-fish, which is a second; 

 and these two alternating. In the latter case, when a jelly-fish produces a jelly- 

 fish, one generation — the stock — has been suppressed. This is important, since there 



