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 inouth, 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 Coelenterata. 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 an}’thing 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 
