THE CQKLENTERATES 35 
fishes may be liberated in such multitudes that the upper 
surface of the ocean for many miles may be closely packed 
with them in numbers reaching far into the millions. In 
these positions they are carried both by oceanic currents 
and through the alternate expansion and contraction of the 
bell, a movement resembling the partial closing and open- 
ing of an umbrella. In the jelly-fish the contraction is the 
more vigorous and rapid, and as it takes place the opening 
in the velum or veil (Fig. 18, 6) is so narrowed that the water 
in the subumbrella space (a) is driven through it with con- 
siderable force, which results in driving the body in the 
opposite direction. 
The life of a jelly-fish is perhaps of short duration, last- 
ing not more than a few hours in some species, up to two 
or three weeks in others, but during that period they pro- 
duce multitudes of eggs which develop into minute free- 
swimming young. These settle down on some rock or sea- 
weed, and soon develop a Hydra-like body which, after the 
fashion described above, grows into another tree-like colony. 
36. Alternation of generations.—It will be noticed that 
the offspring of the jelly-fishes are not jelly-fishes, but stalked 
colonies, and these latter forms give rise to jelly-fishes. 
This is known as the alternation of generations, the jelly- 
fish generation alternating with the colonial form. This 
characteristic is of the greatest service in preventing the 
extermination of the race. Were the stalked forms to 
give rise directly to other stationary colonies, it is obvious 
that before long all the available space in the immediate 
locality would be filled. The food supply, always lim- 
ited, would not suffice, and starvation of some or imper- 
fect development of all would result ; but by means of the 
free-swimming jelly-fish new colonies are established over 
very extensive areas, and favorable situations are held 
by all. 
37. More complex types—As mentioned above, there are 
perhaps upward of a thousand species of Hydrozoa, all with 
