LAW OF ASSOCIATION IN THE ANIMAL KINGDOM. 
33 
at first simple swellings, into tlie middle of which, penetrates 
a prolongation of the body-cavity of the parent ; this swelling 
enlarges, and speedily puts forth some tentacles ; a mouth then 
opens in the middle of the circlet formed by the latter. The 
young Hydra, like its parent, is a simple sac, the wall of 
which is composed of a double series of cells, and of which the 
cavity, playing the part of stomach, communicates directly with 
the stomach of the parent, so that the contractions of the body 
may transmit any food captured by the one into the stomach 
of the other, and vice versa. The parent and child live for a 
longer or shorter time in this close union ; but when the young 
animal has attained a certain size, it detaches itself, adheres to 
some neighbouring leaf, and begins to hunt for prey on its 
own account. Yery soon the parent and the young Hydra 
cannot be distinguished from each other, and both of them 
continually produce new Hydras throughout the summer. 
In water abounding in food, and often in captivity, how- 
ever, things go on rather differently. Each Hydra retains its 
progeny ; the young ones increase in size, and produce new 
Hydras without separating from their parent. A true society 
is founded. Trembley for a long time kept a Hydra, which 
bore as many as twenty- two young ones of different generations. 
It was a living genealogical tree. 
What is only occasionally produced in the common Hydra, 
becomes absolutely normal in another fresh- water species, 
Oordylophora lacustris, and in most of the marine Hydroids, in 
which the colonies are often composed of innumerable in- 
dividuals. But then a new phenomenon occurs — the social life 
becomes complicated ; an actual division of labour is effected 
between the members of the same colony. At first all were 
similar, all performed the same functions in the same maimer; 
but speedily each individual becomes specialized. One devotes 
itself exclusively to the capture of food; another to the elabora- 
tion of the nutritive material ; and a third to reproduction ; so 
that in the end all these individuals,, which originally had no need 
of one another, become mutually necessary ; and by this means 
the society acquires a greater coherence, all its members being 
henceforward bound together as partners. Among the Hy- 
dractiniae we may reckon no fewer than seven kinds of indivi- 
duals, namely, — 
1. The nutritive individuals, or Gastrozooids ; 
2. The prehensile individuals, or Dactylozooids, furnished 
with bunches of urticating capsules ; 
3. Dactylozooids without urticating organs ; 
4. Defensive individuals ; 
5. Reproductive individuals, destined to produce the sexual 
individuals ; 
NEW SERIES, VOL. IV. NO. XIII. 
D 
