HYDRIDZA. TI9Q 
towards its mouth, swallows it. Trembley amused himself by feeding 
the Hydra, while he observed the manner in which it devoured its 
prey. 
“When its arms were extended, I have put into the water a 
woodlouse or a small worm. As soon as the woodlouse feels itself 
a prisoner it struggles violently, swimming about, and drawing the 
arm which holds it from side to side ; but, however delicate it may 
appear, the arm of the polyp is capable of considerable resistance ; it 
is now gradually drawn in, and other arms come to its assistance, 
while the polyp itself approaches its prey ; presently the woodlouse 
finds itself engaged with all the arms, which, by curving and con- 
tracting, gradually but inevitably approach the mouth, in which it is 
soon engulfed.” 
Frédol also notices a singular fact. ‘The small worms, even 
when swallowed by the polyp,” he says, “frequently try to escape ; 
but the ravisher retains them by plunging one of its arms into 
the digestive cavity! What an admirable contrivance, by which the 
worms are digested while the arm is respected |” 
The food of the fresh-water Hydra influences the colour of their 
bodies in consequence of the thinness and transparency of their 
tissues ; so that the reddish matter of the woodlouse renders them 
red, while other food renders them black or green, according to its 
prevailing colour. 
The multiplication of these creatures takes place in three different 
ways :— 
1. By eggs. 
2. By buds, after the manner of vegetables. 
3. By separation, in which an individual may be cut into two 
or many segments, each reproducing an individual. 
We shall only say a few words on the first mode of reproduction. 
At certain seasons of the year ova are found to develop themselves 
in the substance of the lower part of the Hydra’s body. These subse- 
quently separate from the body, and appear to be capable of inde- 
pendent movements ; but whether these be caused by cilia or not 
is not quite determined. After a time the sac of the ovum becomes 
ruptured, and the young Hydra bursts its way out. as 
Trembley has studied with great care the mode of reproduction 
by budding—a process which seems to prevail in the summer months. 
The buds which are to form the young polyp appear on the surface 
of the body as little spherical excrescences terminating in a point. 
A few steps further towards maturity, and they assume a conical, and 
‘finally a cylindrical form. The arms now begin to push out at the 
