HYDRZE, OE FRESH- WATER POLYPES. 
273 • 
The motion of a hydra is very slow. Its ordinary mode of 
locomotion is like that of a leech, but changes of position may 
be made by a gliding motion of its disc. Sometimes this disc 
is protruded above the surface of the water, when it acts as a 
float, and the animal is borne along by any movement in 
the water. Hydras may be found in the spring, summer, and 
autumn, towards the end of which last-named season they 
give birth to eggs and then die. In very mild winters I have 
occasionally found a few specimens of the H. viridis, but for 
the most part these polypes do not survive the winter. The 
mode of increase is two-fold, (1) by gemmation, and (2) by 
the ordinary manner of reproduction. The first takes place 
throughout the summer, the latter only at the end of autumn. 
In the case of increase by gemmation a small swelling is first 
observed on the hydra's body ; this grows larger, and divides 
at its apex into several minute papillae, which are the tentacles, 
but they do not appear simultaneously. The other mode of 
reproduction is by ova. About the end of autumn the ob- 
server will notice certain peculiar elevations on the body of 
the hydra ; some round, and situated about the middle of the 
body ; others of a conical shape, and close to the bases of the 
tentacles. Generally there are about one or two of the round 
bodies, and the same number of the conical ones. When 
there is more than one, the second is found on the opposite 
side of the body. These are the reproductive organs, the 
round elevations containing ova, the conical ones the sper- 
matozoan bodies. The ovum, when ripe, is pushed through the 
body- wall, and having been impregnated, becomes attached to 
some water-weed, awaiting the warm weather of spring to be 
developed into a young hydra. Though I have frequently 
seen the ovigerous and spermigerous elevations, and the ovum 
in the body of the hydra, I have never succeeded in meeting 
with the detached ova. They appear to have been seen by few 
observers. This mode of generation in the hydra does not 
appear to have been observed by Trembley and Baker. 
Trembley thus describes one of his attempts to turn a hydra 
inside out. 
I begin by giving a worm to the polype on which I wish to make an 
experiment, and when it is swallowed I can begin operations. It is well 
not to wait till the worm is much digested. I put the polype, with which 
the stomach is well filled, in a little water in the hollow of my left hand ; I 
then press it with a small forceps, nearer to the posterior than to the anterior 
extremity. In this way I push the swallowed worm against the mouth of 
the polype, which is thus forced to open : and by again slightly pressing the 
polype with my forceps, I cause the worm partly to’ emerge from its mouth, 
and thus to draw out with it an equal part of the posterior end of its stomach 
