HYDRIDE, 113 
Figs. 43, 46, 49, 80, 51. Aeduside, where there is but a single 
polyp, and it is free and oceanic (Figs. 52, 53). And lastly, Lucer- 
nartde, which are easily distinguished from the previous order by 
their umbrella (see Figs. 54, 55, 56, 57, 58). 
HypRID&. 
The order Hydride comprehends but the single genus Hydra, of 
which many species are known, whose habits and-metamorphoses it 
will be our object to particularise. The best known of these so- 
called species are A. vulgaris, H. viridis, H. fusca, and H. rubra; 
these differ for the most part from each other in size, colour, the 
form of the body, or in the relative proportions of the polypite and 
tentacles. 
In this order the polyps are attached by a base which can be 
detached during locomotion. The integument does not secrete any 
horny or calcareous covering, but is easily resolved into two 
portions—an outer called the ectodermic layer, and an inner called 
the enodermic layer. 
Hydra vulgaris (Fig. 49) inhabits stagnant ponds and slowly-running 
waters. It is of an orange-brown or red colour, the intensity of the 
colour depending on the nature of its food, becoming almost blood- 
red when fed on the small crimson worms and larva to be found in 
such places. M. Laurent even succeeded in colouring them blue, * 
red, and white, by means of indigo, carmine, and chalk, without any 
real penetration of the tissue, the buds from them acquiring the same 
colour as the parent, while the colour of the ovum retains its natural 
tint, even when the mother Hydra has been fed with coloured 
substances during the progress of this mode of reproduction. The 
tentacula, usually seven or eight in number, never exceed the length 
of the body, tapering insensibly to a point. 
Hydra viridis, another fresh-water polyp, being more immediately 
within the sphere of our observation, naturally presents itself to our 
notice. It is common in ponds and still waters. It was noticed by 
Pallas, who was of opinion that its young were produced from every 
part of its body. De Blainville, on the contrary, was of opinion that 
the young were always produced from the same place; namely, at 
the junction of that part which is hollow and that which is not. 
Van der Hoven, late Professor of Natural History at Leyden, 
agrees with Pallas, and Dr. Johnston’s opinion is the same. The 
green Hydra is common all over Europe, inhabiting brooks filled 
with herbage—attaching itself particularly to the duckweed of 
I 
