HYDRIDE. \ I 3 



Figs. 43, 46, 49, 50, 51. Medusida, where there is but a single 

 polyp, and it is free and oceanic (Figs. 52, 53). And lastly, Lucer- 

 narida, which are easily distinguished from the previous order by 

 their umbrella (see Figs. 54, 55, 56, 57, 58). 



HYDRIDE. 



The order Hydrides, 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 H. vulgar is, 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. 40) 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 riridis, 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. Johnstan'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 



