ACALEPH^E. 201 



tractile stems, mutable in form, mouth encircled with a single series of granulous fili- 

 form tentacula. 



II. SEIITULARIADJS, plant-like and horny, rooted and variously branched, filled with 

 semi-fluid organic pulp, the polyps contained within sessile cells disposed along the 

 sides of the main stem or branchlets, but never terminal. 



III. MEDUSAD^. Umbel hemispherical, with marginal tentacula; having eight 

 eyes covered by lobes, four ovaries, four cells, four fringed arms, a central opening, 

 and four lateral openings. 



IV. SLPHONOPHORA, having the animals double, and bell-shaped, one fitting into the 

 cavity of the other ; in Dyphyes the animal has a large air-vessel with numerous tenta- 

 cula ; in Physalla, the animal stretches over a cartilaginous plane. 



The true form of the Medusa does not appear in the two first orders. 



The Hydraidae are, according to modern naturalists, Discophorse 

 arrested in their development. They comprehend the single genus 

 Hydra, of which many species are known, whose habits and metamor- 

 phoses it will be our object to particularise. 



Hydra vulgar is inhabits stagnant ponds and slowly-running waters. 

 It is of an orange-brown or red colour, the intensity of the colour de- 

 pending on the nature of its food, becoming almost blood-red when fed 

 on the small crimson worms and larvas 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 mother, 

 while the colour of the ova retains its natural tint, even when the 

 Hydra mother has been fed with coloured substances during the pro- 

 gress of this mode of reproduction. The tentacula, usually seven or 

 eight in number, never exceed the length of the body, tapering insen- 

 sibly to a point. 



Hydra viridis, the 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 offspring was produced from every part 

 of the body. De Blainville, on the contrary, was of opinion that offspring 

 was always produced from the same place ; namely, at the junction of 

 that part which is hollow and that which is not. Yan der Hoven, 

 .the Leyden professor, agrees with Pallas, and Dr. Johnston's opinions 

 accord with Pallas. The green Hydra is common all over Europe, in- 



