HYDROIDA. 201 



surface of the water, it becomes dry, and in this state exerts a repulsive 

 action on the liquid ; so that when dragged below the level of the sur- 

 face, by the weight of the body, it still remains uncovered, and occupies 

 the bottom of the cup-shaped hollow in the fluid, thereby receiving a 

 degree of buoyancy sufficient to suspend it at the surface. The principle 

 is the same as that by which a dry needle is supported on water, in the 

 boat-like hollow which is formed by the cohesive force of the liquid, if 

 care be taken to lay the needle down very gently on the surface. If, 

 while the hydra is floating in this manner, suspended by the extremity 

 of the foot, a drop of water be made to fall upon that part, so as to wet 

 it, this hydrostatic power will be destroyed, and the animal will imme- 

 diately sink to the bottom." 



The Hydroida, with the exception of the genera Hydra and Cordy- 

 lophora, are all marine ; and vary in height from a line to that of a foot 

 or more. A few of them are naked ; but the remainder are invested 

 with a transparent horny-sheath, or skeleton, termed the Polypidom, 

 which is of a tubular form, investing the soft parts. In the families 

 Sertidaria, Tubularia, Plumularia, Antenmdaria, and Campamdaria, 

 their names serve to a certain extent to explain the shape of the poly- 

 pidom. We proceed to notice briefly a few individuals of the species 

 Hydra. 



Hydra vidgaris, or Common Polyp (No. 3, fig. 100), is often found 

 upon plants, branches of trees, pieces of wood, rotten leaves, stones, and 

 other substances in the water. They are of an orange-brown or oil- 

 green colour, and round in shape. The number of its arms, or, as they 

 are technically called, tentacula, vary, and are usually a little longer 

 than the outstretched body of the animal itself. On moving from place 

 to place, they dispose themselves in an arched position, grasping some 

 object with their arms ; they then draw the tail towards the hea'd, fix it, 

 throw their head out again, and seizing some other substance, thus 

 proceed onwards. A young one is seen sprouting from the body of 

 this Hydra. 



Hydra viridis, or Green Polyp (No. 2, fig. 100), differs little from 

 the common polyp, except as regards its beautiful light-green colour, 

 and its having shorter arms. 



Hydra fusca, Brown or Long-Armed Polyp, is, like other polyps, 

 composed of a jelly-like matter, formed into cells, which perform certain 

 functions with a large mouth and a sucker at the opposite extremity j 

 the difference between it and the others named consists in the length of 

 its arms, which will elongate several inches (see No. 1, fig. 100). In the 

 extraordinary foresight of nature, it ia provided that during the cold 



