ORDER I.-HYDRA-FORM POLYPES.

HYDROIDA.

The Zoophytes or Polypes may be arranged in four great divisions. The first consists of those which in structure bear a resemblance to the common fresh-water Hydra, a singular being, whose history is more. strange than that of the strangest fairy tale. One species has claspers or tentacula, capable of being distended to many times the length of its body (Fig. 8). In its contracted state, it seems a little piece of jelly not larger than the half of a pea: but when engaged in the capture of its food, with the tail at the surface of the water and the head underneath, it stretches its tentacula like so many fishinglines; and there is reason to believe that it possesses the power of giving an electric shock, or otherwise stunning by its touch the minute inhabitants of the water.

Fig. 8.-HYDRAS.

Trembley, of Geneva, found that a Hydra might without injury be turned inside out, like the finger of a glove; and if one were cut into a number of pieces, each piece would become a perfect Hydra.

The Polypes of the next division are not naked like the Hydra, but are in tubes from which the head projects.

a

Any one who has seen the dredge bring up, from the depth of many fathoms, number of these tubes bent and twisted together, each surmounted by what presents the appearance of a head of scarlet flowers, will not readily forget the beauty of these sea-born blossoms.

The Polypes of another group have the power of altogether withdrawing into their cells. Their common habitation presents a tree-like aspect (Fig. 9). These are the Corallines, whose feathery tufts may be seen on the shell of the oyster or the mussel. Each cell is occupied by a

Polype, and all are connected together in such a way that the food taken by one contributes to the nourishment of all. So great are the numbers of these Polypes, that a single sea-weed may bear upon it a population greater than that of London or Pekin.

ORDER II.-STAR-SHAPED POLYPES.

ASTEROIDA.

"There, with a light and easy motion,

The Fan-coral sweeps through the clear deep sea;
And the yellow and scarlet tufts of ocean
Are bending like corn on the upland lea."

The animals of the present order live only in the sea. They are never found singly, but in a community, forming altogether a polype-mass, variable in form, strengthened in different ways, and having on its surface the cells in which the polypes live, and which open on the surface in a star-like figure, whence the order takes its name (Fig. 10).

To this order belong the Sea-pens and the Sea-fans. The central part of the Sea-fans is formed of a substance resembling horn, which bends under the force of streams and currents, and is, by this beautiful arrangement, preserved unbroken. To this circumstance the poet has referred in the lines above quoted. In the Isis, the stem is composed in part of a horny and in part of a calcareous substance, arranged in alF. 10.—STAR-SHAPED POLYPES. ternate joints, thus combining strength

and flexibility. When recently taken, the stem is covered with a living membrane, in which are the polype-cells. The common Red Coral resembles the Isis in having a living rind in which the polypes find shelter (Fig. 11). Inside of this is found the substance known as the Red Coral of the Mediterranean. Its growth is slow, and its short, stunted stems do not require, for their protection, the beautiful and effectual contrivances exhibited in the Sea-fan and the Isis.

ORDER III.-SEA-ANEMONES.

HELIANTHOIDA.

"Seas have

As well as earth-vines, roses, nettles, melons,
Mushrooms, pinks, gilliflowers-

As very fishes, living in the seas."

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The scientific name of the present order denotes that the animals it includes bear a resemblance to such flowers as the daisy, the marigold, and others, which the botanist terms compound" (Fig. 7, 12). The most common native species are single, with a fleshy body, live only in the sea, and have the mouth encircled with claspers or tentacula.

The common Sea-anemone, which is generally to be seen in the rockpools round our shores, may be taken as a familiar example, and one which will illustrate some of the most striking peculiarities of structure.

Viewed when the tide has receded, and the rocks are left dry, the Sea-anemones, which adhere to them, appear as fleshy, inert, hemispherical bodies, of an olive tinge, or of a liver-coloured vermilion, the tint being variable. But when