SECTION III.
HYDROZOA, ZOOPHYTES.
Zoophytes are animals of a much higher organization than the Protozoa, inasmuch as they are furnished with special organs of prehension, offence and defence, of attachment, and in many of locomotion. For the most part they consist of numerous individuals called Polypes, united in a community, and living together in intimate sympathy and combined action, so as to form one single compound animal.
Zoophytes are divided into two groups, namely the Hydrozoa, whose type is the common fresh-water Hydra, and the Actinozoa, which are composite animals, including the reef-building corals, whose polypes are formed according to the type of the Actinia, or common Sea Anemone. The Hydrozoa consist of seven orders, the first of which are the Hydridæ, inhabitants of fresh water; the next constitute the oceanic Hydrozoa, some of which, though extremely varied in form, are connected by the most wonderful relations.
The solitary Hydra that lives in fresh-water pools and ditches, consists of a soft cylindrical muscular bag, capable of being stretched into a slender tube, shrunk into a minute globe, or widely distended at will. At one end there is a circular mouth, which is highly sensitive, opening, closing, or protruding like a cone, and surrounded at its base by six long flexible arms called tentacles, arranged symmetrically. The mouth opens into a cavity extending throughout the length of the body, which is the stomach; the other end of the sac is narrow, and terminates in a disk-shaped sucker, by which the Hydra fixes itself to aquatic plants, or floating objects, from whence it hangs down, and the tentacles float in the water.
Fig. 109. Thread-cells and darts.—A, B, C, D, Thread-cells at rest; E, F, G, H, appearance of the darts when projected.
The sac or body is formed of two layers, an inner and an outer layer, of firmer texture, formed of cells imbedded in a kind of sarcode, and the space between the two layers is filled with a semifluid substance, mixed with solid particles and full of vacuoles. The inner and outer layers are united at the mouth, and the tentacles are closed tubes in communication with the cavity of the stomach. The exterior layer of the tentacles is beset with wart-like excrescences, formed of clusters of cells, with a larger one in the centre filled with a liquid. In all of them a long spicula, or sting, often serrated at the edge, is coiled up like a thread, and fixed by one end to a kind of tube, like the inverted finger of a glove, that the animal can dart out in an instant.
Thus armed, the tentacles are formidable weapons; they are highly contractile and wonderfully strong, tenaciously adhering to the small worms and aquatic insects on which the Hydræ feed, and they are aided by the roughness of their surface. They transfix their prey, and are believed to infuse a liquid poison from the dart, or thread-cells, into the wound, then twisting their other tentacles round the victim, it is instantly conveyed to the mouth, and slowly forced into the digesting cavity, where it is seen through the transparent skin to move for a short time, but as soon as the nutritious juice is extracted, the animal ejects the refuse by its mouth. In the inner layer, enclosing the cavity of the stomach, there are cells containing a clear liquid with coloured particles floating in it, which is supposed to perform the part of a liver; and, as the Hydræ have no respiratory organs, their juices are aërated through their skin. They have no perceptible nerves nor nerve centres, yet they are irritable, eminently contractile, and are attracted towards the light—all these being probably sympathetic motions.
Though in general stationary, the Hydra can change its place; it bends its body, stretches to a little distance, and fixes its anterior extremity firmly by its tentacles; then it detaches its sucker and brings it close to its mouth, fixes it, and again stretches its fore part to a little distance along its path, and repeats the same process, so that it moves exactly after the manner of certain caterpillars. It can even move along the water by attaching the expanded disk of its sucker to the surface, where it soon dries on being exposed to the air, and becomes a float, from whence the Hydra hangs down with its tentacles extended like fishing lines, as in [fig. 110]; or it can use them as oars to row itself along under the surface of the water.
On account of their simple organization, the Hydræ are endowed with the most astonishing tenacity of life. As the whole animal is nourished from the surface of the digestive cavity, they appear to suffer no inconvenience from being turned inside-out, the new cavity performing all the functions of digestion as well as the old one. They may be cut into any number of pieces, and, after a little time, each piece becomes a perfect Hydra. The head may be cut off and they get a new one; or it may be split into two or three parts or more, and the animal becomes many-headed; and, what is still more marvellous, two Hydræ may be grafted together direct, or head and tail, and they combine into one animal.