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These organs are doubtless a rudimentary form of circulatory apparatus.
There are no distinct organs of digestion, nor breathing organs, nor nervous system, and no traces of a circulatory system, beyond the little contractile chambers just mentioned.
Paramoecium has the power of multiplying itself by dividing into two parts, either transversely or longitudinally (fig. 2, B, C); but it can also produce young by means of eggs.
RECAPITULATION OF ESSENTIAL CHARACTERS. — The body is usually composed of three distinct layers, and is generally provided with the little vibrating filaments which are known as "cilia.” A mouth is present, but there is no distinct stomach or body-cavity. There are no digestive or respiratory organs, and no nervous system, and the only traces of a circulatory system are to be found in one or more little pulsating sacs or chambers. These characters distinguish the class Infusoria as a whole.
The animals which compose this class are for the most part inhabitants of the sea; and, from their small size, or their plant-like appearance, or again, their living far from land in the open ocean, they can hardly be said to be at all known in general. As the type of this class we shall take a small and abundant form which occurs in many ponds and lakes in Europe, namely, the common Fresh-water Polype (Hydra vilgaris), from which the name of the entire class is derived (Greek, hudra, a water - dragon, hence a Fresh - water Polype; zoön, an animal).
The body of the Hydra (fig. 3, A) is in the form of a simple cylindrical tube, the wall of which is composed of two distinct layers, an outer and inner (shown by the dark and light lines in fig. 3, B). At its base the body
Fig. 3.-A, The Common Hydra (Hydra vulgaris), carrying young Hydro
which it has produced by budding, considerably magnified (after Hincks); B, Diagrammatic section of the Hydra, showing the mouth surrounded by the tentacles, and the disc of attachment; the dark and light lines indicate the two layers of the integument, and on one side of the body is shown a single large egg.
forms a little disc or sucker, by means of which the animal can attach itself to stones, floating pieces of wood, or the stems of water-plants. It can also, by means of this disc, glide slowly, like a snail, over the surface to which it is fixed; but it can detach itself entirely when it chooses.
At the opposite end of the body is placed the opening of the mouth, surrounded by from seven to twelve flexible
and extensible processes, which are termd the “tentacles” (Latin, tento, I touch). By means of these feelers the animal seizes its prey.
The mouth opens into a long cylindrical cavity, which occupies the whole length of the body, and which receives the food. This cavity may therefore be regarded as the stomach; though it is really the general cavity of the body, since it is enclosed by the general integument. Nothing further can or need be said about the internal anatomy of the Hydra; since the undigested food is simply expelled through the mouth, and there are no traces of a nervous system, of breathing-organs, or of any circulatory apparatus.
The walls of the body in the Hydra are very soft and contractile, and the animal can pull itself together into a shapeless lump when irritated, or can expand itself and thrust out its tentacles when in search of food. The outer layer of the body, and especially of the tentacles, is roughened by innumerable little microscopic bodies, which are known as "thread -cells" or “ nettle-cells.” These curious little organs (fig. 4) are seen, when highly magnified, to consist of a little bladder filled with fluid, and carrying at one end a long filament or thread. This thread can be darted out with great rapidity and force, and it is used by the animal in capturing its prey. The thread-cells are too weak to pierce the human cell of the Hu. skin, but they wound the soft bodies of the dra;. much worms and other minute animals upon which magnified. the Hydra feeds, and they appear also to exercise some poisonous or benumbing effect.
In the summer time, the Hydra produces young ones by a process of budding, as is seen in fig. 3, A, much as a plant throws out buds; but these young are detached to lead a separate life, when they are fully grown. In the autumn the Hydra also produces eggs.
The Common Hydra is found abundantly in most
streams and ponds. Though not very much larger than the head of a large pin when contracted, the animal can push itself out to a considerable length, and is generally easily recognised by its brownish-red or orange colour. The Hydra, lastly, has a great power of resisting mutilation or mechanical injury. If cut up with a knife, all the pieces will grow and develop themselves into fresh Hydro, and the animal may even be turned inside out, without appearing to suffer thereby.
RECAPITULATION OF ESSENTIAL CHARACTERS. — The body is composed of two distinct membranes or layers, an outer and an inner, of which the outer is furnished with the offensive weapons known as thread-cells. There is a mouth, surrounded by tentacles; but the mouth opens into a large chamber, which may be regarded as the stomach and body-cavity in one. There are no distinct organs of circulation, or respiration, and no traces of a nervous system. These characters distinguish the Hydrozoa as a whole.
The chief animals comprised in this class are the so-called Sea-Anemones and Corals; and the scientific name of the class is derived from the fact that the body generally shows a distinctly star-like arrangement of its parts, all of which “radiate” from a common centre (Greek, aktin, a ray; zoön, an animal). They are therefore “radiated” animals, and a good example of the class may be found in the Actinia mesembryanthemum, one of the commonest of the British Sea-anemones.
This familiar Sea-anemone (fig. 5, A) has, when undisturbed and in a state of activity, the form of a short
cylinder or column, the base of which forms a broad muscular disc, by means of which the animal fixes itself
Fig. 5.-A, Actinia mesembryanthemum, one of the Sea-anemones (after
Johnston); B, Section of the same, showing the mouth (a), the stomach (b), and the body-cavity (c); the dark and light lines show the two integments of the body.