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wonderful regularity of structure. Their forms are extremely various, the species being very numerous. Some are simple, and of these some

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1-7, Various species; 8, Part of two chambers of one; 9, Vertical Section of a fossil species.

are orbicular, others curiously flask-shaped; but many are chambered, and thus composed of parts variously arranged, sometimes in a straight line, sometimes spirally, but always regularly. These chambered shells seem to be formed in consequence of what is called gemmation,1 or the growth of new individuals from an existing animal, as of buds on a tree. The shells of all the Foraminifera are pierced with numerous pores, through which long delicate processes-extensions of the soft body of the animal-are protruded when occasion requires.

Sponges. Sponges, the only Protozoa that attain a large size-which they do, however, merely in virtue of the formation of aggregate or compound animals by gemmation-are perhaps the lowest of all creatures in the scale of animal life. The movements of a living sponge suggest the idea of animal rather than of vegetable life, but its form and manner of growth are plant-like. It is fixed by its base like a sea-weed; it seems utterly insensible to touch; it may even be pinched with a forceps, or torn to pieces, or bored with a hot iron, without a symptom of suffering. No wonder, therefore, that sponges should long have been regarded as plants. When the life-history of a sponge, however, is considered, its animal nature at once becomes certain. A sponge in its mature state generally consists of a horny fibrous framework, which is all we see in the

1 From Latin, gemma, a bud.

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sponges of commerce, and which may be regarded as the skeleton of the animal: sometimes the skeleton is formed of calcareous or siliceous needles, which are beautiful microscopic objects, admirably grouped and arranged. In a living sponge, the whole framework is covered and its cavities filled with a gelatinous or glairy substance, which is the really living part. A mature sponge is, in fact, composed of a vast number of creatures somewhat resembling Amabæ, aggregated together, and having their framework in common. A young sponge, as it comes forth from the egg of its parent, has the faculty of moving about in the water by shooting out processes of its soft body, like the Rhizopoda. By-and-by, it attaches itself to some rock or other fixed substance, and becomes itself immovably fixed, ceasing to possess the power of locomotion. Its plant-like framework begins to grow, and the animal mass rapidly increases. Every one knows that the surface of a piece of sponge is perforated with a multitude of very small openings, amongst which are a number of larger ones, and that the inside is full of little cavities. In the live sponge there are currents of water continually flowing in at all the smaller openings, which are carried through the whole mass, conveying to every part of it the particles of matter which serve for nourishment, and also the air for its respiration. These currents of water are collected into canals, and pass out by the larger openings, carrying with them all the refuse, and many sponge buds, as they may be termed, which become detached from the parent, and go forth to found new colonies, as well as the eggs which are occasionally produced.

Infusoria. The Infusoria are a class of Protozoa of somewhat higher organisation than the Rhizopods and Sponges. They have a mouth and a gullet, which, however, does not convey the food into a stomach, but into the general internal cavity of the body. Like the other Protozoa, they are composed of a gelatinous substance, but it is covered with a skin, and does not exhibit that variability of form so remarkable in the Rhizopoda. Many of them are also furnished with hair-like organs, called cilia,1 the motion of which carries them with great rapidity through the fluid in which they live, and also creates currents to bring food to the mouth. The Infusoria often multiply by spontaneous division, and each half speedily acquires a mouth and cilia for itself. The multiplication of some Infusoria in this way is extremely rapid. One species has been observed to undergo division every twenty-four hours, from which would result 16,384 individuals in a fortnight, and 268,435,456 in four weeks. It is from the fact of their occurring in all infusions of animal and vegetable substances which stand for a short time exposed to the air, that the Infusoria receive their name. There are very many different kinds, exhibiting great variety of form and structure. Their multitudes are so

1 From Latin cilium, an eyelash.

great, that leagues of the ocean are sometimes tinged with them, although they are individually so minute that the number contained in a single cup of water may exceed the whole human population of the globe.

Cœlenterata.

Superior in organisation to the Protozoa, and generally regarded as ranking next above them, are the groups which unitedly receive the name of Colenterata.1 A great similarity of structure appears in all the Cœlenterata. Their bodies are composed of two layers, one of which forms the external covering, and the other lines the internal cavities. The body is generally soft, and has a digestive cavity with one opening or mouth, which is generally surrounded by a fringe or circle of arm-like tentacles 2 or feelers, employed to capture prey and convey it to the mouth-the first organs of prehension that have come under our notice in this survey of the animal kingdom. The mouth is not furnished with jaws, teeth, nor any organ of mastication, but the food which it receives is conveyed

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Fig. 64.-Campanularia dichotoma,

magnified.

Fig. 65.-Plumularia falcata (natural size):

a, the ovarian vesicle and four of the polype-cel's of P. falcata, magnified.

at once by a short canal into the digestive cavity. The Colenterata, however, are provided with other organs besides their tentacles for the purpose of obtaining prey. Deadly poisoned lances, long and slender, lie coiled

1 From Greek coilos, hollow, and enteron, an intestine.

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2 From Latin tento, to feel.

up

in cells around their tentacles, or in some part of the outer layer of the body, and are darted out to wound and sting to death any worm or other small animal that may come within their reach. The Cœlenterata are all aquatic animals, most of them marine. Some are attached by a base to a rock or other fixed object, the mouth being at the opposite extremity, while others swim freely in the water. Many are compound animals, increasing by gemmation, the common stem branching in very various and beautiful forms in the different species, and the individuals—if they may be so termed-which compose the community, being arranged with as perfect regularity as the flowers of any plant. From their plant-like appearance, many of them have been called Zoophytes,1 and some have, for a similar reason, received the popular names of Animal-flowers and Sea-anemones. From the circle of tentacles surrounding the mouth, many are termed polypes or polypi.2 The common stem of many of the compound species is protected by a horny or calcareous secretion, which gives it strength to resist the action of waves and currents, and having a minute cup or cell for each polype to retire into upon the approach of danger. Coral reefs are formed by the calcareous secretions of polypes, which remain as stony masses after the polypes have died, and afford a foundation on which other polypes may proceed to build. Great tracts of country and many whole islands of the Pacific Ocean consist of coral rock, the work of these tiny and apparently insignificant animals.

The Hydra. The polypes of the genus Hydra may be regarded as the type of one of the chief groups of Colenterata. They are inhabitants of fresh water, and may often be found adhering to stones, submerged leaves, or twigs. When not looking out for its prey, the Hydra has the appearance of a small gelatinous button; when active, it extends itself into the form of a hollow cylinder, less than an inch long, with a circle of slender

Fig. 66.

tentacles round the mouth (a, fig. 66, represents a Hydra in this state). The body may be described as consisting merely of an outside skin and the lining of the cavity that traverses its whole length from the mouth. This cavity is all the digestive apparatus the Hydra has; indeed, its structure is so simple, that it may be turned outside in without being destroyed, for the outer layer just adapts itself to do the duty of the inner, and the inner to do that of the outer.

It will be noticed that on the side of the Hydra in the figure, there is a little one growing. It is thus that the young are produced by gemmation. A wart-like knob begins to grow; and as it grows it becomes more and more like the parent animal, till at last it becomes contracted where it is

1 From Greek zōon, an animal, and phyton, a plant.

2 From Greek polys, many, and pous, a foot.

joined to the parent, and finally is cut off altogether, and commences an independent existence.

The Actinia. The Actinia, or Sea-anemone, may be taken as a type of another chief group of Colenterata. There are many species of Actinia, and some of them are common on the British shores. It is in tropical seas, however, that they are most numerous, and attain the greatest size and beauty. An Actinia consists of a cylindrical body, the lower end forming a disc by which the animal is generally attached to a rock or other object, whilst at the other end is the mouth surrounded by numerous tentacles arranged in circles, like the petals of a flower. Many of the Actinia vie with the finest flowers in the delicacy and brilliancy of their colours. When left dry by the receding tide, or when rudely touched, they contract into a jelly-like mass. In this condition, the most common British species must be familiar to every one who has gathered periwinkles, or searched for shells on a rocky shore. It is far from being the most beautiful of our British Actiniæ, but it is one of those most easily kept in the aquarium. It is capable of subsisting for a considerable time without supplies of food, but is very voracious, and readily accepts morsels of almost any animal substance. When food comes within reach of the tentacles, those nearest to it lay hold of it, and convey it not only into the mouth, but into the stomach, not letting go till it is fairly lodged there. Many animals seem to have an instinctive horror of the tentacles of the sea-anemone. The hermit crab instantly flees out of its shell, if the shell is caught by them; as any one may readily observe for himself, who, having found a sea-anemone of good size in a rock-pool, brings a hermit crab cautiously near to it.

Radiata.

The characters upon which Cuvier founded the primary division of the animal kingdom which he called Radiata, or Radiated Animals, may be seen in their greatest perfection in star-fishes. These creatures belong to a class to which the name of Echinodermata is given from their generally rough or spiny integument. They are much higher in organisation than any of the groups hitherto noticed; they have a distinct digestive system, and a distinct vascular system, although for the former many of them have only a single orifice. A circular and radiating nervous system has been observed in many. They are especially distinguished from all the Protozoa and Calenterata by their well-organised skin, which is often strengthened by calcareous plates, and sometimes has the additional protection of numerous long spines, as in Sea-urchins; and are further characterised by their water-vascular system, or apparatus for watercirculation, a peculiarity of the radiate animals. This water-vascular

1 From Greek echinos, a hedgehog, and derma, skin.

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