Animals That Approach the Simplest Forms of Life Worms, Leeches, Sea-Anemones, Coral-Polyps, Jelly-Fishes, Sponges
Several groups of the lower animals are collectively known as Worms, though most of these groups are but remotely related. Ringed worms are elongated creatures in which the body is made up of a considerable number of rings or segments, most of which are, on the whole, much alike. There is often a well-marked head, but no distinct thorax and abdomen, as in an insect or crayfish. Two subdivisions are recognized: (a) Bristle-worms (Chætopoda) and (b) Leeches (Discophora).
Bristle-worms include a host of marine worms, together with some that live in fresh water, and also the earthworms. Their average characters are best understood by examining one of the commonest shore-worms, known as the sea-centipede (Nereis). Here the segments are very clearly seen, and almost every one of them bears a pair of unjointed conical foot-stumps, used for crawling.
Imbedded in the foot-stumps of the sea-centipede are bundles of strong bristles, which give a hold on the underlying surface and prevent slipping. The head-region is fairly distinct, and bears a number of feelers of various kinds, as well as four simple eyes. Sea-centipedes and many of their allies are highly carnivorous, and seize their prey by means of a pair of horny jaws which can be protruded at will.
Earthworms are found in all parts of the world, though naturally they do not thrive in arid tracts; and their effect upon the fertility and drainage of the soil can hardly be calculated. Burrowing into the ground, they cast up the earth they have swallowed, and so pursue a constant and thorough system of ploughing. Though eyeless, they evade the light and only come out of their burrows at dusk, often remaining, even then, with their tails in the holes and their bodies working round and round.
Darwin long since demonstrated, the earthworm is one of the farmers’ best friends. Its burrows drain and aerate the soil, while the earth which has passed through its body is finely divided and constantly being brought to the surface from lower levels.
Not far from the front end of an earthworm a thickening will be seen, often erroneously supposed to be the result of injury. From it exudes a fluid which hardens into the egg-cases.
Leeches live in the sea, fresh water, or even in damp, tropical forests. The flattened body of the leech is divided by grooves into a number of narrow parts, several of which go to make up a segment. Foot-stumps and bristles are entirely absent, and progression is effected by means of suckers, one at each end. They effect a looping movement, but the animal can also swim by undulations of its body. The freshwater leech is a bloodsucking parasite. The mouth is situated in the middle of the front sucker, which serves to fix the animal to its victim. Three saw-edged jaws are then brought into play, a three-rayed cut being made, and a fluid poured out which prevents the blood from clotting. Digestion is slow, and the food is stored in a large crop, drawn [243] out into numerous pairs of pouches. The head possesses eye-spots, but no feelers.
Animals Like Plants
ANIMALS LIKE PLANTS (Cœlenterata) of which sea-anemones, corals, and jelly-fishes are examples, are distinguished by the ray-like symmetry of starfishes and their kind, though here, as a rule, it is more perfect. In structure they are much simpler, than any of the animals so far considered. For such a creature is to all intents and purposes simply a stomach, the wall of which is made up of two layers of cells, one (ectoderm) external, and the other (endoderm) internal. In higher animals a third layer (mesoderm) is interposed.
Sea Anemones are common between tides and lower on all coasts. They are cylindrical animals, with a mouth surrounded by tentacles at one end. Inside there is a single cavity which serves as a stomach and whose branches run to all parts of the body, thus distributing the food like a blood vessel. The colors, especially in the tropics, are variable, and often gorgeous.
Coral-polyps are closely related to sea-anemones, but differ from them by secreting a hard, limy skeleton in the base of the body. They are either simple or compound. The well-known mushroom coral may be taken as an example of the former. Its skeleton is a shallow cup, exhibiting numerous radiating plates. If we look at the upper surface of such a coral in the living state we shall see a mouth surrounded by circlets of tentacles, much as in a sea-anemone.
A compound coral consists of a number of individuals, relatively small in size, connected together by a common flesh, and formed by the budding or splitting of a single original polyp, the results of the process remaining united.
Many corals branch, while others form compact masses, as in the kind above described, and also in the brain coral, where the boundaries between the individuals are not clearly marked.
Corals are widely distributed, some living even in cold latitudes, and others on the floor of the deep sea. Coral reefs, however, made up of the skeletons of such animals, are only found in the warmer parts of the ocean, where the water is clear, particularly favorable conditions being afforded by the Pacific and Indian Oceans. (See also Coral Reefs and Islands).
Jelly-fish (Medusæ).—All agree in having a more or less bell or umbrella shaped body, with a proboscis hanging down in the place of the handle of the umbrella or the tongue of the bell. The mouth is at the end of the handle and leads into a stomach which divides and sends out branches, like the ribs of the umbrella, to the margin. The common name is due to its gelatinous consistency. Most of the species start in life as buds from attached animals, which later separate and henceforth lead a free existence, swimming by opening and closing the bell.
Sponges
SPONGES (Porifera) are animals of peculiar structure, which resemble zoophytes in many respects, but possess neither tentacles not thread-cells. Some are simple, but most of them are compound. A simple sponge may be compared to a cup or vase with a wall perforated by numerous small holes, through which currents of sea-water stream into the central cavity, to make their exit by the main opening. They are set up by ciliary action.
Venus Flower-basket.—In the majority of cases the skeleton of a sponge is mostly or entirely made up of sharp needles of lime or flint, which may be welded together, as in this form. Often the opening of the vase is provided with a convex perforated covering. Another elegant form is the Glassrope Sponge native to the Japanese seas. It is rooted in the mud by a bundle of long, glassy spicules, which are slightly twisted.
Most sponges are marine, and, despite their fixed habit and apparent helplessness, are pretty free from the attacks of most other creatures, partly because of the innumerable sharp spicules they contain, and partly because their taste and smell are unpleasant. These deterrent qualities are often associated with bright warning colors, generally red, yellow or orange.
Most of us little realize that the sponges we see or use daily are in reality dead animals.
CROSS-SECTION OF LIVING SPONGE
THE SPONGE IN ACTION
The arrows show how the water enters by the small pores, to pass out by the large openings. Food is thus brought to the cells which line the channels. At the right the currents of water are seen passing from the outer openings, as seen under a powerful microscope.
THE BORDER-LINE BETWEEN ANIMAL AND PLANT LIFE
On the left are represented highly magnified animal skeletons of a class of Protozoa called Radiolarians. Most of them are under one-twenty-fifth of an inch in size. Millions upon millions of these little shells are found upon the floors of the ocean, and upon its shores. They are marvels of form and color—so wonderful, indeed, that man with all his skill cannot imitate them. When alive they consist of but a single cell, and live in colonies with the plant forms, called algæ, pictured on the right. The algæ are also single-celled, and of rare beauty of form, and the strange association of the simplest of animal forms with the simplest of plant forms has up to the present time proved the supreme enigma of science.