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.