CRABS, insects, centipedes and spiders together form a group called the Arthropoda or animals with jointed legs. The first three of these form quite a definite series and appear to be sharply separated from the spiders. In addition there is another animal called Peripatus—a primitive, worm-like creature, retiring in habit and living in damp places in the tropics of Africa and America, in Australia and New Zealand. It is an interesting animal, because it shows affinities with worms on the one hand and Arthropoda, especially the insects, on the other; we shall say no more about Peripatus, but it should be remembered as one of those synthetic sort of animals which combine the qualities of two or more widely separated groups and whose very existence enables us to build up a fairly accurate knowledge of how higher groups may have evolved from the lower.
The arthropod body consists of a series of rings and, theoretically, each ring or segment carries a pair of legs. This is well shown in the centipedes. When we come to the higher forms, these legs tend to disappear from some of the segments, fusion of the segments takes place and specialisation for various purposes sets in. Some of the legs are no longer used for walking but become modified as jaw-feet. On account of changes of this kind, we shall find it convenient to describe the legs and similar structures as appendages, a term which can include antenna? or feelers, claws of crabs, claw horns or cheliceras of spiders as well as the modified jaw-feet of Crustacea (lobsters, crabs, shrimps, etc.) and of insects.
Arthropods may be recognised by their strong, horny armour which would greatly hamper their movements were it not for the many-jointed limbs from which the group gets its name. The possession of antennas or feelers is quite characteristic. Like practically all other groups of the animal kingdom, and as we should expect from their great diversity and numbers, the Arthropoda has representatives in every sphere of animal activity; they flourish in the air, on and in the earth, and in the seas, rivers and ponds. It is interesting
to recall that even the typically water-loving Crustacea have sent a few representatives to the land as in the case of the land crabs and wood lice. This almost universal distribution is the outcome of success in evolution brought about by a remarkable adaptability on the part of the organisms—there is, for example, a shrimp which lives in concentrated brine !
In the life histories of two great sub-groups of the Arthro-poda—the Crustacea and the Insects—there is rarely a direct change over from egg to adult comparable to the way in which a hen’s egg gives rise to a chick of more or less the same form as its parents; there are phases interpolated, each phase being marked by a stage in the life history which is quite characteristic in form and structure. In other words, the young arthropod may look very different from its parents. Such a series of changes is referred to as metamorphosis and the complexity of the metamorphosis may vary very much in character and degree in the different members of the group. Unlike many of the lower animals, the sexes are usually separate.
The Crustacea, which includes the Crabs and Lobsters, is typically a water-inhabiting group breathing by gills. The Insecta, which typically dwells on the land, breathes by means of air-tubes.
The arthropod group has a particular claim to man’s attention, for it is important to him in a variety of ways, pleasant and otherwise—it may afford him food like crabs and shrimps; it may destroy his possessions as the termites do; it may ruin his crops in the way the Colorado beetle does; it carries disease through the house-flics, mosquitoes and lice; it supplies the bees which pollinate his flowers.