Page:Insects - Their Ways and Means of Living.djvu/56

 CHAPTER II THE GRASSHOPPER'S COUSINS

tendency is to produce groups rather than individuals. Any animal you can think of resembles in some way another animal or a number of other animnals. An insect resembles on the one hand a shrimp or a crab, and on the other a centipede or a spider. Resemblances amnong animnals are either superficial or fundamnental. For example, a whale or a porpoise resembles a fish and lives the life of a fish, but has the skeleton and other organs of land-inhabiting mammals. Therefore, notwithstanding their form and aquatic habits, whales and porpoises are classed as mammals and not as fishes.

When resemnblances between animals are of a fundamental nature, we believe that they represent actual blood relationships carried down from some far-distant common ancestor; but the determination of relationships between animals is not always an easy matter, because it is often difficult to know what are fundamental characters and what are superficial ones. It is a part of the work of zoologists, however, to investigate closely the structure of all animnals and to establish their true relationships. The ideas of relationship which the zoologist deduces from his studies of the structure of animals are expressed in his classification of them. The primnary divisions of the Animal Kingdom, which is generally likened to a tree, are called branches, or phyla (singular, phylum).

The insects, the centipedes, the spiders, and the shrimps, crayfish, lobsters, crabs, and other such creatures belong to the phylum Arthropoda. The name of this phylumn means