mimic poisonous species. And this mimicry is found even among birds.
Active defense implies such organs and methods of defense as are under control of the animal's will, or matters of conscious action. We shall here find much greater variety.
The homes of animals—nests, houses, burrows, etc.—are protection from the storm and for the young as well as from foes. This is a most interesting and extended field, and requires separate treatment.
But many small creatures build individual shells or cases wholly for defense against enemies. These are frequently carried about with the creature, as armor, wherever it goes. A familiar example, found in any brooklet, is seen in the case of the young caddis-fly. To hide and protect itself from the ever-hungry fishes, the larva of this insect incloses its body in a tube formed by gluing together bits of wood, shells, sand, and all sorts of matter that may be found at the bottom of a stream. This case has a silken lining, and out of the end the larva protrudes its head and legs for locomotion, or wholly withdraws out of sight and danger. Other water larvae reside within a bit of hollow straw or plant-stem. 
Fig. 8.—Caddis-Worm, with its Case. A similar habit characterizes a group of sea-worms, to which belongs the Serpula. Thin tubes may be formed of a limy secretion, or built by cementing sand, shells, etc. One of the tentacles of the Serpula is terminated by an expansion which, when the worm withdraws into its case, serves as a stopper (operculum) to securely close the opening (see Monthly, February, 1882, page 452, Fig. 4). The silk pupa-cases of the moths are very wonderful, even if very common, examples of artificial covering.
The singular hermit-crabs are obliged, on account of their lack of a hard epidermis, to inhabit some empty mollusk-shell. And they are exactly fitted for that sort of life: the tail-fin is changed into hooks for holding the shell; some of the legs are strong levers for dragging its heavy house; and one of its claws is disproportionately large, in order to close the opening of the shell.
Keen senses combined with swiftness of locomotion are the chief reliance of birds and mammals. The rabbit is a fair example. Innocent and timid, entirely without weapons, it is always on the alert. With its large eyes, ears sensitive to the slightest sound, and a delicate sense of smell, it is as difficult to surprise as it is to "catch a weasel asleep." Every deer-stalker knows he must approach his game on the side opposite the wind. Most mammals, especially the herbivorous, scent danger, and flee away. Many of them use their natural weapons only when brought to bay, and in despair. This is true, indeed, of many carnivorous beasts when they are not bold with hunger. Birds rely, for warning of foes, more exclusively upon their eyes.
