The above outline will be sufficient to show that insects are not very unlike the crustaceans in their general characteristics; and, indeed, when we examine certain forms, noting the distinct segmentation of the body, the hardened exo-skeleton of chitinous material, and the unhardened skin between the segments to admit of freedom of movement, we see a striking resemblance in external appearance to some of the typical crustaceans. Insects are divided into several orders, and some of these are fairly well represented on the sea coasts, though it must be understood that but few species are strictly aquatic and marine in their habits. Fresh-water pools and streams teem with insect life, and quite a large number of the insects that live in these situations are peculiarly adapted for a life of submersion, their general form being often such as to allow of rapid progress through the water, their appendages modified into admirable swimming organs, and, in many cases, their breathing apparatus adapted for the direct absorption of oxygen dissolved in the water. However, one would hardly expect to find similar forms of life abundant in the water that washes our shores, the disturbing action of the waves, even in calm weather, being more than such fragile creatures could withstand. And this is really the case, for there are but few insects that may be described as marine in the strictest sense of the word; and of these the species that have been observed are mostly inhabitants of warmer seas. It is noteworthy that all the insects which exhibit marine tendencies are small, and they seldom, if ever, live permanently below the surface. But few of them can swim. A few run on the surface of the water, supporting themselves on the surface film after the manner of water-gnats, whirligig beetles, &c., without ever being wetted; and these are said to feed on different kinds of floating matter, and occasionally to dive below the surface. A rambler on the sea shore in the summer time will always meet with plenty of insect life, but the number of species observed may not be large: and omitting all those which show no decided preference for the coast, but are found in inland districts as well, we find that by far the larger proportion live at or near the high-water mark, where they feed on the refuse washed up by the waves. Some species, however, live among the stones, or burrow into the sand, between the tide-marks; and these, as a rule, are not driven inland by each advancing tide, but allow the sea to wash over them, having at first protected themselves from disturbance by burrowing or seeking other suitable shelter.