I trust that none of my readers may suddenly faint on reading this page, but should be glad to hear of any experience of the kind. It is readily understood when the profound impression produced by the colour of man's blood is considered, that the great inquirer Aristotle and a good many uninquiring people of the present day should overlook the fact that the lower animals have blood. The insects, crustaceans, mussels, clams, snails, and cuttle-fish, and many worms have true blood and a heart and blood-vessels, but in most of them the blood is colourless, or of a very pale blue tint. Hence, like the lymph described in the preceding chapter, it escapes attention, and Aristotle called them all "blood-less animals." The fact is, however, that not only do they possess colourless or pale blue blood, but that the bristle-footed worms (earth-worms and river-worms and marine Annelids) and even the leeches possess bright red blood contained in a complete branching network of blood-vessels, whilst here and there among the otherwise colourless-blooded molluscs and crustaceans and insects we find isolated instances of the possession of red blood. Thus the flat-coiled pond-snail, Planorbis, has bright red blood, so have one or two bivalve clams, so, too, has an insect larva (known to boys as a blood-worm) that of the midge (Chironomus), so, too, have some small fresh-water shrimps, and also a single species of star-fish and one kind of sea cucumber! I explained in the previous chapter that the blood of the vertebrates may well be called hæmolymph, since in them the colourless, slightly opalescent fluid called "lymph" is continually poured through certain openings into the red blood, and mixed with it. In the earth-worm and other lower animals the red-coloured blood, or its equivalent--the "hæma," as distinguished from the "lymph"--is held in a closed system of vessels, and does not receive any of the lymph. When examined with the microscope, the blood, or hæmolymph, of man is found to consist of an albuminous, slightly sticky liquid, in which float an immense number of "corpuscles"--minute bodies, some rounded, some irregular, some bun-like, and some spherical. The most abundant of these are the "red corpuscles," of the shape of buns, slightly depressed on each surface. Three thousand two hundred of them could be placed lying flat side by side along the space of a measured inch. They appear pale greenish-yellow in colour under the microscope, but in quantity, lying one over the other, they allow only red and some blue light to pass through them, and so have a fine red colour. They consist of a small quantity of albuminous matter and water, and of a large proportion of a red-coloured, crystallizable, chemical substance dissolved in them, called hæmoglobin, or blood-red. It is this hæmoglobin which performs one of the most important duties of the blood, since it combines with the oxygen of the inspired air when the corpuscles are flowing