In man and the warm-blooded quadrupeds, in birds, reptiles, and fishes, the blood is of a splendid red colour, and the transparent vessels can be easily traced in their graceful ramifications and intricate networks, in consequence of the red blood showing through their walls. The red colour is due to a peculiar body, which can be easily separated from the blood as crystals. It has the special duty of carrying oxygen gas dissolved and attached to it; and of giving up that essential element to cause slow burning or oxydation in all parts of the body whilst taking up fresh supplies of oxygen on its passage through the lungs or the gills. In many of the lower animals (for instance, the oyster) the blood is devoid of this red crystalline substance (which, by the bye, is called hæmoglobin), and accordingly we cannot easily catch sight either of the heart or the blood-vessels (see, however, Fig. 30). But in shell-fish the blood has a very pale blue tint, and this colour is due to a substance like hæmoglobin, which also can be crystallised, and is the oxygen-carrier. Some sea-worms have a green substance of a similar nature dissolved in their blood, and one can trace their blood-vessels as a beautiful green network. A good many worms, for instance the common earth-worm and the leeches (a discovery made by Cuvier, and referred to by him on his deathbed), and many sea-worms have deep-red-coloured blood, due to the presence of the same crystalline substance which we find in man’s blood. And even a snail, common in the ponds at Hampstead and such places—the flat coiled snail known as _Planorbis_—has blood of a fine crimson colour, due to the presence of the same red oxygen-carrier, as an exception to the colourless or pale-blue blood found in most shell-fish. Perhaps if oysters, too, had red blood, there would be a prejudice against eating them in the uncooked condition.