1 48 SCIENCE FROM AN EASY CHAIR 



branch from it like a tree into the living tissues of the 

 body, whence it returns by other vessels (the veins) back 

 to the heart. 



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 conse- 

 quence 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 haemoglobin), 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 sub- 

 stance like haemoglobin, 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 excep- 

 tion to the colourless or pale-blue blood found in most 



