If we look at the structure of an earth-worm or of one of the graceful marine worms (Nereis or Arenicola), we gain a good deal of light as to the nature of the lymphatic system of Vertebrates. Suppose you have killed a large earth-worm with chloroform! Then pin it out on a cork plate, and open it by a cut along the back with a fine pair of scissors. The point of your scissors passes through the muscular body-wall of the worm into a great chamber filled with a clear liquid. This chamber is the "cœlom," and is the same structure as the pleural and peritoneal chambers of the Vertebrate. But it holds (proportionately) more liquid. The liquid is "lymph," like that of the Vertebrate, and has numerous protoplasmic cells floating in it. There is comparatively little connective tissue in the earth-worm. The cœlom is free and unblocked--the great viscera lie in it. There are some delicate, transparent bands of connective tissue, but not much nor bulky. The wall of the cœlom itself is lined with connective tissue, and if that tissue grew greatly in bulk, and bound all the organs and muscles together, it would reduce the large cavity, filling it up with spongy tissue in the small interstices of which there would be lymph. And so we should get a lymph system resembling that of Vertebrates, instead of one large chamber. But what about the opening of the lymphatics into the blood-vessels? This is one of the interesting differences between the earth-worm and the Vertebrate. The earthworm and many marine worms have a beautiful system of vessels, containing a bright red blood, and forming true capillaries, connecting arteries and veins. The heart is a long, rhythmically beating tube, extending along the whole length of the animal just above the intestine. There is no opening into it of the lymph-cavity. It is purely a respiratory blood-system, pumping its fluid, coloured red by oxygen-seizing hæmoglobin into every part of the body. It passes along the fine capillaries of the skin, where it seizes oxygen from the outside air or water and carries it to all the tissues. The fact is that the red respiratory element of the blood which we call the "hæma" or hæmal portion (the Greek word for red blood is αἷμα) is here kept separate from the nourishing and elaborating element, the lymph or lymphatic portion. So that we should, to be explicit, describe the blood of a vertebrate as "hæmolymph," a conjunction of hæma and lymph, which in the more primitive earth-worm and sea-worm have never effected a junction! In some closely allied marine worms, however, a junction of these two is effected in another way. We know that in the Vertebrates the red blood corpuscles are formed by detached bits of the same tissue, which becomes converted into capillaries, the finest blood-vessels. Now in several marine Chætopods or bristle-footed worms (Glycera, Capitella, etc.) the tissue which should form the blood-vascular system and its red liquid