Transparent contractile vesicles of a totally different nature from the vacuoles are peculiarly characteristic of such Infusoria as have a digestive cavity. They exist either singly or in even numbers, from 2 to 16, according to the species, and never change their places; but they dilate and contract rhythmically at pretty regular intervals. When dilated, they are filled with a clear, colourless fluid, the product of the digestive process which they are supposed to diffuse through the body of the animal. The Euglena, a very extensive genus of Infusoria, have smooth bodies and green particles imbedded in the sarcode, which fills their interior; and M. Wöhler discovered that the green mantle covering the saline springs at Rodenberg and Königsborner, which consists of three species of these green Infusoria, gives out bubbles of pure oxygen; thus indicating a respiratory process in these animals, the same with that in plants, namely, fixing the carbonic acid of the atmosphere and exhaling oxygen, a singularly close analogy, if not identity, of action. The Euglenæ are also distinguished by an irregular oblong space in the head filled with a red liquid; but, as it does not contain a crystalline lens, it can only be regarded as the very earliest rudiment of an eye, totally incapable of distinguishing objects, though probably sensible to the influence of light. They swim with a smooth gliding and often rotatory motion, producing a kind of flickering on the surface of the water by the lashing of a long filament attached in front, and supposed to be their only organ of locomotion; nevertheless, Mr. Gosse thinks that they are covered with most minute cilia from their manner of swimming. The Euglena acus is one of the prettiest of these little animals; it is long and slender, of a sparkling green with colourless extremities, a thread-like proboscis, and a rich crimson spot. When it swims it rotates, and a series of clear, oblong bodies are seen towards the head, and another at the tail, as if they were imbedded in the flesh round a hollow. The Loxades bursaria, which is a giant among its fellows, has an ovoid body with green particles imbedded in its interior. The outer skin is spirally grooved, so as to form a kind of network, the elevated points of which support the cilia with which its body is beset. It has a mouth and gullet lined with cilia, which force the food in balls into the soft matter in the interior, where both the food and the green particles circulate, being carried along by a gyration of the gelatinous matter in which they are imbedded.