. Le microscope ; un simple manuel. Les microscopes. 132 LE MICROSCOPE créature, lias aussi un grand disque-comme la surface où il n'est pas sldii- automatiquement à sa gelée. Les cils qui vibrent autour de ce disque cause un puissant courant de couler, et il est étonnant de voir les plus petits types de protozoaires, d'être bousculés dans le corps de la créature, où ils nagent environ une seconde ou deux et sont alors encore. Comme une étape finale nous trouver les créatures avec seulement une petite ouverture où la nourriture peut être absorbée, et le développement complet de la bouche est ici terminée. L'une des premières figures montre le délicat l

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. The microscope; a simple handbook. Microscopes. 132 THE MICROSCOPE creature, lias also a large disc-like surface where there is prac- tically no sldii to its jelly. The cilia which vibrate around this disc cause a powerful current to flow, and it is amazing to watch the smaller kinds of Protozoa being hustled into the creature's body, where they swim about for a second or two and are then still. As a final stage we find creatures with only one small open- ing where food can be absorbed, and the complete development of the mouth is here concluded. One of the earlier figures shows the dainty little Collared Monad, which consists of a single cell with a vibrating whip, or flageUum, and a very perfect little cup or collar of transparent material. They are often found in large colonies on the surface of weeds. To them we owe our sponges. A microscopic examina- tion of one of the holes of a growing sponge reveals a colony of these little organisms, closely arranged aU round the interior of its surface. These have the power of creating instead of shells a fibrous material, which forms the matrix in which they are embedded. In place of shells they deposit hard, flinty spines in the substance of this matrix, and these are called spicules. Thus a sponge consists of myriads of colonies of Collared Monads, their vibrating flageUse causing a current of water to rush through every cavity of the entire sponge, in order to provide the food and oxygen necessary Fig. 128.—Sponge. for the support of the community. The Protozoa show in a series of inter- esting stages the gradual development of creatures of one cell. Each ceE is complete in itself, though, as in the case of the sponge, an approach to a more elaborate form is seen. Never- theless, here each organ eats for itself, breathes for itself, re- produces itself by splitting in half, and is an individual. Later stages of development show creatures of more than one cell, in which some cells perform one function and some anoth

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