Insulated groups of germs covered with cells called swarm-cells seem to form parts of the sponges; they lie completely within the mass of the living sponge. They have the form of a hen’s egg, are visible to the naked eye, and when they come into the water they swim in all directions for a day or two; become fixed; a white spot within is enlarged; and the constituents of young sponges appear.[16] The generic forms of fossil sponges augment in number and variety from the Silurian to the Cretaceous beds, where the increase is rapid; but all the sponges which had a stony reticulated form without spicules passed away with the Secondary epoch, so that the family has no representatives in the Tertiary deposits or existing seas. The calcareous sponges which abound in the Oolite and Cretaceous strata, and attain their maximum in the Chalk, are now almost extinct, or are represented by other families with calcareous spicules. Siliceous fossil sponges are particularly plentiful. In England extensive beds of them occur in the Upper Greensand, and in some of the Oolitic and Carboniferous Limestones; and some beds of the Kentish Rag are so full of their siliceous spicules, that they irritate the hands of the men who quarry them. Since every geological formation except the Muschelkalk is found in England, the number and variety of fossil sponges are very considerable. The horny sponges are more abundant now than they were in the former seas. According to M. D’Orbigny the whole number of fossil sponges known and described amount to thirty-six genera and 427 species, which is probably much below the real number.[17] CLASS IV.—INFUSORIA. The Infusoria, which form the second group of the Protozoa, are microscopic animals of a higher grade than any of the preceding creatures, although they go through their whole lives as isolated single cells of innumerable forms. They invariably appear in stagnant pools and infusions of animal and vegetable matter when in a state of rapid decomposition. Every drop of the green matter that mantles the surface of pools in summer teems with the most minute and varied forms of animal life. The species called Monas corpusculus by the distinguished Professor Ehrenberg, has been estimated to be 1/2000 part of a line in diameter. ‘Of such infusoria a single drop of water may contain 500,000,000 of individuals, a number equalling that of the whole human species now existing upon the face of the earth. But the varieties in size of these animalcules invisible to the naked eye are not less than that which prevails in almost any other natural class of animals. From the Monad to the Loxades or Amphileptus, which are the fourth and sixth part of a line in diameter, the difference in size is greater than between a mouse and an elephant; within such narrow bounds might our ideas of the range in animal life be limited if the sphere of our observation was not augmented by artificial aid.’[18]