556 FOSSIL ANIMALCULES. nally soft tissue of the body in a dry state. By exposing this substance to a white-heat, the animal matter is driven off, and the delicate siliceous shields are obtained in a perfectly unaltered state. By this process it is found that about 25 per cent. of dry animal matter is contained in the earth. In some of these collections, the animalcules are of large size, their siliceous shields measuring as much as Too of an inch in length. In the older strata of the tertiary formation extensive deposits are found. One of the most remarkable of these, is that which is termed the Polierschiefer (polishing slate) of Bilin; this supplies the tripoli used by artisans in metal for polishing their work, and also the fine sand employed to form moulds for 110 FIG. 627.-FOSSIL REMAINS OF ANIMAL. more casting small articles of Berlin iron. For these purposes, Ehrenberg estimates its consumption in Berlin only, at from 50 to 60 cwt. yearly. This deposit occupies a surface of great extent, probably the site of an ancient lake, and forms slaty or laminated strata of fourteen feet in thickness. It is almost entirely composed of an aggregation of the siliceous shields of one of the minute forms of fossil Infusoria, the Gaillonella distans. The length of one of these is about of a line, which is about of the thickness of a human hair. About 23 millions of animalcules are contained in a cubic line of the Polierschiefer, and 41,000 millions in a cubic inch. A cubic inch of the Polierschiefer weighs 220 grains; and thus 187 millions go to a grain; or the siliceous shield of each animalcule may be regarded as weighing about the T700oooo of a grain. The minuteness of these is surpassed by that of the Animalcules of the Marsh Ochre, which are only Too of a line in diameter, TT part of the thickness of the human hair, of the diameter of a globule of the human blood. A cubic line of such animal iron-ochre would thus, in the same proportion, contain 1000 millions, and a cubic inch upwards of 1,000,000,000,000 of living beings 0000 FOSSIL ANIMALCULES. 557 1136. In the Polierschiefer are found hard nodules of an Opaline character, which are pretty obviously composed of the siliceous shields of Gaillonella, united together by an inorganic cement, formed by the partial solution of some portions of the mass,just in the same manner as compact limestones may be formed by the solidification of coral-sand (§ 1105). By the similarity in the character of these nodules to the Semi-opal of older formations, Ehrenberg was led to examine various specimens of this mineral; and he has succeeded in obtaining distinct organic remains of animalcules from their interior: so that it is probable that its nodules have been entirely composed, like those of the Polierschiefer, of the siliceous remains of Animalcules, which have subsequently undergone a partial metamorphosis. In the white and opaque bands of Chalk Flints, he has also found spherical bodies, which he considers to be of organic origin; these are especially abundant in the white siliceous crust which forms the exterior of the flints, and in the mealy siliceous powder sometimes found within their cavities. As the same flints often exhibit remains of the structure of Sponges, it would not seem improbable, that the Animalcules may have been drawn into the canals of the Sponge whilst alive, and may have thus been imbedded within it. It is not a little interesting to remark that, whilst the microscopic contents of more recent strata are all freshwater Infusoria, those of the chalk are species which must, or at least can, live in the waters of the ocean. It is also a curious and important fact, that, of about eighty species of fossil Infusoria, discovered in various strata, almost half are species which still exist in the waters; and thus these forms of life, so long overlooked as invisible specks of brute matter, have a constancy and durability through the revolutions of the earth's surface, which is denied to animals of more conspicuous size and higher organisation, the continuance of whose existence depends upon a greater variety of favourable conditions. CHAPTER XXVI. OF THE CLASS OF PORIFERA. 1137. THE lowest of the classes usually comprehended under the Radiated type, is one whose claim to a place in the Animal Kingdom must be regarded as very doubtful. Not only is the radiated disposition of parts altogether wanting, but even that definiteness of form is absent, which so peculiarly distinguishes the higher groups of Animals from the members of the Vegetable Kingdom. The absence of charac FIG. 628-SPONGE. teristic structure does not extend to external form alone; it is equally remarkable in the internal arrangement of the parts, of which these beings are composed. There is no stomach or digestive cavity for the reception of food,-no nervous system or organs for sensation or locomotion, and nothing beyond the very simplest apparatus for reproduction. In the vital actions performed by these beings, there is nothing distinctly characteristic of an Animal nature. No movements can be observed, either in the whole structure, or in portions of it; neither is there any indication of the possession of sensibility. In fact, the only obvious vital action which can be observed in their ordinary state, is a movement of fluid through their channels; and this does not differ, except in rapidity, from what may be seen in Plants. 1138. There can be little question, then, that if the Class PORIFERA, consisting of the Sponges and their allies, is to be ranked in the Animal creation, the lowest place in the scale is justly assigned to it; since the beings which it contains exhibit so few indications of any but organic life, that it has been, and perhaps still may be, doubted, whether they should not PORIFERA, OR SPONGE TRIBE. 559 rather be assigned to the Vegetable Kingdom. Perhaps the strongest argument for their Animal nature, is to be found in the resemblance of their structure to a part of that, which constitutes beings whose animal character is undoubted. The group of Alcyonia (§ 1087) consists of Sponges with Polype-mouths; and in the growth of these structures, the spongy mass is first formed, the polypes not appearing until a subsequent period. The substance commonly known as Sponge is in reality but the framework or skeleton of the being in its living state. When growing beneath the waters it inhabits, this framework is covered by a living flesh, which not only envelopes its exterior, but lines all the canals that ramify through its substance. This enveloping flesh is so extremely soft, as to drain away when the mass is removed from the water, like the white of an egg, or the vitreous humour of the eye; hence its existence was for a long time overlooked, and the mode in which the harder texture grows was not at all understood. It is principally to the observations of Dr. Grant (which have been confirmed by other naturalists), that we owe the elucidation of the real character of the spongy structure, and of its concern in the actions performed in the living state. 1139. When we examine the exterior of a piece of common Sponge, we observe that it is covered with minute orifices or pores (whence the name which has been given to the class), thickly set together; and that larger openings are disposed at intervals amongst these. If these larger orifices, or vents, be traced into the substance, it will appear that they are the mouths of canals or vessels which ramify through it; these have definite walls formed by the firm tissue of the sponge, but perforated with a large number of orifices, connecting them with the spaces that lie between their net-work of branches. The pores, on the other hand, open into a less regular arrangement of small tubes and cells, of which the spongy mass is principally composed. These spaces are produced by the interlacement of the fibres that form the solid framework; they communicate with one another throughout the mass; and the canals seem to take their origin from the midst of them, arising by small tubes 560 STRUCTURE AND ACTIONS OF SPONGE. which unite into larger ones, and these again meeting to form the wide channels which terminate in the vents. If, after making such an examination of the structure of the Sponge, we tear a small portion of it into fragments, its fibrous nature becomes very apparent. If we submit these fragments to a low magnifying power, it is perceived that they form a complete network, inosculating with each other in every direction. By applying a higher power, it is ascertained that they are of tubular structure. In the Common Sponge, the whole framework is composed of these tubular fibres, which in some parts are set very closely together, whilst in others they are loosely arranged. They consist entirely of an organic texture, rather approaching the horny substance of Animals (as is made evident by the smell emitted on burning it), than anything which we meet with in the Vegetable Kingdom; and it is to their elasticity, that the Sponge owes those properties, which make it so useful to Man. 1140. In other forms of the group, however, spicula of earthy matter are disposed amongst these; which add to the firmness of the structure, and diminish its elasticity, according to the amount in which they are present. These spicula are in some instances composed of carbonate of lime; in others they consist of pure silex; and their form varies according to the material. The earthy spicula are disposed at intervals through the whole mass; but they are especially abundant in the neighbourhood of the canals, and around the external orifices, both of these and of the pores. In fact each of these orifices is strengthened by a regular framework of spicula disposed around it; and a high magnifying power enables us to see, that across the entrance to the pores, there is also a very delicate membranous network, which acts as a filter or sieve, preventing the entrance of improper particles along with the fluid which is drawn in through them. In many instances, the sharp ends of the spicula project from the surface of the Sponge, as if for its defence. 1141. When Sponges are examined in their living state and natural condition, a constant and rapid stream of water is seen to issue from the larger orifices or vents. This stream is made apparent by the movement of the minute particles contained in |