organization. The single cell which embodies vitality in the yeast-fungus or the monad is governed by the same immutable organic laws which preside over the complicated structure of Man and the other vertebrata: and the single cell which is the embryotic condition of the Mammal has no more relation to the single cell which is the permanent condition of the monad, than has the perfect animal into which the mammalian cell is ultimately developed. The cell that forms the germ of each species of organism is endowed with special properties, which can result in nothing but the fabrication of that particular species. There is an analogy between the human embryo and the monad of the Volvox, in that each consists of simple cells; but there is no more identity between the human and infusorial cells than between the perfect Man and the mature Animalcule.

12. THE BRYOZOA.-From this digression we return to the consideration of the Zoophytes, and especially of the Bryozoa, which stand first in order among the plant-like animals, and are well called Moss-animals on account of their aggregated masses incrusting other bodies like moss. They present numerous fresh-water species,† as well as an extraordinary abundance of marine forms. Their polypes, though exceedingly minute, are highly organized, their digestive organs being more complicated than those of the other tribes of Zoophytes. I will select one of the most simple and common forms in illustration of the subject. Most persons in their rambles along the sea-shore must have noticed on the seaweeds, shells, and pebbles patches of a white reticulated substance, which, when closely examined, appear like delicate lace-work: these, apparently mere earthly particles, are clusters of the compound zoophytes termed Flustra§ or "Sea-mat." (See Plate V. figs. 5, 6, 7, 8.)

* See Notes to "Thoughts on Animalcules;" and Westminster Review, No. XC. Art. "The Microscope and its Revelations."

+ Most philosophically described and most beautifully illustrated by Prof. Allman in his "Monog. Fresh-water Polyzoa," 1856 (Ray Society). See above, p. 601.

The word Flustra is here intended to comprise the closely-allied genus Membranipora, as formerly, for the sake of convenience of reference and description.

The Flustra,* when taken alive out of the water, presents to the naked eye the appearance of fine net-work, coated over with a glossy varnish (Pl. V. fig. 5); with a glass of moderate power, the surface is discovered to be full of pores, disposed with much regularity (Pl. V. fig. 8). If a magnifying glass be employed while the Flustra is immersed in sea-water, very different phenomena appear; the surface is found to be invested with a fleshy or gelatinous substance, and every pore to be the opening of a cell, whence issues a tube with several long feelers or arms (Lign. 140); these expand, then suddenly close, withdraw into the cells, and again issue forth; the whole mass being studded with these hydra-like forms. The Flustra thus constitutes, as it were, a family of polypes, each individual of which is permanently fixed in a calcareous cell, and the whole united by a common integument, by which the hard frame-work was secreted. Under a powerful microscope the form and structure of the animal of the Flustra may be easily distinguished. Pl. V. fig. 6, represents a single polype with its tentacula extended; and fig. 7, the same animalcule, withdrawn into its cell. The figure, Lign. 140, shows a polype protruded, under a much higher power.†

The body of the animalcule consists of a transparent sac or pouch, doubled on itself, and having at the mouth, or large external opening, ten or twelve tentacula fringed with cilia, which have the power of extending and contracting themselves with great celerity.

The Flustroid Bryozoa present considerable variety in the

* For the latest technical descriptions, and detailed illustrations of the recent Flustræ and their allies, see Johnston's Brit. Zoophytes, 2nd Edit. p. 342; Busk's Polyozoa Brit, Mus. p. 46; and Alder's Cat. Zoophyt. p. 139.

This figure is from a memoir by Mr. Lister (Philos. Trans. 1834) on the structure and functions of the Tubular and Cellular Polypes. The observations were made at Brighton, and I had the pleasure of seeing the live zoophytes under the fine instrument constructed by Mr. Lister.

form and arrangement of their cells. Some of the most common species are found attached to marine plants, which they enclose, as it were, in a living sepulchre (Pl. V. fig. 5); others spread into foliated expansions, and have both sides covered with cells. The prevailing hue is white, or a light-fawn colour, but some species have a tinge of pink or

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LIGN. 140.-POLYPE OF FLUSTRA PILOSA; WITH ITS MOUTH AND CILIATED ARMS PROTRUDED FROM THE CELL.

(Magnified 150 times.)

yellow. Numerous species † resemble in miniature the branching tree-like forms of the Corals, or other coralloid and spongoid shapes. They abound in every sea, are not restricted by climate, occur in profusion along the sea-shores, and are found attached to the fuci that are thrown up from below low-water mark.‡ The small parasitical species, when dried, appear like spots of a chalky substance on the sea-weed.

* Membranipora, Busk, Polyz. Brit. Mus., p. 56, pl. 71. + Medals, p. 268, fig. 89.

A concise notice of the distribution of Molluscous animals in the several zones into which naturalists have divided the bed of the sea, will be found in Woodward's "Manual of the Mollusca," p. 439: and in Mr. J. Alder's "Catalogue of the Zoophytes of Northumberland and Dur. ham" (Trans. Tyneside Nat. Field Club, vol. iii. p. 98), the reader will find upwards of 200 species of British Anthozoa and Bryozoa arranged according to their distribution into zones of depth.

The increase of the Flustra is thus described by Lamouroux. -When the animal has acquired its full growth, it flings from the opening of its cell a small globular body, which fixes near the aperture, increases in size, and soon assumes the form of a new cell; it is yet closed, but through the transparent membrane that covers its surface the motions of the polype may be detected; the habitation at length bursts, and the tentacula protrude, eddies are produced in the water, and conduct to the polype the atoms necessary for its sub

sistence.

Innumerable specimens of Bryozoa or Moss-corals* are found in the fossil state, in every geological formation; and sometimes they occur in such abundance as to form whole stratified masses, as in the case of the Suffolk Crag,† and the upper part of the Maestricht Chalk. The White Chalk of the South of England, and of France and Germany, contains a large proportion of Moss-corals, but chiefly in a fragmentary condition.

The Graptolites,§ found in the Silurian rocks, have been referred by some authors to the Bryozoan group; by others, however, they have been placed with the Virgularia of the Alcyonarian order.

13. THE FOOD OF ZOOPHYTES.-However improbable it may appear to the mind unaccustomed to investigations of this nature, that beings so minute as those under examination should prey upon living forms yet more infinitesimal, the fact is nevertheless unquestionable. It is even possible to select the food of animalcules much smaller than the

Medals, p. 265. From the imitative resemblance of many of their little polyparies to those of the larger Corals, the Bryozoa are often referred to by the above name.

+ This was formerly termed "Coralline Crag," owing to the then imperfect state of the nomenclature of the Zoophytes. See vol. i. p. 224.

See vol. i. p. 331. Also D'Orbigny's "Paléontol. Française ;" and Hagenow's "Die Bryozoen der Maastrichter Kreidebildung."

§ Medals, p. 255,

polypes of the Flustra, and thus exhibit their internal structure. This experiment is easily shown under a good microscope, and the animalcules termed Vorticella, a very abundant family of Infusoria, are best adapted for the purpose. Immediately on a minute particle of a very attenuated solution of pure carmine or indigo being applied to a drop of water containing a group of the Vorticella, the most beautiful phenomena are observable. Currents are excited in the fluid in all directions by the rapid motion of the cilia, which form a crown round the anterior part of the body of the animalcules, and the particles of indigo are seen moving in different directions, but generally all converging towards the orifice or mouth, which is situated not in the centre of the crown of cilia, but between the two rows of these organs, which exist consecutive to one another. The attention is no sooner drawn to this interesting spectacle, than presently the bodies of the animals, which were before quite transparent, become dotted with distinctly circumscribed spots, of a dark-blue colour, exactly corresponding to that of the moving particles of indigo. In some species, particularly in those which are provided with an annular contraction or neck, separating the head from the body, the molecules of indigo can be traced in a continuous line in their progress from the mouth to the internal cavities. The animalcules termed Monads may be considered as the lowest term of animal organization recognisable by man, being only from the 1,200th to the 24,000th part of an inch in diameter, and the powers of the microscope extend no farther; yet it is impossible to doubt that there are myriads of living forms more infinitesimal, some of which serve as food to these miniatures of life.†

* The Vorticellæ are hyaline vase-shaped or bell-like animalcules, attached by a slender peduncle at the base, and having rows of cilia disposed in zones round the margin; these, when seen in some directions while in rapid motion, appear like wheels. See "Thoughts on Animalcules," Pl. III.

"The size of the ultimate particles of matter must be small in the