ture eluding observation. In the simplest animal organisms they are the organs by which motion and respiration are effected, and the food necessary for nutrition obtained. But they exist also in Man and the higher orders of vertebrata, and are the instruments by which many of the most important functions of the animal economy are performed. As we cannot separate the idea of muscular fibre from animal motion, it is conjectured that the Cilia are impelled by definitely arranged muscles: and Ehrenberg believes he has detected these muscles in some of the larger Infusoria. The number of the cilia, even in an animalcule invisible to the naked eye, is almost incredible; Dr. Grant calculates that a single polype of a Flustra has 400 millions.

9. THE HYDRA, OR FRESH-WATER POLYPE.t-Before describing the zoophytes which are the immediate subject of this Lecture, I would call your attention to one of the most simple forms of animal life that abounds everywhere in freshwater streams, and, being relatively of considerable size, will afford a convenient illustration of some of the vital phenomena exhibited in the coral-zoophytes which give rise to reefs and islands in the seas of warm climates, and of one group of which (the Hydroida or Sertularida) it is the simplest form and type. This is the Hydra, or fresh-water polype, of which two or three kinds inhabit our ponds, rivulets, &c. They are generally attached to the stems and leaves of aquatic plants; and the largest species, when in an expanded state, is from a quarter to half an inch long, and of the size of a hog's bristle; it is constricted at the end attached to the plant, and has an aperture or mouth at the free extremity, from around which proceed from six to ten

* See the representation of the Cilia on the rotatory organs of the Rotifer or Wheel-animalcule in "Thoughts on Animalcules," p. 35. The continuance of the ciliary motion in parts separated from the rest of the body of Mytilus and many other animals, and even for some time after death, proves that the action is automatic, or independent of the will of the animal.

+ Polype, or polypus (many feet), is a name derived from the tentacula or processes which in some species serve for progression, in others for respiration. The name has been also applied to some of the manyarmed Cephalopods.

long delicate tentacula (Lign. 139, fig. 4). The Hydra present an example of a highly endowed organism of the simplest structure; the whole animal consisting merely of a gelatinous, transparent, open cylinder, or tube, contracted at one extremity, and having the margin of the other prolonged into tubular tentacula. It is, in fact, a stomach, or digestive sac, with no appendage but the instruments for

Fig. 1. Hydra fusca magnified, the tentacula partially expanded. 2. Two Hydræ on the same base, one contracted, the other expanded. 3. Hydra viridis, natural size. 4. Hydra, with the body enlarged from its containing food. 5. Vertical section of Hydra viridis, highly magnified. 6, 7, 8. Hydra in various states of progression. 9. A double Hydra, produced by the vertical section of a single one.

seizing its prey. A vertical section (Lign. 139, fig. 5), highly magnified, shows the interior of the receptacle for the food, the relative thickness of its substance, and the manner in which the tentacula are formed by an extension of the upper margin. The Hydra is endowed with vitality in a very extraordinary degree, and its substance is highly sensi

tive and contractile in all its parts. It fixes itself to other bodies by the small end of the tube, and expands and contracts at pleasure. These enlarged drawings (Lign. 139) represent the polypes in different positions and states of contraction. The mode of progression is shown in figs. 6, 7,8; it is effected by the bending of the body into a curve, and holding by the tentacula; the base is then brought into contact with the fixed point, and the tentacula are again projected forwards. The Hydra* can greatly elongate the body, and also contract itself into a small globular mass, as in the uppermost polype in fig. 2.

The most extraordinary vital endowment possessed by these fresh-water polypes is that of the reproduction of lost parts to an almost unlimited extent; even to the formation of several perfect animals from the separated pieces of a single individual. If the body be split in half, each portion grows into a complete Hydra, as is shown in this drawing (fig. 9); and, as if there were no limits to its transformations, the creature may be turned inside out, and that which was the surface of the stomach will become the epidermis, and the outer skin form the lining of the new stomach, and carry on the process of digestion!†

10. ELEMENTARY ORGANIC STRUCTURE; CELLS. - The interpretation of these phenomena is to be found in the peculiar organization of the Hydra, its entire structure being nothing more than an aggregation of cells.

A vertical section of the Polype (Lign. 139, fig. 5) shows the internal cavity or digestive sac, the relative thickness of the substance of the body, and the manner in which the arms are formed by a prolongation of the upper part into hollow processes. This animal is, in fact, a simple sac or pouch formed of a congeries of cells, for the reception and assimilation

See "Thoughts on Animalcules,” Pl. I., for coloured figures of the Hydræ.

† Abraham Trembley of Geneva, in 1740, first observed this wonderful property of the Hydra. See his work, "Mémoires pour servir à l'Histoire d'un Genre de Polypes d'Eau douce." 4°. Leyde, 1744.

of food. The cells lining the stomach select and absorb the nutritious particles, and the tube then spontaneously contracts and casts out the residue of digestion. This organization is analogous to that of the simplest condition of the vegetable kingdom, the Cellulosa: for even the large Fuci, or sea-weeds, consist only of cells. The fresh-water Confervæ are merely jointed films composed of cells, containing granules or lesser cells. A cell bursts, the granules escape, and float in the water until they become fixed to some other body; they then reproduce cells, which are aggregated after the same pattern as in the parent plant. The common mould or mustiness is a cluster of plants formed of cells only; and there are some vegetables in which the entire plant consists of but one isolated cell; such as the red snow (Leparia nivalis); whilst in the yeastfungus (Tortula cerevisia), the plant consists of but one, two, three, or a few more. In these examples we see that all the functions of vegetable life, namely, absorption, assimilation, the fixation of carbon from the atmosphere, respiration, exhalation, secretion, and reproduction, can be effected by one single cell. Even in the highest and most complicated orders of vegetables, in which there is a variety of organs adapted for the performance of different offices, these functions are effected by the agency of cells, which obtain materials of formation and support from the ordinary chemical agents around them. Thus an aggregation of simple cells forms the cellular tissue; a fusion or blending of several cells produces the vessels, and so forth; and by cells are elaborated the gum, resin, oil, starch, gluten, &c.: and by cells specially endowed are secreted the narcotic substance of the poppy, the deadly poison of the nightshade, and the stimulant aromatic of the clove.

In like manner, in animal structures, all the various processes of vitality are performed by cells or globules, varying in size from infinite minuteness to forms visible to the unassisted eye. Thus one system of cells secretes the bile, another the adipose substance, another the nervous matter, and so forth; but how these special products are formed by cells apparently of similar organization we know not. Whether the special endowment belonging to the system of cells of a particular organ depends on the intimate structure of the wall or tissue of such cells, and this structure be so attenuated and infinitesimal as to elude our observation,or whether it results from the transmission of some peculiar modification of that mysterious vital force we term nervous influence,-are questions to which, I apprehend, no satisfactory reply can be given.

In fine, a minute globular cell is typical of the common germ from which all organic fabrics proceed. All animals and plants, therefore, may iustly be regarded as definite aggregations of cells, endowed with specific properties in the different types, and subjected to a never vary.

ing law of development; and in animals, as well as in plants, there are certain kinds in which the entire organism consists of but a single cell; and others in which each individual is but a cluster of such cells arranged in a definite manner. These mere aggregations of cells perform all the functions of animal life, viz. the maintenance of a particular form for a certain period, the elaboration of materials of support from food, locomotion, and the perpetuation of the species; hence these animals, like the simplest plants, may be divided without losing their vitality, and every part may become a perfect individual. To this class of organization belongs the Hydra; and the above exposition of its structure renders the production of several animals from the vivisection of an individual perfectly intelligible.

11. ANALOGY NOT IDENTITY.—And here I must briefly comment on the doctrine of the law of development, as it is termed, so speciously, but unphilosophically, advocated in a late work;† in which it is attempted to show, that all the varied forms of organic life are the result of a law by which is produced an unbroken chain of gradually exalted organization, from the crystal to the globule, and thence through successive stages of the polype, mollusk, &c., up to Man.‡

The following remarks will serve to show the fallacy of this reasoning. Though it is an established physiological axiom, that cells are the elementary basis, the ultimate limit to which we can trace all animal and vegetable structures, and that the varied functions in which organic life essentially consists are performed by the agency of cells not distinguishable from each other by any well-marked characters, there is not the slightest ground for assuming any identity between the primary cells of the simplest species of animals and vegetables, much less those of higher

*The Monads: see "Thoughts on Animalcules," Plate II. † Vestiges of the Natural History of the Creation.

The following remarks of Sir John Herschel on this theory are too important to be omitted. "The transition from an inanimate crystal to a globule capable of such endless organic and intellectual development is as great a step-as unexplained a phenomenon-as unintelligible to us— and in any human sense of the word as miraculous, as would be the immediate creation and introduction upon earth of every species and every individual. Take the amazing facts of Geology which way we will, we must resort elsewhere than to a mere speculative law of development for their explanation."-Brit. Assoc. Report for 1845, p. 42.