this animal appears to present a new and singular mode of internal gemmiparous generation. Since these germs, or sonamed ova, are evolved within the body of the parent, and are detached without injuring or affecting its general form, this may be considered as a more complicated, or more perfect, kind of generation, than that by spontaneous division, exhibited by animalcules, where the form of the parent's body suffers materially during the process, and half of its substance is removed. As the ova of the sponge, however, are not fully formed individuals at the time of their separation, but require to undergo a further change to bring them to the fixed and perfect state of the parent, this mode of generation is less perfect than the true external gemmiparous generation of the hydra, where the new individual falls off from the body of the parent in a state of perfect maturity. Many other zoophytes exhibit the same kind of internal gemmiparous generation by the detachment of imperfectly formed portions of their soft substance; their ova require to undergo the same metamorphosis to bring them to the perfect state, and they exhibit the same singular spontaneous motions during the intermediate state between the time of their forming a part of the parent's body, and that of their existence as new individuals. Mr Ellis observed similar spontaneous motions in the ova of the Campanularia dichotoma, Cavolini in those of the Gorgonia verrucosa and Caryophyllia calycularis, and I have observed them in those of the Plumularia falcata (See Ed. New Phil. Journ. vol. i. p. 155). The power of spontaneous motion is not given in vain to these minute portions of gelatinous matter on which the propagation of the species depends. As the fecal orifices open into the general cavity in such cup-like sponges as the S. ventilabrum and S. patera, which sometimes appear to grow erect in the still recesses of the deep, the spontaneous motions of the ova in these, and, in all erect tubular species, will aid their escape, and prevent them from destroying the parent, by a parasitic growth in the interior. The power of spontaneous motion will prevent the ova of such species as the S. oculata, S. panicea, S. palmata, and S. compressa, which hang vertically from the roofs of caves, from sinking by their gravity to the bottom, where they could not fail to be crushed or buried among the moving sand, and will enable

them to seek and to take that vertical position which seems necessary to their future development; and, by this locomotive power, produced by the vibrations of the cilia, the ova are suspended for a longer period at the mercy of the waves, the tides, and the streams of the ocean, by which the species are gradually spread over the globe. Thus the S. communis, S. lacinulosa, S. usitatissima, and other horny species, which seem to be confined to warm climates, and abound in the Red Sea and the Indian ocean, appear to have been gradually wafted by the Gulf Stream from the shores of the east to corresponding latitudes of the new world. The S. fulva, S. fistularis and fine varieties of the S. officinalis, Pall. are among the horny species which abound on the tropical shores of America, and their elastic filaments form a beautiful transition to the cartilaginous threads which wind round the cells of Alcyonia. All the known calcareous sponges are inhabitants of the British coasts; the delicate and minute S. compressa has been seen on the shores of Greenland, Shetland, Scotland and England, and I have found it along with the S. nivea abundant and extensively distributed over the Western Islands. The S. botryoides, S. nivea, and S. compressa, are calcarious species, inhabiting the Frith of Forth. An immense number of silicious species inhabit our northern latitudes; and from their peculiar habits, their simple structure, and their tenacity of life, they are probably the animals which exist nearest to the poles. The S. coalita, S. oculata, S. dichotoma, S. prolifera, S. palmata, S. suberica, S. papillaris, S. panicea, S. cristata, S. tomentosa, and S. cinerea, Gr., (fig. 3.) are found in the Frith of Forth. The S. papillaris and S. tomentosa I have found common on the coasts of Britain, Ireland, and the Western Islands; and I have observed the S. panicea roofing the excavated basaltic cliffs of the island of Staffa. The S. sanguinea, Gr., (fig. 9.) a remarkable blood-red sessile species, I have found growing, like the S. panicea, on the under surface of the seabeaten rocks of Islay, Staffa, Iona, and, along with the S. nivea, at the entrance of the spar caves on the shores of Skye. The S. tomentosa is said to occur on the shores of Europe, North America, Africa and India (Lamouroux, Hist. des Polyp. p. 30.); but I believe it has not been authentically shewn that the same silicious species occur in the corresponding lati

tudes of the two hemispheres; indeed the geographical distribution of the species cannot be satisfactorily ascertained till their characters are better described and defined. This animal, however, seems eminently calculated for an extensive distribution, from the remarkable simplicity of its structure, and the few elements required for its subsistence. Its inertness, its soft gelatinous structure, its want of organs for seizing prey, the incessant currents through its body, and the growth of its ova, when nourished only with sea-water, shew that it subsists either on the elements of that fluid, or on the minute particles of organic matter suspended in it. Its canals present the first rudiments of an internal stomach; by these simple organs it extracts a mass of gelatinous matter from the waters of the ocean, and organises it for the digestive organs of animals higher in the scale. Its interior affords a domicil and a magazine of food for myriads of minute marine animals. It extracts silicious matter from the ocean, and precipitates it in regular and beautiful crystalline forms. It precipitates, in the form of an insoluble carbonate, the calcareous matter continually poured by rivers into the bed of the ocean in a soluble state; it thus assists in purifying the vast abyss of a corrosive ingredient, and prepares it for the maintenance of the various tribes of vertebral inhabitants that people its boundless expanse. And it has probably aided in the formation of silicious and calcareous rocks.

I have now given a brief outline of the natural history of the Sponge as a genus, and stated the laws which regulate its external form, in so far as I have been able to observe the living characters and habits of the species in the Frith of Forth. I have endeavoured to trace to their sources the discoveries which have been successively made in its structure and economy, and have shewn, that the true nature of this singular being, and the uses of all its parts, were as well known to the ancient Greeks as to the naturalists of modern Europe,-that the description of it given by Aristotle is more correct and complete than that of Lamarck. I have detailed a series of experiments to determine the uses of the pores, canals and orifices; and have shewn, that the incessant currents through these passages, which are subservient to

the nourishment, respiration, and reproduction of the animal, are not produced by the alleged irritability of its axis, nor by the supposed systole and diastole of its apertures, but by certain minute organs disposed over the whole surface of the internal canals. I have described the most striking differences which I have observed in the chemical constitution and microscopical forms of the minute parts composing the skeleton of this animal, in the three great tribes of horny, calcareous, and silicious species, and their beautiful arrangements to maintain the general form of the zoophyte, and to support and defend its soft parts. I have stated the characteristic properties and appearances of the connecting matter of the spicula, the parenchymatous, or general cellular substance of the body, the gelatinous net-works of the pores, and the granular bodies of the internal canals. I have examined the successive changes which the ova undergo from the time of their first appearance in the parenchymatous substance of the parent till their full development, and their expulsion from the fecal orifices, the causes of the singular spontaneous motions they exhibit, from the time of their expulsion till their metamorphosis into fixed inert zoophytes, and the progress of their growth in this fixed state, till they attain the perfect form of the parent. And, lastly, I have stated a few observations on their geographical distribution, and their purposes in the economy of nature. The uses of the central cavities in the horny fibres, and in the earthy spicula, and the different forms of these elementary parts, in all the known species; -the mode in which the animal imbibes nourishment through the parietes of the internal canals, and the chemical changes produced on the fluid by its transmission through these passages;-the particular tribes of infusoria and more perfect ani.. mals that infest the different species, and depend on them for subsistence, and the applications of the earthy species of this animal to useful purposes in the arts, are still unknown. No one has yet excited to action any part of the adult animal, and the moving organs of the currents have never been seen. The mode of generation of this animal, and the structure of its soft parts, have yet been examined only in a very few species. The characters and the geological distribution of its organic remains have yet to be investigated, and probably not a tenth part of

the existing species have yet been brought to light from their recesses in the depths of the ocean. This animal still affords

many curious and interesting subjects of inquiry to those who have leisure and opportunities of examining the more perfect species of tropical seas; and, though probably the simplest of animal organisations, the investigation of its living habits, its structure and vital phenomena, and the distinguishing characters of its innumerable polymorphous species, is peculiarly calculated to illuminate the most obscure part of zoology, to exercise and invigorate our intellectual and physical powers, and to gratify the mind with the discovery of new scenes of infinite wisdom in the economy of Nature.

PLATE II

Fig. 1. Silicious, double-pointed, curved spiculum of the Spongilla friabilis. (See Edin. Phil. Jour. vol. xiv. p. 279.) This and the following 19 figures are magnified 50 times. Fig. 2. Silicious, fusiform, curved spiculum of the Spongia papillaris. (See Edin. New Phil. Jour. vol. i. p. 346). This spiculum occurs in Spongia tomentosa, or urens, S. cristata, and large in S. coalita..

Fig. 3. Silicious, double-pointed, curved, short spiculum of the Spongia cinerea, Gr. (See zoological notices at the end of the present Number.) This spiculum occurs half as large in S. oculata, S. palmata, S. dichotoma, S. prolifera, and S. cancellata, Sowerby.

Fig. 4. Silicious, single-pointed, straight spiculum of the Spongia panicea. (See Edin. New Phil. Jour. vol. i. p. 347.) This spiculum occurs slightly curved in the S. parasitica, Mont.

Fig. 5. Silicious, long, waved filament, obtuse at both ends, of the Spongia ventilabrum (see Edin. New Phil. Jour. vol. i. p. 349.), occurs along with another silicious spiculum, similar to fig. 18.

Fig. 6. Silicious, single-pointed, curved, thick spiculum, with a round head on its obtuse end, of the Spongia patera. (See Edin. New Phil. Jour. vol. i. p. 348.)

Fig. 7. Silicious, single-pointed, curved, slender spiculum, with a round head on its obtuse end, of the Cliona celata. (See Edin. New Phil. Jour. vol. i. p. 80.)