tina, especially when seen brightly illuminated upon a black ground; since (for the reason formerly explained, § 62) their

"solid forms" become much more apparent than they are when these objects are examined by light transmitted through them. And the "black ground illumination," either by the "spotted lens" or by the "paraboloid" (§ 61), is much to be preferred for this purpose, to the ordinary mode of illuminating opaque objects by incident light from a condenser, although this may be advantageously had recourse to, by the Microscopist who is unprovided with these appurtenances. No class of objects is more suitable than these to the "Binocular Microscope" (§ 40); the stereoscopic projection of which causes them to be presented to the mind's eye in complete relief, so as to bring out with the most marvellous and beautiful effect all their delicate sculpture, reminding the observer (to compare small things with great) of the finest specimens of the hollow ivory balls carved by the Chinese.'

295. Sponges. Although this tribe has been bandied from the Animal to the Vegetable kingdom, and back again, several times in succession, yet its claim to a place among the Protozoa may now be considered as pretty certainly determined, by the information derived from Microscopic examination of its minute structure. For in the living Sponge, the skeleton, usually composed of a fibrous network strengthened by spicules of mineral matter, is clothed with a soft flesh; and this flesh has been found by Dujardin and all subsequent observers to consist of an aggregation of Amoeba-like bodies (Fig. 217, B), some of which (as Mr. Dobie has shown) are furnished with one or more long

1 For a fuller description of this group, see Prof. Ehrenberg's Memoirs in the "Transac tions of the Berlin Academy" for 1846, 1847, and his recently published" Microgeologie;" also "Ann. of Nat. Hist." 1847.

2 Goodsir's Annals of Anatomy and Physiology," No. 2, May, 1852.

cilia, closely resembling those of Volvox (Fig. 70, 1), by the agency of which, a current of water is kept up through the passages and canals excavated in the substance of the mass.

Structure of Grantia compressa:-A, portion moderately magnified, showing general arrangement of triradiate spicules and intervening tissue;-B, small portion highly magnified, showing ciliated cells.

And from the observations of Mr. Carter upon the early development of Sponges, it appears that they begin life as solitary Amoebae, and that it is only in the midst of aggregations formed by the multiplication of these, that the characteristic Sponge-structure makes its appearance, the formation of spicules being the first indication of such organization. The ciliated cells seem to form the walls of the canals by which the whole fabric of the Sponge is traversed; these canals, which are very irregular in their distribution, may be said to commence in the small pores of the surface, and to terminate in the large vents; and a current is continually entering at the former, and passing forth from the latter, during the whole life of the Sponge, bringing in alimentary particles and oxygen, and carrying out excrementitious matter.

296. The skeleton which gives shape and substance to the mass of sarcode particles that constitutes the living animal, is composed, in the Sponges with which we are most familiar, of an irregular reticulation of fibres. The arrangement of these may be best made out, by cutting thin slices of a piece of Sponge submitted to firm compression, and viewing these slices, mounted upon a dark ground, with a low magnifying power, under incident light. Such sections, thus illuminated, are not merely striking objects, but serve to show, very characteristically, the general disposition of the larger canals and of the smaller areola with which they communicate. In the ordinary Sponge, the fibrous skeleton is almost entirely destitute of spicules, the absence of which, in fact, is one important condition of that

1 "Annals of Natural History," Ser. 2, vol. iv.

flexibility and compressibility on which its uses depend. When spicules exist in connection with such a skeleton, they are either altogether imbedded in the fibres, or they are implanted into them at their bases, as shown in Fig. 218. In the curious and beautiful Dictyochalix pumiceus of Barbadoes, however, the entire network of fibres is composed of silex, and is so transparent that it looks as if composed of spun glass. There are many Sponges in which no fibrous network can be discerned, the spicules lying imbedded in the midst of the sarcode mass; such is the case in Grantia (Fig. 217, A), whose triradiate spicules are composed of carbonate of lime. Sponge spicules are much more frequently siliceous than calcareous; and the variety of forms presented by the siliceous spicules is much greater than that which we find in the comparatively small number of species in which they are composed of carbonate of lime. The long needle-like spicules (Fig. 219) which are extremely abundant in several Sponges, lying close together in bundles, are sometimes straight, sometimes slightly curved; they are sometimes pointed at both ends, sometimes at one only; one or both ends may be furnished with a head like that of a pin, or may carry three or more diverging points, which sometimes curve back so as to form hooks (Fig. 334, H). When the spicules project from the horny framework, they are usually somewhat conical in form, and their surface is often beset with little spines, arranged at regular intervals, giving them a jointed appearance (Fig. 218). Spongespicules frequently occur, however, under forms very different from the preceding; some being short and many-branched; and the branches being themselves very commonly stunted into mere

FIG. 218.

tubercles (some examples of which type are presented in Fig. 334, A, c); whilst others are stellate, having a central body with conical spines projecting from it in all directions (as at D of the same figure). Great varieties

Fig. 218. Portion of Halichondria () from Madagascar, with spicules projecting from the fibrous network.

present themselves in the stellate form, according to the relative predominance of the body and of the rays; in those represented

in Fig. 219, the rays, though very numerous, are extremely short; in other instances the rays are much longer, and scarcely any central nucleus can be said to exist. The varieties in the form of Sponge-spicules are, in fact, almost endless; and a single sponge often presents two or more (as shown in Fig. 219), the stellate spicules usually occurring either in the interspaces between the elongated kinds, or in the external crust. In one curious Sponge described by Mr. Bowerbank (the Dusideia fragilis), the spicules are for the most part replaced by particles of sand, of very uniform size, which are found imbedded in the horny fibre. The spicules of Sponges cannot be considered, like the "raphides" of Plants (§ 230), simply as deposits of mineral matter in a crystalline state. For the forms of many of them are such as no mere crystallization can produce; many of them possess internal cavities, which contain organic matter; and the calcareous spicules, whose mineral matter can be readily dissolved away by an acid, are found to have a distinct animal basis. Hence it seems probable, that each spicule was originally a cell or segment of sarcode, which has undergone calcification, and by the self-shaping power of which, the form of the spicule is mainly determined.

297. Of the Reproductive process in Sponges, much has yet to be learned. The following is perhaps the most probable account of it:-Multiplication by gemmation is effected by the detachment of minute globular particles of sarcode from the interior of the canals, where they sprout forth as little protuberances, whose footstalks gradually become narrower and narrower until they give way altogether; these gemmules, like the zoospores of Algæ, possess cilia, and issuing forth from the vents, transport themselves to distant localities, where they may lay the foundation of new fabrics. But according to the recent observations of Mr. Huxley,' a true sexual generation also takes place, as might be anticipated; both ova and sperm-cells being found imbedded in the substance of the Sponge. The bodies distinguished as capsules, which are larger than the gemmules, and which usually have their investment strengthened with siliceous spicules very regularly disposed, are probably the products of this operation. They contain numerous globular particles of sarcode, every one of which, when set free by the rupture of its envelope, becomes an independent Amoba-like body, and may develope itself into a complete Sponge.

298. With the exception of those that belong to the genus Spongilla, all known Sponges are marine; but they differ very much in habit of growth. For whilst some can only be obtained by dredging at considerable depths, others live near the surface, whilst others attach themselves to the surfaces of rocks, shells, &c., between the tide-marks. The various species of Grantia, in which alone of all the marine Sponges has ciliary movement

1 "Ann. of Nat. Hist." Ser. 2, vol. vii.

yet been seen, belong to this last category. They have a peculiarly simple structure, each being a sort of bag whose wall is so thin that no system of canals is required, the water absorbed by the outer surface passing directly towards the inner, and being expelled by the mouth of the bag. The cilia may be plainly distinguished with a 1-8th inch objective, on some of the cells of the gelatinous substance scraped from the interior of the bag; or they may be seen in situ, by making very thin transverse sections of the substance of the Sponge. It is by such sections alone, that the internal structure of sponges, and the relation of their spicular and horny skeletons to their fleshy substance, can be demonstrated. In order to obtain the spicules in an isolated condition, however, the animal matter must be got rid of, either by incineration, or by chemical reagents. The latter method is preferable, as it is difficult to free the mineral residue from carbonaceous particles by heat alone. If (as is commonly the case) the spicules are siliceous, the Sponge may be treated with strong nitric or nitro-muriatic acid, until its animal substance is dissolved away; if, on the other hand, they be calcareous, a strong solution of potass must be employed instead of the acid. The operation is more rapidly accomplished by the aid of heat; but if the saving of time be not of importance, it is preferable on several accounts to dispense with it. The spicules, when obtained in a separate state, should be mounted in Canada balsam. Sponge-tissue may often be distinctly recognized in sections of agate, chalcedony, and other siliceous accretions, as will hereafter be stated in more detail (Chap. XIX).

1 See Dobie, loc. cit.; and Bowerbank in "Transactions of Microscopical Society," 1st Ser. vol. iii, p. 137.