The living organism is dark violet-grey, passing into yellowish brown where the light does not reach it freely. The surface is beset with minute elevations or conuli (A: c), and is marked by fine close-set ridges which divide it into areae. Within these areae are situate the microscopi pores (A: p.) by which the sea-water finds its entrance. The number of oscula or exhalent apertures varies in a given specimen. The pores lead either into the dilated commencement of an afferent water-canal or into a system of 'subdermal spaces' (A: s.s.). The afferent canals (A and B: a.c.) branch, and penetrate the substance of the sponge. The ultimate branches, -oi mm. in diameter, open two to four together, into the globular portion of pyriform ampullae or ciliated chambers (A: a., B.). The stalk of the pear opens either directly into an efferent water-canal (A and B: e.c.) or into one of its rootlets. The branches of the efferent canals unite inter se, and finally open into an oscular canal. This canal is straight, two to three mm. wide, and rises vertically to the surface, where it opens by an osculum, which is limited by an iris-like membrane.

The sponge-substance is composed of three tissues: ectoderm, endoderm, and mesodermic tissue. The ectoderm consists of flat nucleated cells covering the outer surface. It is not certain whether the cells coating the subdermal spaces and afferent water-canals belong to the ecto- or to the endo-derm. They appear at any rate to absorb solid particles and hand them over to the cells of the mesodermic tissue. The ampullae are lined by cylindrical granular endoderm cells - about sixty to each ampulla - with a basal nucleus, single cilium surrounded by a hyaline protoplasmic collar (B: c.c). The currents of water which traverse the sponge are due to the motion of these cilia. The efferent water-canals and'the oscular canals are lined by flat cells (B: ep.), which are endodermic apparently in origin. The cells near the mouth of an ampulla have frequently a transitional character. The mesodermic tissue is present in mass. It consists of a matrix (A and B: m.) hyaline save in the region of the ampullae, where it is opaque from the presence of many rounded granules.

It imbeds (i) stellate (C: s.c.) or fusiform connective cells with processes which appear to anastomose; (ii) cells containing pigment, most numerous near the surface of the sponge and round the large afferent canals; (iii) cells devoid of all processes, which often contain globular masses of a refractile substance, fatty or starchy in nature (?); (iv) long fusiform contractile fibre-cells, either isolated or in bundles, on the surface of the sponge, or disposed circularly in the annular constrictions of the water-canals, and the iris-like membrane of the oscula, both parts which show contractility; (v) the fibrous skeleton. This skeleton consists of thick principal fibres (A:f.), which radiate from the base of attachment and end with fine points (A: f.) in the conuli, and of more delicate branched secondary fibres (A: f".), which connect the principal, fibres. Each fibre consists of a central granular axis surrounded by hyaline strongly refracting lamellae of Keratin or Spongin, a substance near akin chemically to silk, and which in most instances polarises light. The principal fibres have a rough exterior, and their axes contain foreign bodies, chiefly sand-grains, apparently taken in by the mass of irregular cells surrounding the tip of the fibre wherever the fibre is lengthening.

The hyaline lamellae are secreted by pear-shaped cells or spongoblasts (C: sp.), which are probably modified connective tissue or mesodermic cells. The secondary fibres arise independently, and become connected subsequently to the principal fibres.

Euspongia may be propagated artificially from fragments. The sexes appear to be separate, and the male sponge rare. The sexual elements, derived probably from mesodermic cells, undergo development in spaces of the mesodermic matrix lined by flat cells. The ripe ovum is .25 mm. in size, is impregnated and developed into a larva in situ. The free larva is .4 mm. long, .35 mm. broad, shaped like a conical bullet and composed of an external layer of ciliated, and pigmented cells, an internal layer of small cells, and a central mass of cells separated inter se by well-formed membranes.

Spongilla appears to possess a distinct ectoderm, and a mesodermic tissue containing little matrix, but many cells and silicious spicules. Of the latter, some are scattered as flesh-spicules irregularly in the matrix, others - the skeleton spicules - aggregated by a cementing material into fibres. There appears also to be a system of subdermal spaces. Afferent and efferent water-canals lead to and from ampullae lined by the collared cell characteristic of sponges.

For the vexed question as to the homology of the tissues here termed ectoderm, endoderm, and mesodermic tissue, with the parts so termed in other Metazoa, see a paper by Marshall, Ontogenie von Reniera filigrana, Z. W. Z. xxxvii. 1882. He appears to prove that afferent and efferent canals are alike lined by endoderm. Schulze, on the other hand (Die Plakinidae, Z. W. Z. xxxiv. 1880, pp. 416 and 436), appears to find that a part at least of the afferent canals are formed by invaginated ectoderm. Ganin's observations on the development of Spongilla show that in that organism the afferent and efferent canals with ampullae are lined by endoderm, but the subdermal spaces (= Leibeshohle) are lined by an epithelium derived from neither ecto- nor endo-derm. Gotte has recently published an account of observations quite at variance with Ganin's, and hard to reconcile with facts observed in the development of other sponges. Cf. Z. A. vii. 1884 (A. N. H. (5) xv. 1885); Z. A. viii. 1885.

Porifera, Vosmaer, Bronn's Klass. und Ordnungen des Thierreichs, ii. (in progress). A monograph of Australian Sponges, von Lendenfeld, Proc. Linnean Soc. of New South Wales, ix. 1884.

Spongilla and its fresh-water allies. The following references are for convenience given to one work, where the titles of the originals will be found. Carter, A. N. H. (5): gemmule of Carterella, vii. 1881; ix. 1882; fossil spicules, x. 1882; xii. 1883. Notes, etc, xv. xvi. 1885. Dybowski, ibid., xiv. 1884. Hilgendorf, ibid., xii. .1883. Marshall, ibid., xii. 1883;(gemmule), xiii. 1884. Potts, ibid., ix. 1882; xiii. 1884; xv. 1885; xvii. 1886. Vejdovski, ibid., xiii. 1884; xv. 1885.

Retzer, Deutschen Susswasserschwamme, Inaug. diss., Tubingen, 1883.

Reproduction of Spongilla. Marshall, SB. Natf. Gesellsch., Leipzig, 1884, (Journal Roy. Micr. Soc. v. 1885, p. 1011); Ganin, Z. A. i. 1878; Gotte, Z. A. vii. 1884 (A. N. H. (5) xv. 1885); Id. Z. A. viii. 1885; Id. Abhandl. Entwick.-geschichte der Thiere, iii. Leipzig, 1886.

ChlorophyI corpuscles and amyloid deposits. Ray Lankester, Q. J. M. xxii. 1882. Chromatology. Sorby, Q. J. M. xv. 1875.

Euspongia, Schulze, Die Spongiden, Z. W. Z, xxxii. 1879.

Growth of spongin fibres, Von Lendenfeld, Z. W. Z. xxxviii. 1883, pp. 265, 285, PL xiii. Figs. 25-32.

Colouring matter of Sponges. Krukenberg, Vergleich. Physiol. Vortrage, ii. 1882, p. 51. Digestion in Sponges. Id., op. cit. iii. 1884, p. 125. Von Lendenfeld, Proc. Lin. Soc. N. S. W. ix. 1884, p. 434. Vital phenomena of Sponges, Solger, Biol. Centralblatt,'iii. 1883-84.

Coelenterate nature of Sponges. Marshall, A. N. H. (5) xvi. 1885 (transl.). Relation to Choano-flagellata. Schulze, A. N. H. (5) xv. 1885 (transl.).