They surround in the Tetillae in question 12-20 ampullae, but in the Tethya ampullae grow out among the cells and then multiply in number. The bud-mass is gradually extruded either along a fascicle of spicules belonging to the parent, or by an outward movement of the fascicle. It has an ectoderm; spicules develope in it and in the Tethya named subcortical spaces. In Tetilla japonica the mass of cells contains no ampullae, nor does it in Tethya lyncurium, where it originates by the division of a single cell. Buds of a similar external character, the histology of which is not known, occur also in two or three other species of Tethya, in a Suberites, and in Rinalda (=Polymastia?) arctica. They are developed in the last-named at the apex of hollow conular processes of the sponge, the walls of which are perforated by pores1; and there are usually two or three buds in a line, one beyond the other, connected by spicules, and a small quantity of sponge-substance, a phenomenon seen also occasionally in a Tethya from the White Sea. In Craniella Muleri (=Tethya s. Tetilla cranium) buds of the same kind develope into sponges within the parent.

The freshwater Spongillidae, with one or two exceptions, develope resting buds, the seeds or gemmules, at the commencement of the winter in cold regions, of the hot season in tropical. They possess a protective envelope or case, an organic membrane hardened, at least in some instances, by silica, generally strengthened by siliceous spicules as well, often complex in structure and provided with a single aperture, or with a principal and several secondary apertures. A hydrostatic apparatus, or vesicular dilatation of the special membrane or cuticle, which immediately invests the contents of the case, closes the aperture in two instances. The envelope is said to be formed by the outer layer of cells of the gemmule in Meyenia (Spongilla) fluviatilis, by mesoglaeal cells in Spongilla lacustris. Its contents are granular cells, closely packed, derived from mesoglaeal and transformed endodermal cells. When climatic conditions are favourable, they escape and reproduce the sponge. See p. 2502. The production of gemmules brings about the death of the sponge.

1According to Ganin, the larval ectoderm persists; the endodermal cavity becomes the internal cavity, and outgrowths of it give rise to the ampullae, but the subdermal cavities originate independently as spaces between the ectoderm and mesoglaeal cells.

2 Whether or no buds are formed by the solid branching stolons of Esperia stolonifera from the White Sea, described by Merejkowsky (Mem. Imp. Acad. St. Petersburg (7), xxvi. No. 7, p. 23), seems uncertain.

3The ampullae probably multiply by division, or by the separation of a small part: see Keller on Reniera semitubulosa, Z. W. Z. xxx. p. 579. Schulze states that isolated ampullae are met with on the walls of the oscular tube where they are very thin in Oscarella lobularis: see Z. W. Z. xxviii. p. 23, Pl. II. Fig. 9. The view held by some authorities that the ampulla represents an individual, is negatived by the facts of embryology and the undoubted homology existing between ampullae and radial cones.

A peculiar mode of propagation has been observed in Oscarella lobularis. The papillae of the surface, or the whole sponge, if small, may be converted into globular vesicles or brood-buds, 2-3 mm. in diameter. The wall of such a bud is composed of ectoderm, mesoglaea and endoderm, It contains semi-globular ampullae, which open externally by narrow pore-canals, internally by a single aperture. Its internal cavity is the dilated exhalent canal system of the papilla or sponge, as the case may be. These brood-buds float about, but finally settle down, flatten out, and give rise each to a new sponge.

1Merejkowsky thinks that these conular processes are converted into oscular tubes, after having served as supports for the buds: Mem. Imp. Acad. St. Petersburg, (7), xxvi. No. 7, p. 10.

2Goette, Wierzejski, and Marshall do not quite agree in their accounts of the mode in which the gemmule and its case are formed. See the summary by Vosmaer, Porifera, Bronn's Thierreich, ii. pp. 428-9; Goette, Abhandl. Entwickelungsgeschichte der Thiere, Leipzig, pt. 3, 1886, p. 21; Wierzejski, Archives Slaves de Biologie, i. 1886; Marshall, SB. Naturf. Ges. Leipzig, 1884, pp. 22-9, or Journ. Royal Micr. Soc. (2), v. 1885, p. 1011.

Gemmules were said by O. Schmidt to occur in a marine Reniera (Z. W. Z. xxv. Suppl. p. 139): but Keller states that he observed similar structures in a dead Chalinula fertilis, and that they proved to be the egg-masses of a Dinophilus (Z. W. Z. xxxiii. p. 341).

With the exception of Spongillidae, the Porifera are marine. They are found in all seas. The Calcarea are cosmopolitan. The majority of Hyalospongiae live at depths greater than 150 fathoms, often descending below 400, as do the Lithistina below 100. Most Halichondrina and Ceratina are restricted by a zone of 50 fathoms, the Calcarea of 100, but Ascetta (Leucosolenid) blanca has been dredged at 450. Sponges frequently shelter animals of other groups, especially Crustacea, within their canal systems. As to size, they vary extremely, some attaining a maximum limit, others differing much. The Calcarea are small; e. g. the mean size of the Asconidae is 1-3 mm., of the Syconidae and Leuconidae 15-20 mm. (i.e.

(3-4)/5 in.). The goblet sponges, Poterion, are the largest known. P. Amphitritae may attain a height of 31/2 ft., and its cup a diameter of 21/2 ft. The Calcarea are colourless; other sponges are variously tinted, but usually of one colour. The Spongillidae possess chlorophyll; see pp. 242-4, 251. The yellow pigment Aplysinofulvin turns blue, then black on exposure to air. In some instances coloration is due solely to algae living in the mesoglaea 1. Many sponges exhale a strong and peculiar odour somewhat resembling oxydising phosphorus or ozone2. Fossil forms are numerous. As to Calcarea, a Syconidan, Protosycon, has been found in Jurassic strata, and an extinct group, the Pharetrones of Zittel, represented by a single species in the Devonian, is greatly developed in the Mesozoic period, and dies out in the Eocene3. Of the Non-Calcarea, the Hyalospongiae and Lithistina appear in, the Silurian, and attain their maximum development in the Upper Chalk; the Tetraxonina are sparingly represented in Carboniferous strata and the Chalk. Shells, bored, as it is supposed, by a Clione, occur in Jurassic, Cretaceous and Tertiary formations, and the spicules of a Spongilla (Sp. Purbeckensis) have been detected in freshwater limestones of the Purbeck series (Jurassic).

1See the table in Vosmaer, Porifera, p. 458; Brandt, Mittheil. Zool. Stat. Naples, iv. p. 223, and Carter, A. N. H. (5), ii. 1878. According to Brandt, Zooxanthella (see p. 243, ante) is found in Hircinia variabilis and Reniera cratera.

2See Krukenberg, Vergleich. Physiol. Studien (1), 2, p. 37, and cf. p. 44. The smell is pro-bably due to an ethereal oil.

3Inasmuch as lime often replaces silica and vice versd in the fossil spicules of sponges, the position of the Pharetrones has been much debated. But the complete accordance of their spicules with those of living Calcarea, the fact that they may be oval or rhomboidal in section, whilst true siliceous spicules are round, has led the most recent authorities to support Zittel's view. See von Dunikowski, Palaeontographica, xxix. 1882-3; Hinde, A. N. H. (5), x. 1882, and Catalogue of Fossil Sponges, Brit. Mus. 1883, p. 157; Sollas, Sci. Proc. Royal Dublin Soc. iv. 1885, p. 387, and cf. pp. 389-90. Calcarean spicules occur in Pleiocene beds (Hinde, Quart. Journ. Geol. Soc. xlii. 1886, p. 214). For lime replacing silica, see Sollas, A. N. H. (5), vi. p. 437 et seqq., and vice versa, Id. Quart. Journ. Geol. Soc. xxxiii. pp. 252-4, 813-19, 835; cf. Hinde, Ph. Tr. 176, pp. 425-33.