Structures may occur which fall under none of the four types, e. g. the amphidiscs of the gemmules of Spongilla (p. 250)1.

1 For the origin of the calcareous spicule within a cell, see Metschnikoff, Z. W. Z. xxxii. pp. 361, 369, and Balfour's Comp. Embryology, i. note p. 117; similarly for the siliceous, Keller, Z. W. Z. xxxiii. p. 334; Schulze, Z. W. Z. xxxiv. p. 421; Deszo, A. M. A. xvi. p. 640; Goette, Abhandl. Entwick. der Thiere, pt. 3, Leipzig, 1886, p, 16; Sollas, infra. Polejaeff has described cells (calcoblasts) lying upon the calcareous spicules of Ute argentea and Leuconia multiformis, which, he suggests, may be concerned in their growth (Calcarea, Challenger Reports, viii. p. 32, Pl. VI. Fig. 3 c.). The statements in the text relative to the calcareous spicule are derived from von Len-denfeld's paper on the 'Histology of the Calcispongiae' in Proc. Lin. Soc. New South Wales, ix. pp. 979-80. Sollas states (A. N. H. (5), ix. p. 159), that he not only observed the origin of siliceous spicules within cells in the young Craniella s. Tetilla cranium, but that he found in this and some other sponges an association of a cell with 'all not fully developed spicules.' The relative proportions of silicified and organic layers to one another is variable, and the spicules may even be bendable; see Bowerbank, British Spongiadae, Ray Soc. i. p. 7 et seqq.

The calcareous spicule is doubly refractile according to Haeckel. Max Schultze found that the organic layers of the spicules in Hyalonema were so: Die Hyalonemen, Bonn, 1860, pp. 17-18. For the proofs of the statement that the silica is in a colloidal condition, and the lime carbonate in the crystalline form of Calcite, see Sollas, Sci. Proc. Royal Dublin Soc. iv. 1885, p. 374. The siliceous spicule is often attacked by a minute vegetable parasite, the Spongiophagus Carteri of Martin Duncan (A. N. H. (5), viii. p. 120), and sometimes, perhaps, by Achlya penetrans so common in corals; see Id. Journ. Royal Micr. Soc. (2), i. 1881, p. 557.

The siliceous spicules may be united in one of three ways; (1) by interlocking processes as in Lithistidae; (2) by a deposit of silica involving the bundles as a whole or only their ends as in most Hyalospongiae; and (3) by spongin or pseudo-keratose2, variable in amount, e.g. in Halichon-dridae, completely surrounding them as in many Desmacidonidae, or completely surrounding some, and only one end of others forming a hispid fibre as in Ectyonidae. Spicules united together are often termed 'skeleton' spicules, whilst the loose spicules which occur in the same sponge are known as 'flesh' spicules. When the spicules are all loose, they may lie in a confused manner, or disposed in regular bundles as in many Tetractina, or in a definite order as in Calcarea.

The organic skeleton, especially characteristic of the Ceratina ( = Kera~ tosa), is composed of spongin, a substance close akin chemically to silk. It takes the form of triaxile spicules in Darwinella, side by side however with fibrous structures, the usual shape in which it occurs. The fibrous skeleton may consist of isolated and slightly branched stems, rooted to some foreign object, Aplysilla; of a much branched tree, Dendrilla; of a regular network in which the fibres are nearly equal in size, Velinea; or Of principal radial or vertical fibres connected by more slender transverse fibres, e.g. Spongidae. The free extremities of the principal fibres lie immediately beneath the surface of the sponge which they raise into conuli (Fig. 12, A. c. p. 251). The fibre has a cortex and a medulla; the former laminated, yellowish, clear, refractile, polarising light to a variable degree (note 2, infra), the latter more or less granular and opaque. The relative proportions to one another of the two parts is not the same in all sponges, nor even in fibres of the same diameter in the same sponge. The medulla is plentiful in the Darwinellidae (=Aplysillidae) and Aplysinidae; so scanty in some Spongidae that the fibre appears homogeneous, though it is doubtful if it is ever really so.

The formation and growth of the fibre are due to spongoblasts; see Fig. 12, C, p. 251. The apex of the fibre is capped by a mass of polygonal cells: its surface clothed with pyriform cells, placed radially to it, and the two sets of cells are in continuity. The cortex increases by the addition to the apex of the fibre of caps of spongin prolonged laterally into lamellae. A certain proportion of medulla, derived probably by the metamorphosis of some of the polygonal cells in the apical cap, is inclosed between successive caps of spongin. The medulla becomes as a rule continuous throughout the system of fibres. A structure closely resembling that of the fresh formed fibre is retained in Aplysilla, Aplysinay and Dendrilla1.

1 For a shorthand mode of describing the spicular skeleton, see Vosmaer, Tidschrift Nederl. Dierk. Vereen. v. p. 197.

2 The term 'pseudokeratose' has been proposed by Ridley (J. L. S. xv. p. 481) for a spongelike material which does not polarise light. Vosmaer found great variability in the polarising power of spongin; sometimes none at all, as in Aplysilla sulftirea and Siphonochalina coriacea (Mitth. Zool. Stat. Naples, v. p. 491-2).