PORIFERA. 797 



of an axis, as appears to be the case with the triaxiles of Calcarea and some 

 other Porifera : (4) polyaxile ; as in the stellate, or the globular, spicule ; 

 the stellate globule, &c. The ends of the axes may be pointed, knobbed, 

 or branched. Most sponges possess a variety of spicules when they are 

 present : some Calcarea^ Renieridae, and Suberitidae, have needle-shaped 

 monaxiles alone. Structures may occur which fall under none of the four 

 types, e. g. the amphidiscs of the gemmules of Spongilla (p. 250) \ 



The siliceous spicules may be united in o*ne of three ways ; (i) 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-keratose 2 , variable in amount, e.g. jn Halichon- 

 dridae, completely surrounding them as in many Desmacidonidae^ or com- 

 pletely 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- 

 tosd), 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 



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 sponge- 

 like material which does not polarise light. Vosmaer found great variability in the polarising power 

 of spongin ; sometimes none at all, as in Aplysilla sulfurea and Siphonochalina coriacea (Mitth. Zool. 

 Stat. Naples, v. p. 491-2). 



