796 THE ANIMAL KINGDOM. 



The skeletal structures are either inorganic or organic, separate or in 

 union, or partially, very rarely completely, represented by foreign inorganic 

 bodies. They are rarely absent entirely, as in Oscarella, Halisarca, Bajalus, 

 and Chondrosia, and are always lodged in the mesoglaea. The proper 

 inorganic skeleton is made up of spicules. A spicule consists of a very 

 scanty organic basis or spiculin, laminated, hardened by Calcium carbonate 

 as Calcite in the Calcarea, by colloidal silica in the Non-Calcarea, and 

 inclosing an axial thread of organic matter, the so-called canal, said to 

 be absent sometimes. It is formed in the first instance within a cell 1 , 

 beyond which it soon projects. It continues to increase in length and 

 thickness. The calcareous spicule is covered by a sheath or cuticle, by 

 the calcification of which the spicule grows, and outside the cuticle by 

 a layer of mesoglaeal cells except when it projects freely, the covering cells 

 in this case being derived from the ectoderm (or in the gastric cavity from 

 the endoderm?). The siliceous spicule appears to remain in close con- 

 nection with a cell until it is of full size. When it projects above the 

 surface it is, so far as is known, naked. It sometimes attains a great 

 length, especially in the rooting spicules of Hyalospongiae^ e. g. of Hyalo- 

 nema> which reach even two feet or more. As to shape, four leading 

 types are recognised in accordance with the number of axes traceable. 

 (i) Monaxile ; straight and then pointed, blunt, or knobbed, smooth or 

 spinulose ; curved, and then of very various shapes hook, anchor, bow, 

 &c. : (2) triaxile, characteristic of Hyalospongiae and only present in 

 rudiment elsewhere ; the three axes are those of an octohedron, equal 

 or unequal in length, one even suppressed in some instances : (3) tetraxile ; 

 the four axes are lines drawn from the centre to the vertices of a tetra- 

 hedron ; of equal or unequal length ; becoming triaxile by the suppression 



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, PI. 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 propor- 

 tions 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. 



