PORIFERA. 155 



so that the skeleton-fibre consists essentially of minute flinty needles 



(% 48, D). 



3. In a third group of sponges, the skeleton is destitute of horny 

 matter, and consists wholly of siliceous spicules, which may be fused 

 with one another into a continuous framework, or may be so inter- 

 locked by their ends as to produce practical rigidity, or may be simply 

 held in position by the fleshy substance of the sponge. In both this 

 group and the preceding, in addition to the spicules of the proper 

 skeleton, there are generally developed in the mesoderm numerous 

 still more minute microscopic needles of flint, which are known as 

 " flesh-spicules." 



4. Lastly, there is a group of sponges in which the skeleton is 

 wholly made up of spicules of carbonate of lime. 



In all sponges which possess hard structures of flint or lime, the 

 " spicules " consist, each, of concentric layers of mineral matter de- 

 posited round a central fibre of organic matter which occupies an 

 " axial canal " in the spicule. In the Calcispongice each spicule may 

 be regarded as essentially a single crystal of carbonate of lime. In 

 the ordinary recent siliceous Sponges, on the other hand, the spicules 

 are composed of colloidal silica, which is as clear as glass, wholly 

 non-crystalline, and entirely unaffected by polarised light between 

 crossed Nicols. The silica of recent sponge-spicules is also soluble 

 in hot solutions of caustic potash. 



As regards the distribution of the Sponges in space, the great 

 majority are marine, but representatives of the Spongillidce are found 

 in fresh waters in all the great continental regions except Australia. 

 Of the marine Sponges, the Cakispongicz and Ceratospongicz are 

 principally inhabitants of shallow water. On the other hand, the 

 Hexactinellid Sponges are mostly found in deeper water, the major- 

 ity living at depths of from 100 to 2500 fathoms. The Lithistid 

 Sponges, on the contrary, are found in comparatively shallow water, 

 being most abundant between depths of 10 to 150 fathoms. 



In connection with the subject of the distribution iii time of the 

 Sponges, it is necessary to briefly consider the different modes in which 

 the skeleton of the Sponges may be preserved in the fossil condition. 

 Taking first the Sponges which secrete a siliceous skeleton, it has been 

 pointed out above that the silica of the spicules or spicular network of 

 these, in the recent condition, is colloidal and non-crystalline, glassy, and 

 entirely unaffected by polarised light. Though unaffected by acids, the 

 spicules undergo solution in hot caustic potash, and there is reason 

 to know that they readily become changed, or even dissolved, under 

 certain natural conditions which it is difficult or impossible to imitate in 

 the laboratory. While the above is the condition of the siliceous spicules 

 in recent Sponges, these structures in fossil Sponges have, as a rule, under- 

 gone more or less change during the process of fossilisation, the following 

 being the principal forms of alteration which have been observed. 



(a.) The silica of the spicules may be changed into an amorphous 



