316 A MONOGRAPH OF THE AUSTRALIAN SPONGES, 



Within these horny fibres sometimes cells are to be found, 

 which like the Osteoklasts of the vertebrate animals destroy the 

 hard substance, here the Spongiolin or horn, and cause the skeleton 

 to appear as a system of tubes filled with a soft core. (Aplysillidse, 

 von Lendenfeld.) It is perhaps possible that these cells are 

 Algfe symbyotic in the Sponge. 



Silicious spicules may be formed within the horny fibres, and 

 these always belong to the Monactinellid type, a rod with one or 

 two points. They are small in number and size at the beginning 

 (Veluspa, Miklouho-Maclay), but get larger and more numerous 

 until the horny fibres are nearly entirely replaced by dense bundles 

 consisting of Monactinellid spicules only. 



At the same time in all of the stages from a soft non-skeletous 

 Sponge (Halisarca), to a Sponge with a strong skeleton of silica- 

 cords (Suberites), silicious spicules may be produced outside the 

 fibres in the ground substance of the Sponge. These iiever belong 

 to the Monactinellid type. They are always originally charac- 

 terized by possessing more than two ends — more than one axis. 

 These " flesh spicules " attain very extraordinary shapes. If they 

 appear in Sponges, which already possess a strong fibrous skeleton, 

 they remain small and loose (Desniacidonidas, O. Schmidt) ; if on 

 the other hand they make their appearance in Sponges which are 

 destitute of a fibrous skeleton they attain a larger size and 

 coalesce to form a hard, continuous skeleton (Hexactinellidse, 

 Tetractinellida?, Lithisticlre. Monactipyalea.) Originally these 

 spicules were small and scattered. Such ancestral forms were 

 probably allied to the Plakinidas (F. E. Schulze) ; or to those 

 Chondrosida?, which possess silicious bodies in their outer layer. 



The Sponges are therefore, as far as the skeleton is concerned, 

 to be grouped along two divergent directions, both of which take 

 their origin in the Sponges without a skeleton. The one row 

 comprises the calcareous, the other the horny and silicous Sponges. 

 (Grant, Vosma^r.) The latter group appears as a straight row 

 (stem) culminating in those fibrous Sponges, in which all the 

 horny substance has been replaced by Monactinellid silicous 

 spicules, and comprising the horny Sponges, Ceraospongige (0. 



