142 PROTOZOA. 



or with crystalline carbonate of lime. Though this dissolution 

 of a flinty skeleton, with or without replacement by lime, is 

 at variance with all preconceived ideas on this subject, and 

 though it is very difficult to give any adequate or satisfactory 

 explanation of the way in which it is effected, the researches 

 of Prof. Zittel and Mr Sollas hardly allow us to doubt its 

 actual occurrence in nature. It is impossible here to pursue 

 this intricate and still controverted subject further ; but it 

 may be pointed out that in this dissolution of the skeletons 

 of siliceous Sponges (and of other flinty organisms) by waters 

 percolating through sediments in course of formation, we find 

 a sufficient source for supplying the amorphous and gelat- 

 inous silica of the chalk-flints. A similar origin may with 

 all probability be ascribed to at least some of the chert- 

 nodules so common in many formations. 



As regards the classification of the Siliceous Sponges, natu- 

 ralists now universally accept the division of the group into 

 the two primary sections of the Hexactinellidce and Litliistidce, 

 first proposed by Oscar Schmidt, and subsequently very 

 variously subdivided by different authorities. It is not 

 necessary to consider the minor subdivisions here, but we 

 may define the two primary sections above named, and 

 glance at some of the leading types of each, taking these 

 in geological rather than zoological order. 



(A.) Section Hexactinellidce. Siliceous Sponges, the fun- 

 damental elements in the skeleton of which are six -rayed 

 spicides, the rays of ivhich are almost invariably at right 

 angles to each other. In the centre of each spicule is an 

 axial canal, consisting of three tubes cutting each other at 

 right angles. The spicules may be only united by sarcode, 

 or they may be fused together by amorphous silica, in either 

 case being so disposed as to form a trellis- work (fig. 38, B) 

 with rectangular or polyhedral meshes. Besides the true 

 " skeleton-spicules," the sarcode contains (in the living forms 

 at any rate) numerous detached " flesh-spicules," which are 

 also fundamentally six-armed, but which give off secondary 

 branches, so as to form a " rosette." 



Until of late years, even the living Hexactinellidce were 

 very little known, the most familiar example of the group 



