Form and Distribution of Silica in Sponges

Opaline sclerites, spicules composed of hydrated silica, comprise all or part of the skeleton of three of the extant classes of the phylum Porifera, namely, the Demospongiae, Sclerospongiae, and Hexactinellida. The occurrence of mineralized spicules is generally regarded as one of the most distinctive characteristics of the phylum, although three of the orders of the Demospongiae lack them entirely, and in scattered species among other demosponge orders as well, spicules are absent. Furthermore, some of the genera of sclerosponges and Calcarea with non- spicular massive skeletons lack spicular elements. A majority of living sponge species do possess spicules, however, and these exhibit a great variety of forms characteristic of species and hence useful in taxonomic endeavors. Within a single sponge species from one to eight or more kinds of spicules may occur. In a number of lines of demosponges and hexactinellids there is a tendency toward the deposition of secondary deposits of silica on the basic spicule forms with the resulting formation of rigid skeletal frameworks composed of fused or interlocking spicules. Figures 16-13 to 16-30 in this chapter appear sequentially in the section entitled “Evolution of Kinds of Spicules in Sponges.”