SPONGES. 



397 



the fibres, however, in these cases are represented ly sili- 

 ceous needle-shaped spicula, and the horny matter serves 

 the important office of binding them firmly together, as 



Fig. 216. 



1. Transverse section of a branch of Myriapore. 2, A section of the stem of 



Virgularia mirabilis. 3, A spiculum from the outer surface of a Sea-pen. 4, 



Spicula from crust of Isis hippuris. 5, Spicula from Gorgonct elongata. 6, 

 Spicula from Alcyonium. 7, Spicula from Gorgonict, umbraculum. 



shown in fig. 213, ~No. 1. There are, however, some re- 

 markable exceptions to this rule, one, Dictyochalix pumi- 

 ceus, described by Mr. S. Stutchbury, in which the fibrous 

 skeleton is composed of threads of silex quite as trans- 

 parent as glass ; another, the Hyalonema, Glass-rope. 



The mineral portion, as before stated, consists of spicula 

 composed either of silica or carbonate of lime ; the first 

 kind is the most common and likewise most variable in 

 shape, and presents every gradation in form, from the 

 acuate or needle-shaped to that of a star. The calcareous 

 spicula, on the contrary, are more simple in their form, 



