SPONGES. 



397 



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

 ceous needle-shaped spicula, and the horny matter serves 

 the important office of binding them firmly together, as 





-■ftU&S\ 



M>#.33 



Fig. 215. 



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



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



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

 Spicula from Alcyonium. 7, Spicula from Gorgonia 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. 



