SPONGES. 



397 



tlie fibres, how3ver, in these eases are represented by sili- 

 ceous needle-shaped spicula, and the horny matter serve?^ 

 the important office of binding them firmly together, as 



Fig. 215. 



1. Transverse section of a branch of Myrlapnre. 2, A section of the stem n[' 



VirgiUarla mirahilis. 3, A spiculum from the outer surface of a Sea-pen. v, 



Spicula from crust of Isis hlppiiris. 5, Spicula from Gorgona elongata. 0, 

 Spicula from Alcyoniuvi. 7, Spicula from Gorgonia umhraculum. 



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

 markable exceptions to this rule, one, Dlctyochalix piimi- 

 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 tho 

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

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



