SPONGES. 



287 



ceous needle-shaped spicula, and the horny matter server 

 the important office of binding them firmly together, as 

 shown in fig. 170, No. 1. There is, however, one remark- 



Fig. 172. 



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 Gorgona elongata. 6, 

 Spicula from Alcyonium. 7, Spicula from Gorgonia umbraculum. 



able exception to this rule, viz. Dictyochalix pumiceus, 

 described by Mr. S. Stutchbury, in which the fibrous ske- 

 leton is composed of threads of silex quite as transparent 

 as glass. 



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, 



