188 THE MICROSCOPE. 



l-8fch of an inch in length, others are as small as the 1-2 00th of an 

 inch. (Fig. 103, No. 6.) 



The purple-coloured spicula of the outer surface of the Sea-pen 

 (Renilla Americana) are cylindrical, a little twisted, and seem to have 

 a central canal ; they are the l-70th of an inch long, by the 800th in 

 the short diameter. (Fig. 103, No. 3.) 



The dull lake or dirty-white spicula of the crust of Gorgonia elon- 

 gata are bowed with projecting^ from one side (fig. 103, No. 5) ; but 

 they are commonly elongated, covered with tubercles arranged in rings. 



Muricca elongata are yellowish-brown, of a pine-apple shape ; one 

 extremity covered with short sharp-pointed tubercles, the other ex- 

 panded with spines ; the free extremities of which all point in one 

 direction. 



Spicula from the fleshy crust of Isis hippuris, mostly of a quadrate 

 or clavate figure, covered with large modulated tubercles, are repre- 

 sented in fig. 103, No. 4. 



The Gorgonia umbraculum present two kinds, one found in the 

 crust, of large stellate angular figure and rich brown colour ; the 

 other much smaller, and covered with modulated tubercles. (Fig. 103, 

 No. 7.) 



When a section is made of the Myriapora, Nullepora alciformis, it 

 is seen, as in fig. 103, No. 1, to be of a reticulated structure, the diameter 

 of each cell of which is about the l-300th of an inch. 



Professor Quekett states that the skeletons of sponges are composed 

 principally of two materials, the one animal, the other mineral ; the 

 first of a fibrous horny nature, the second either siliceous or calcareous. 

 The fibrous portion consists of a network of smooth, and more or less 

 cylindrical threads, of a light-yellow colour, and, with few exceptions, 

 always solid ; they frequently anastomose, and vary considerably in 

 size ; when developed to a great extent, needle-shaped siliceous bodies 

 termed spicula (little spines) are formed in their interior j in a few 

 cases only one of these spicula is met with, but most commonly they 

 occur in bundles. In some sponges, as those belonging to the genus 

 Halichondria, the same horny kind of material is present in greater or 

 less abundance, but its fibrous structure has become obscure ; the fibres, 

 however, in these cases are represented by siliceous needle-shaped spicula, 

 and the horny matter serves the important office of binding them firmly 

 together, as shown in Plate IV. No. 1. There is, however, one remark- 

 able exception to this rule, viz. Dictyochalix pumiceus, described by 

 Mr. S. Stutchbury, in which the fibrous skeleton is composed of threads 

 of silex quite as transparent as glass. 



