PHYLUM AND CLASS PORIFERA 



111 



The elements of the skeleton differ in character in the different 

 classes. In the Calcarea they consist of calcareous spicules, usually 

 triradiate in form. Each of these spicules is developed from a single 

 cell the sclcrdblast. In the Non-Calcarea the skeleton either con- 

 sists of spongin fibres alone (Fig. 80, A), or of siliceous spicules alone, 



B.Pachychaiina 



FIG. 80. Microscopic structure of the skeleton in various sponges. A, Euspongla, network 

 of spongin fibres ; B, Pachychalina, spongin strengthened by siliceous spicules ; C, 

 Spongelia, spongin strengthened by various foreign siliceous bodies, fragments of spicules 

 of other sponges, &c. (After Vosmaer.) 



or of a combination of spongin fibres with siliceous spicules (B) : in 

 some MyxospongiaB skeletal parts are altogether absent. Spongin 

 is a substance allied to silk in chemical composition : the fibres are 

 exceedingly fine threads, consisting of a soft granular core and an 

 outer tube of concentric layers of spongin. These threads branch 



