Ill 



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 sckroblast. In the Non-Calcarea the skeleton either con- 

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



A.EusJDongia 



B. Pachychalina 



FIG. SO. Microscopic structure of the skeleton in various sponges. A, Eusponiria network 

 of spongin fibres; li, Pachychaliiia, spongin strengthened In- sili.vnus .-.pirnlcs: C, 

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

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



or of a combination of spongin fibres with siliceous spiciilrs ( 15) : in 

 some Myxospongise skeletal parts are altogether absent. Spminin 

 is a substance allied to silk in chemical composition : the fibn s aiv 

 exceedingly fine threads, consisting of a soft ^ranul;ir core and an 

 outer tube of concentric layers of spongin. These ilnvads 



