126 



ZOOLOGY 



SECT. 



being members of existing groups. 1 Some of the orders of existing 

 Sponges such as the Myxospongise are incapable of being 

 preserved as fossils, and the fossil forms belong, as we should 

 expect, to the more highly silicifiedjgroups and to the more complex 

 groups ofjthe Calcarea. 



FIG. 95. Development of Sycon raphanus. a, ovum ; b, c, ovum segmented b, as seen 

 from above, c, lateral view ; d, blastula ; e, amphiblastula ; /, commencement of invagi- 

 nation ; g, larva attached by its oral face ; h, i, young sponge h, lateral view ; i, as 

 seen from above. (From Sollas, after Schulze.) 



Fresh-water Sponges (Spongillidce) occur in rivers, canals, and 

 lakes in all the great divisions of the earth's surface. Marine 

 Sponges occur in all seas, and at all depths, from the shore 



1 The Archceocyathince, reef-forming "organisms of Cambrian age, may 

 have been nearly related to the Calcareous Sponges ; but this remains 

 doubtful. 



