i 



THE AGE OF INVERTEBRATES OF THE SEA. 49 



sponges may be roughly divided into three groups : 1, those 

 with corneous or horny skeleton, like our common washing 

 sponges; 2, those with skeletons composed of silicious 

 needles of various forms and arrangement; 3, those with 

 calcareous spicules. Of these, the second or silicious group 

 has precedence in point of time, beginning in the Early Cam- 

 brian, and continuing to the present. Two of its subdivisions 

 are especially interesting in their range. The first is that of 

 the Lattice-sponges {Hexactinellidce), in vhich the spicules have 



Fig. 2g. — Portion of skeleton of Hetactinpllid Sponge {Caeloftychium). Magnified. 



After Zittel. 



six rays placed at right angles, and are attached to each other 

 by their points, so as to form a very regular network (Fig. 29). 

 The second is that of the Stone-sponges {LithisfidcB), in which 

 the spicules are four-rayed or irregular, and are united by the 

 branching root-like ends of the rays. The most beautiful of all 

 sponges, the Venus Flower-basket {Euplectelld)^ is a modern 



E 



