so 



THE CHAIN OF LIFE. 



Hexactinellid, and the wonderful weaving of its spicules is as 

 marvellous a triumph of constructive skill as its general form 

 is graceful. The Lithistids are less beautiful, but are the 

 densest and most compact of sponges, and are represented 

 by several species in the modern seas. Both of these types 

 go back to the Early Cambrian, and have continued side by 

 side to the present day, as representatives of two distinct geo- 

 metrical methods for the construction of a spicular skeleton. 



Many years ago the keen eye of thj late lamented Salter 

 detected in a stain on the surface of a slab of Cambrian slate 

 the remains of a sponge ; and minute examination showed that 

 its spicules crossed each other, and formed lattice-work on the 



Fig. 30.- /'rtf/£jj/£7«^/Vty<^«<'s^rrt/rt (Salter). Menevian group. «, Fragment showing the 

 spicules partially displaced, i, Portion enlarged. 



hexactinellid plan. Salter boldly named it Protospongia (the 

 first sponge), and it is still the earliest that we know (Fig. 

 30). Thus the type whose skeleton is the most perfect in a 

 mechanical point of view takes the lead. It is continued in 

 the Silurian in many curious forms, of which the stalkless 

 sponges (Astylospongia) are the most common (Fig. 31). It 

 perhaps attains its maximum in the Cretaceous, from which the 

 beautiful example in Fig. 29 is taken, and it still flourishes, 

 giving us the most beautiful of all recent forms. Before the 

 expiration of the Cambrian there were other sponges of the 

 Lithistid type. Fig. 32 represents a group of spicules from 

 the Calciferous (Lowest Silurian or Upper Cambrian) of 



