38 THE CHAIN OF LIFE. 



the Sack -corals. Both are quite remote from Eozoon in 

 structure, wanting its complexity in the matter of minute 

 tubules, and having greater regularity and complication on the 

 large scale. Archaeocyathus had the form of a hollow inverted 

 cone with double perforated walls, connected by radiating 

 irregular plates, also perforated (Fig. 24). It has been re- 

 garded as a sporge, and some species are certainly accompanied 

 with spicules ; but these seem to be merely accidental, and will 



Fig. 25.— ReceptacuUtes. Restored.— After Billings. 



it. Aperture. /;, Inner wall, c. Outer wall, n, Nucleus, cr primarj' chamber. 



V, Internal cavity. 



be referred to in the next chapter. Archaeocyathus came in with 

 the Later Cambrian, and seems to have died out in the Siluro- 

 Cambrian. The only more modern things which at all resemble 

 it are ;he Foraminifera called Dactylopora, which belong to the 

 Tertiary period. 



ReceptacuUtes is a still more complex organism. It has a 

 sack-like form, often attaining a large size, and the double 

 walls are composed of square or rhombic plates, connected 



