THE AGE OF INVERTEBRATES OF THE SEA. S3 



that Geinitz has described two species from the Permian which 

 he believed to be early precursors of the Spongillae, or fresh- 

 water spopges ; but more recently he seems to regard them 

 as probably Algae. Young has, however, recently found true 

 spicules of Spongilla in the Purbeck beds.^ 



A stage higher than the sponges are those little polyp-like 

 animals with sac-like bodies and radiating arms or tentacles, 

 which form minute horny c- calcareous cells, and bud out into 

 branching communities, looking to untrained eyes like delicate 

 sea-weeds — the sea-firs and sea-mosses of our coasts (Fig. 36). 

 These belong to a very old group, for in the oldest Cambrian 

 we have a form referred to this type (Fig. 33), and in the 



Fig, 2,l.—Dictyonemn IVeisieri (T)n). Niagara formation. 

 a. Enlarged portion {Acadian Geology). 



Upper Cambrian another still more decided example (Fig. 

 34).2 This style of life, once introduced, must have increased 

 in variety and extended itself with amazing rapidity, for in the 

 Siluro-Cambrian age we find it already as characteristic as in 

 our modern seas, and so abundant that vast thicknesses of 

 shale are filled and blackened with the debris of forms allied 

 to the sea-firs, and masses of limestone largely made up of the 

 more calcareous forms of the sea-mosses. As examples of the 

 former we may take the Graptolites, so named from their re- 

 semblance to lines of writing, and of which several forms 



1 Geological Magazine, ■'^'", 1878. 



^ It is regarded as sonk;^./bat doubtful whether these a-? Ilydroids or 

 Bryozoa. 



