PALAEOZOIC AGE 



SPONGES 



SILURIAN PERIOD 



VEGETATION SILURIAN life attested a well-sustained progress. Vegetation 

 was becoming more diversified; club-mosses had greatly 

 increased in variety (Berwynia, Sagenaria) : and some plants 

 had now become so far evolved as to be distinguishable as 

 ferns (Sphenopteridium). All Silurian plants, it would seem, 

 reproduced by means of spores a somewhat circuitous and 

 extravagant process for the end in view. The seed-bearers 

 more straightforward and less prodigal in posterity matters 

 were yet to come. 



The advance in marine life was necessarily attended by 

 the falling out of many old forms and types. Increased com- 

 petition, and other changes in environment, though con- 

 ducive to progress, were bound to bring sectional reverses. 



Sponges with simple rod-like needles so prevalent at the 

 present time are first known in Silurian seas (Monaxida). 

 The flinty supports may in some cases have been worked 

 up into a framework, but probably they were more often 

 scattered about the porous " flesh." A rigid framework must 

 have interfered with a free development of canals ; and the 

 sponges that eschewed it developed, no doubt, highly im- 

 proved canal systems, and so became enabled to confront 

 more varied conditions. Some forms, in course of time, 

 entirely discarded needles. This was a hazardous proceeding, 

 for many animals no longer afraid of a prickly reception 

 took to preying on them. In long after-times this innocuous 

 condition also led to their use as bath-sponges. 



Solitary corals in thick-walled cups were numerous (Zaph- 

 rentis, etc.) ; but what may be called reef-building was now in 

 evidence. Certain forms, presenting in the mass a honey- 



28 



CORALS 



