PALAEOZOIC AGE 



SILURIAN PERIOD 



SPONGES 



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 hfe was necessarily attended by 

 the falUng 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 developrhent 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 prejdng 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 nowin 

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



28 



CORALS 



k 



