LIFE IN PRIMARY PERIOD 209 



and the same can be said of the other forms included by 

 various writers among the Tabulata. 1 On the other hand, 

 the Hexacorallia are noted for the defmiteness of the 

 chambers of their polypary and the regular arrangement 

 into radiate systems of the septa dividing them, and all these 

 characters can be logically deduced from the theory classing 

 them with Hydrocorallia. 



Nothing like this is found among the Tetracorallia of the 

 Primary Period which have septa and chambers of a very 

 rudimentary character. Some fixed Medusae exist, e.g. the 

 Lucernarians and others, and the large Medusae constituting 

 the Acelephae class, also begin here in the form of fixed 

 Scyphistoma. Since certain Hydractinias, and all the Hydro- 

 corallia, which are real hydroid polyps, secrete lime abundantly, 

 there is nothing improbable in the assumption that organisms 

 comparable with Lucernaria and the Scyphistoma were able to 

 do the same, and we should therefore regard as kindred animals 

 the Tetracorallia which constructed around the continents 

 of the Devonian and Carboniferous ages the vast coral reefs 

 which afterwards became marble. Theoretically, these Tetra- 

 corallia should have appeared before the Hexacorallia. 



Pleurodictyum problematicum of the Devonian Epoch, which 

 looks like a deep funnel, with walls formed of elliptical particles 

 united by transverse twigs, at the bottom of which is 

 found a kind of a Serpula bent back upon itself, more nearly 

 resembles a Bryozoan like the Adeona of the present day, which 

 is likewise funnel-shaped, than a hydroid. It would, however, 

 be imprudent to go too fast when certain forms intermediate 

 between the Hydrocorallia and the Bryozoa are under dis- 

 cussion. The latter are easily recognized from the Silurian 

 onwards. They have persisted in large numbers up to the 

 present day without ever playing an important part, or showing 

 the least tendency towards a higher evolution. We shall 

 therefore pass them over like the Sponges. 



We now come to the Artiozoa, which began simultaneously 

 in two kinds of segmented animal organisms, the Arthropods, 

 which have preserved their primitive structure almost intact, 

 and have not attained a high development, and the Worms, 

 which are also segmented but display great plasticity. We 



1 Favosites, Alveolites, Trachypora, Aulopora of the Devonian: Chcstetes, 

 Michelinia of the Carboniferous. 



