﻿84 EVOLUTION IN THE PAST 



appearing in thin and flexible body-coverings (Pelanechinus, 

 Keeping). Successors of these in the next Period (Echino- 

 thuria, S. P. Woodward) closely resembled some sea-urchins 

 now living (Calveria). 

 polyzoans The annals of polyzoans have not been very well preserved 

 in the rocks ; but there is evidence that some of the com- 

 munities had undergone important modifications. Vast 

 numbers of colonies with individuals in open-mouthed tubes 

 were living in the seas ; and were very similar to their fore- 

 runners. The rival colonies, however, exhibited a great 

 improvement. In these communities the orifice of the zooid- 

 tube, instead of being provided as formerly with a small 

 shaft, was furnished with a movable lid. The zooid, there- 

 fore, could at will keep the entrance to the tube open or shut 

 (Chilostomata), and so possessed more control over its 

 destinies. In some forms the lid improvement had only been 

 partially effected. 



The " open-mouthed " colonies, however, in spite of their 

 more primitive tubes, were far more prominent than their 

 rivals. There is little doubt indeed that the latter had 

 undergone great trials in the course of their reconstruction. 

 lampshells Brachiopods continued to decline, but several old families 

 maintained a stubborn front against " the slings and arrows ' 

 of fortune. This was notably the case with well-beaked 

 brachiopods — much in evidence also in the Trias. Brachio- 

 pods in " butterfly " shells— with an ancestry running back 

 to Silurian times — were still in existence (Spiriferina) ; but 

 they had little enough oil, so to speak, left in their lamps ; 

 and before the close of the Period their feeble lights all 

 flickered away. 



bivalved Bivalves were manifesting their adaptive powers in many 



molluscs new developments. 



Oysters of various forms were now in being ; and in some 

 regions they were gathered together in beds in the manner of 

 modern oysters (Ostrea, Exogyra). Many of them were 

 enclosed in shells curiously twisted by the growth of big 

 beaks, developed, it may be supposed, in connection with 

 fixation purposes (Grypfuea). Thorny oysters, vaguely loom- 

 ing for some time past, seem now to have reached a state 



