LIFE IN THE PALEOZOIC EPOCH 



459 



group died out entirely before the end of the epoch. They were 

 very probably related to the existing Hydroid Zoophytes, and 

 most of them belonged to the plankton of their time, and were 

 most likely attached to drifting masses of sea -weed. In fact, 

 Sargasso conditions were probably then more widely spread than 

 now. 



Corals were extremely abundant, and some of them were reef- 

 builders, but they were mostly of a more primitive type than those 

 now existing, and chiefly belonged to the extinct group of Four- 

 Rayed Sea- Flowers (Tetrac- 

 tinia or Rugosa}. 



PALEOZOIC H EDGEHOG - 

 SKINNED ANIMALS (ECHINO- 

 DERMATA). Sea- Lilies (Cn- 

 noidea), now a declining 

 group, were extremely abun- 

 dant, and some of the Palaeo- 

 zoic limestones are mainly 

 composed of their remains. 

 Two other classes of fixed 

 Crinoids are limited to this 

 epoch, and one of them 

 (Cystoidea] is of particular 



interest, because it prO- pig. I 3 i 3 .-Forked Craptolites \Didymograptus] on the 



bably represents the stock J?Jt\r * "^ A sma11 part f ne is drawn 

 from which all other echino- 



derm groups have been derived, directly or indirectly. The 

 other order (Blastoidea) was a sterile side -branch. Star-Fishes 

 (Asteroidea) and Brittle -Stars (Ophiuroidea) were both repre- 

 sented, and one of the former (Pal&odiscus) possessed a biting 

 apparatus like that of many Sea- Urchins. The class (Echin- 

 oided) to which creatures of the last-named kind belong was 

 represented by a number of primitive types, with more numerous 

 plates than later species, and (on the evidence of Palaeodiscus) 

 it has been suggested that Sea- Urchins are descended from Star- 

 Fishes. 



PALAEOZOIC LAMP- SHELLS (BRACHIOPODA). This group of 

 greatly specialized worms, distinguished by the possession of a 

 bivalve shell, at the present time contributes but little to the 

 fauna of the sea. In the Palaeozoic epoch it was extremely 



