24 THE LESSON OF EVOLUTION 



very remarkable that the only extensive pre-Cam- 

 brian fauna is composed of Eadiolarians and 

 Sponges; and, as the Sponges are more complex 

 animals than the Eadiolarians, we must suppose that 

 they are descended from them, and not the Eadio- 

 larians from the Sponges ; consequently the 

 Eadiolarians are the oldest organisms we know. 

 No doubt the Eadiolarians of Brittany are not the 

 first of their class ; nevertheless we seem to have got 

 as near, perhaps, as we ever can get to the first 

 organisms ; and we find that they belong to a group 

 which at the present day floats near the surface of 

 the sea. This pelagic aspect of the early faunas is 

 carried out by the Mollusca of the Algonkian and 

 Cambrian periods, as well as by the great develop- 

 ment of free-swimming Medusa9 in the Cambrian ; 

 and we should remember that these delicate pelagic 

 animals must have been very numerous to have left 

 any record at all. 



The earliest known Eadiolarians are accompanied 

 by the remains of sponges which must have lived on 

 the bottom of the ocean, and these were followed by 

 creeping worms and Trilobites. The early Brachio- 

 pods have diaphanous shells, like the pelagic 

 mollusca ; but it seems probable, from a study of 

 their development in living forms, that at first they 

 had no shell at all, but consisted of the peduncle 

 encased in a sand-tube. The shell was afterwards 

 added to protect the branchire, and in course of time 

 the intestinal tract in the peduncle atrophied. 

 Perhaps the so-called annelid tubes of the Torridnn 

 sandstone represent the first Brachiopods. 



