ANCIENT LIFE ON THE EARTH 25 



From all this we may infer that the first animals 

 were pelagic protozoa, which in time varied and gave 

 rise to pelagic worms and mollusca. At a very 

 early date, however, some of the protozoa followed 

 down the dead organisms and settled on the bottom, 

 giving rise to the sponges. Afterwards worms 

 moved in the same direction, feeding, probably, on 

 the sponges ; and from them are descended the 

 Brachiopods and the Crustaceans. 8 



The remains of the Brachiopods and Trilobites are 

 found chiefly in shallow-water deposits ; but some of 

 them may have pushed their way into the deep sea, 

 feeding on the dead pelagic organisms which rained 

 down from above; indeed, it has been thought that 

 the eyeless conditions of some of the early Trilobites 

 is a proof of this. But the eyes are always placed 

 on the second segment, called the free cheek ; and in 

 several of the earlier forms this free cheek is ventral 

 only, in which case no eyes could appear on the 

 dorsal surface ; the absence of eyes is not, therefore, 

 always a proof of degeneration, but there are some 

 species of Illcenus in which the eyes have 

 disappeared. 



The hard spines of the early Trilobites could not 

 have been for defence, for there were no enemies 

 capable of attacking them ; but, perhaps, they were 

 used indirectly for locomotion. As their weak little 

 leg paddled backwards and forwards in the mud, the 

 spines, all of which are directed backwards, would 



8 This theory was originated, I think, by Biologists, and 

 was first brought prominently before Geologists by Pro- 

 fessor W. K. Brooks, in the American Journal of Geology 

 for July and August, 1894. 



