THE PROGRESS OF LIFE 57 



animal life of the early seas. No recognisable veget- 

 able remains have been found in any pre-Cambrian 

 rock, but pelagic algse must have existed, for otherwise 

 there would have been no food for the animals. 



CAMBRIAN LIFE 



When we pass upward into the Cambrian period 

 we find that life has made considerable progress, in- 

 cluding the appearance of a new sub-kingdom the 

 Echinodermata ; and in the Upper Cambrian we have 

 the first Bryozoa and Pelecypoda. However, the only 

 fossils which show much variety are the Brachiopoda 

 and the Trilobites. 



The Hydrozoa were represented by Sertularians, 

 Graptolites, and Medusa?, the latter being so abundant 

 that the National Museum at Washington has more 

 than 8000 specimens. These Cambrian Medusae 

 belong to a distinct family of the Discomedusse called 

 BrooTcsellidce, and are distinguished by having a lobate 

 umbrella without any marginal tentacles. It is re- 

 markable that such soft things as jelly-fish should 

 have been preserved as fossils ; but although they 

 have no hard parts, their tissues, when saturated with 

 water, are sufficiently firm to make impressions on the 

 mud or sand on which they have been thrown by the 

 waves, and when the umbrella is turned upside down 

 the gastric cavities get filled with the mud or sand 

 and leave star-like marks, which are easily recognised. 

 The Actinozoa are represented only by the curious 

 Archoeocyatliince, which appear to be related to the 

 perforate corals. 



