I30 



THE ORIGIN AND EVOLUTION OF LIFE 



ing lobes which are so beautifully preserved in the fossil cor. 

 respond closely with those of the existing genus Dactylometra 

 of the suborder Semostomae. It is possible that the marginal 

 tentacles may have been lost in Peytoia, as so frequently hap- 

 pens in living jellyfishes when in a dying condition. 



From the Burgess fauna it appears that the pre- Cambrian 

 invertebrates had entered and become completely adapted to 



all the life zones of the 

 continental and oceanic 

 waters, except possibly 

 the abyssal. All the 

 principal phyla — the 

 segmented Annulata, 

 the jointed Arthropoda 

 (including trilobites, 

 merostomes, crusta- 

 ceans, arachnids, and 

 insects), medusae and 

 other coelenterates, 

 echinoderms, brachio- 

 pods, molluscs (includ- 

 ing pelycypods, gastro- 

 pods, ammonites, and other cephalopods), and sponges — ^w^ere all 

 clearly established in pre- Cambrian times. Which one of these 

 great invertebrate divisions gave rise to the vertebrates remains 

 to be determined by future discovery. At present the Annulata, 

 Arthropoda, and Echinodermata all have their advocates as 

 being theoretically related to the ancestors of the vertebrates. 

 The evolution of each of these invertebrate t^^Des follows the 

 laws of adaptive radiation, and in the case of the articulates and 

 molluscs extends into the terrestrial and arboreal habitat zones, 

 while many branches of the articulates enter the aerial zone. 



Fig. 27. Jellyfish, Cambrian and Recent. 



Peytoia nathorsti, mid-Cambrian (after Walcott), 

 and Dactylometra quinquecirra, recent. The 

 thirty-two lobes of the fossil specimen corre- 

 spond with the same number often observed in 

 Dactylometra, and the characteristic marginal 

 tentacles may have been lost in Peytoia. 



