538 



THE QUARTERLY REVIEW OF BIOLOGY 



The pterobranchiates seem to fall between 

 the chjetognaths and the phoronids. 



These four groups are much more closely- 

 similar to each other than any of the 

 other series of four groups except the 

 very first. While the first four groups 

 all agreed in having a considerable degree 

 of radial symmetry, the last four groups 

 agree in the possession of a highly de- 

 veloped notochord. 



These four groups possessing a con- 

 spicuous notochord are, though entirely 

 distinct, so very close to each other that 

 a fifth readjustment would presumably 

 give a final type in which all of the four 

 chief features of the original types would 

 be reunited in the economically most 

 perfected form, and the balanced adjust- 

 ment of the original radiate progenitor 

 would be once more attained. 



The vertebrates appear to occupy this 

 central place. In them we are able to 

 recognize the segmentation of the cestodes, 

 annelids and cephalochordates, combined 

 with the ccelomic structures first indi- 

 cated in the flukes, both enclosed in the 

 undivided body characteristic of the 

 turbellarians, which is provided with 

 limbs possibly to be considered as having 

 had their ultimate origin in budded units. 



In the course of the various readjust- 

 ments which culminated in the reestab- 

 lishment of the original balance in the 

 vertebrates, numerous secondary features, 

 such as the visual and other sense organs, 

 appendages of different kinds, diverticula 

 and other outgrowths from the enteric 

 canal, chitinous and calcareous skeletons, 

 etc., all of which features are found in a 

 rudimentary form in the ccelenterates, 

 attained an extraordinary development, 

 so much so as often to overshadow and 

 almost completely mask the fundamental 

 characters. 



THE FOSSIL RECORD 



If this is a true delineation of the facts, 

 it would naturally follow that at its very 

 first inception on the earth animal life 

 assumed essentially the form in which we 

 know it now, for the various readjust- 

 ments leading from the radial type of 

 animal to the recombination of its charac- 

 ters in a seemingly wholly different form 

 in the vertebrates would presumably be 

 simultaneous, or very nearly so. 



What can we learn in regard to this from 

 the fossil record? 



The earliest aquatic fauna that we know, 

 that of the Cambrian rocks, was in its 

 broader aspects singularly similar to the 

 aquatic fauna of the present day. Every 

 one of the numerous component species 

 falls at once within a definite phylum as 

 outlined by the living forms, and in a 

 definite class within that phylum. Many 

 of the species can be recognized as mem- 

 bers of families still existing, while a few 

 may be assigned even to recent genera. 



In Cambrian times crustaceans were 

 represented by phyllopods, trilobites and 

 merostomes; among the echinoderms there 

 were crinoids, cystideans, and elasipod 

 holothurians; chastognaths, brachiopods 

 and graptolites were present; of the anne- 

 lids we know polynoids, nereids, 

 gephyreans, and Tomopterzs-like forms; of 

 the mollusks there were pteropods and 

 gastropods; and there were sea-anemones 

 and other ccelenterates, and sponges. 



As a supplement to this varied Cam- 

 brian fauna, we know from the Ozarkian 

 rocks cephalopods and bivalve mollusks, 

 and from the Ordovician polyzoans, sea- 

 urchins, brittle-stars, starfishes, and fishes. 

 There is no evidence that these were not 

 also present in the Cambrian. 



The significance of this imposing list of 



