662 INVERTEBRATA CHAP. 



From the Annelida arose Arthropoda, whose members in some 

 degree regained the free -swimming life of their far-off ancestors ; but 

 they never regained domination in the open sea, for, in the meantime, 

 that portion of the Protocoelomate stock represented by the Dipleurula 

 had advanced in development, developing long ciliated tentacles 

 for the prehension of food ; and this section of the stock leads 

 straight on to the victorious Vertebrata, whose leading members 

 have never deserted the free life of the ocean rover, and have been 

 and are now dominant in the seas. 



From time to time weaker brethren have given up the struggle 

 and sought safety in a bottom life ; of these the oldest offshoot are 

 the Echinodermata, later came the Cephalodiscida, and then the 

 Balanoglossida, which, though classed together as Enteropneusta, 

 represent two separate offshoots from the free - swimming Proto- 

 coelomata. Then, much later, the Cephalochorda diverged, later still 

 the Urochorda (Tunicata), and finally the Cyclostome Fishes. 



It is, therefore, broadly speaking, true that Invertebrates collect- 

 ively represent those branches of the Vertebrate stock which, at 

 various times, have deserted their high vocation and fallen to lowlier 

 habits of life. 



When once a stock has so fallen, is there a place for repentance ? 

 Can a dominant position be regained ? At first sight the history of 

 Arthropoda appears to answer this question in the affirmative, for they 

 have certainly progressed far beyond their Annelidan ancestors. As 

 all know, vigorous attempts have been made to prove that Vertebrata 

 are evolved from Arthropoda. The only evidence for this is the 

 superficial similarity between certain early Arachnida and fossil 

 Fishes, coupled with the assertion that Arthropoda were dominant 

 in the seas before Vertebrata, and that a dominant group like 

 the Vertebrata must have arisen from a pre-existing dominant 

 group. 



Now the difficulty in the way of this view, even supposing that the 

 enormous differences between vertebrate and arachnid anatomy could 

 be brushed aside, is that the fish which are compared to Arachnida are 

 degenerate bottom-living forms. They are not the earliest and most 

 primitive fish, whose blade-Like bodies swept Like arrows through the 

 waters above. The Arachnida, with which they are compared, are 

 also almost certainly bottom-living forms, and there is no evidence 

 that the Arthropoda, except in the case of minute-bodied incon- 

 spicuous forms, such as Copepoda and Cladocera, ever really took to 

 active Life in the open sea again. The larger and heavier types are 

 all bottom -living, and it seems perfectly clear that, when the 

 Arthropoda sought to recover their lost birthright, they found the 

 ground preoccupied by the Vertebrata and their opportunity gone 

 for ever. 



In adaptation to life on land, however, Arthropoda were beforehand 

 with their rivals, and in pre-Carboniferous days must have expanded 

 and flourished enormously; but the start was]speedily overhauled\vhen, 



