THE STATEMENT OF THE PROBLEM. 395 



nauplius or an ostracode) and which in turn were derived from rotifer-like trocho- 

 zoans. Neither the annelids nor any other worm-like forms were included in this 

 stock. (Fig. 309.) The first group, the syncephalata, includes together with 

 other arthropods, the phyllopod-arachnid-ostracoderm-vertebrate phylum, or 

 the craniata; the second group, or the acraniata, includes the cirripeds, tunicates, 

 Amphioxus, echinoderms, enteropneusta, pterobranchia, phoronida, polyzoa, 

 chastognatha, and brachiopods. 



Our problem is in a measure clarified, and at once assumes an entirely new 

 aspect, as soon as we recognize that the chordata consist of several phyla derived 

 from the arthropods via as many separate lines, and that some of the striking 

 features in which they resemble one another were independently acquired. We 

 associate, for example, Amphioxus, Balanoglossus, and the tunicates with the 

 vertebrates because of their notochord, perforated gill slits, heemostoma, and 

 atrial chamber. But, as we have already seen, these basic characters, in one 

 form or another, are actually, or potentially present in all primitive arthropods, 

 and presumably, they may be expressed in all their descendants, although, per- 

 haps at different times, and in varying ways, in different phyla. 



The assumption that several independent caudate phyla have arisen from 

 the arthropods is of two-fold value, for it enables us to explain why the tunicate- 

 balanoglossus group resembles at the same time both the arthropods and the 

 vertebrates, without being in the direct line of vertebrate descent; and it enables 

 us to attach these heretofore isolated and obscure phyla to the great trunk line 

 of organic evolution, and to thereby obtain a new basis for the interpretation of 

 their morphology. 



Let us first consider in a summary way the more important features of these 

 two great groups. 



The Craniates. 



We have shown in the preceding chapters that the trunk line of vertebrate 

 descent runs through the dipnoi-arthrodire-ostracoderm-arachnid-phyllopod 

 stock, which almost from the very outset consisted of highly specialized segmented 

 animals. Primarily they were neither sessile nor parasitic, but large, vigorous 

 free swimming forms with highly developed nervous system, sense organs, cephalic 

 appendages, and paired jaws. They were often, at times predominantly, fre- 

 quenters of the warm littoral, of fresh or brackish waters, and of the land. At 

 every stage of evolution, with a few notable exceptions, the animals that consti- 

 tuted the advancing crest of this stock have been on the whole the largest, and 

 the most active, the greatest consumers and spenders of energy, the most highly 

 organized, the most progressive and innovating, and the most widely distributed 

 animals of their day and generation. 



They had a chitenous, dentine-like exoskeleton, a cartilaginous endocranium, 

 gill cartilages, middle chord, gill sacs, voluminous enteric diverticula, a trioc- 



