52 IOWA GEOLOGICAL SURVEY 



with Merostomes (Eurypterids etc.) and Arachnids (Scorpions 

 etc.), it is important to recall that the groups mentioned had 

 already attained a high degree of specialization in the early 

 Pakeozoic, and had diverged widely along certain directions from 

 the primal trilobitic type of organism. Applying here the uni- 

 versal rule that the progenitors of a new type are to be sought 

 not among the more highly modified, but among the more gener- 

 alized members of an old race, we are forced to exclude Meros- 

 tomes and Arachnids as possible ancestors of backboned animals 

 by virtue of the fact that they are already too highly specialized. 

 However plausibly the trilobitic organization may be regarded as 

 ancestral to the higher Crustacea, and even Insects, it does not 

 even remotely suggest affinities with chordates ; and in case the 

 gap between the two phyla cannot be bridged over at this point, 

 we must perforce deny that there is any connecting link be- 

 tween them. The latter proposition is now commonly accepted, 

 and such resemblances as are shared by Merostomes and early 

 fish-like vertebrates are explained as due to mimicry, or to adap- 

 tation of creatures of different grades to a similar environment. 

 On the other hand, there are no theoretical objections to looking 

 upon the worm-like Enteropneusta, by some actually placed 

 among the Protochordates,* as possible ancestors of the verte- 

 brate stem. And it may be suggested that the Cambrian prob- 

 ably affords a sufficient time-interval for the elaboration neces- 

 sary to overcome differences of grade. 



*The terms Chordata and Protochordata are thus distinguished by President 

 D. S. Jordan, in his Guide to the Study of Fishes, vol. I, p. 460. 



"Chordata.— The chordate animals are those which at some stage of life possess 

 a notochord or primitive dorsal cartilage which divides the interior of the body into 

 two cavities. The dorsal cavity contains the great nerve centers or spinal cord; 

 the ventral cavity contains the heart and alimentary canal, in all other animals 

 which possess a body cavity, there is no division by a notochord, and the ganglia 

 of the nervous system, if existing, are placed on the ventral side or in a ring about 

 the mouth. 



Protochordata.— Modern researches have shown that besides the ordinary back- 

 boned animals certain other creatures easily to be mistaken for mollusks or worms, 

 but being chordate in structure, must be regarded as offshoots from the vertebrate 

 branch. These are degenerate allies, as is shown by the fact that their vertebrate 

 traits are shown in their early or larval development and scarcely at all in their 

 adult condition. 



Enteropneusta. — Most simple, most worm-like, and perhaps most primitive of 

 all the [Proto-] Chordates is the group of worm-shaped forms, forming the class 

 of Enteropneusta. . . . With the [lower] Chordates, and not with the worms, 

 this class, Enteropneusta, must be placed if its characters have been rightly inter- 

 preted. It is possibly a descendant of the primitive creatures which marked the 

 transition from the archaic worms, or possibly archaic Echinodernis, to the archaic 

 Chordate type." 



