430 



CHORDATE ANATOMY 



of free-swimming annelids. (Figs. 376, 377) According to Delsman, the 

 fundamental differences between the adults of these three groups are 

 due to alteration in the growth centers of the embryos of the three groups. 

 The ectodermal foregut of the annelid is, he thinks, the homologue of 

 the neural tube of the chordate. 



If any one of these hypotheses is correct, it follows that none of the 

 several extant types of invertebrates may be regarded as vertebrate 

 "ancestors." Whatever resemblances to vertebrates they show do not 

 prove that they are in the direct line of vertebrate ancestry, but simply 

 illustrate the principle of parallelism or that of convergence. That such 

 resemblances should occur in forms which started with similar potencies 

 is not surprising. 



CHORDATE TROCHOPHORE ANNELID 



BLASTOPORE, 

 ENTERON 



NEURAL TUBEv 



FOREGUT 

 \ /MOUTH 



MOLUUSC 



Fig. 377. — Diagram illustrating the theory of Delsman that, by change in the loca- 

 tion of the growth center, a trochophore (A) may be converted into either a chordate 

 (B), or a mollusc (C), or an annelid (D). 



That the protochordates in many ways represent the ancestors of 

 vertebrates seems highly probable. Cyclostomes, more especially their 

 larval forms, lead us in the direction of Amphioxus which, "if it hadn't 

 existed, would have had to be invented." The similarity of Amphioxus 

 embryos to larval tunicates strongly suggests their common origin. Since 

 this seems the most reasonable interpretation of the facts, we may conclude 

 that metamerism has been attained de novo by chordates and not inherited 

 from metameric invertebrates. 



A "family tree" which expresses these conclusions in regard to the 

 phylogenesis of man is given in Fig. 378. The diagram assumes the 

 dichotomy of Metazoa into Protero- and Deuterostomians. Chordates 

 belong to the latter branch of the animal kingdom. There is general 

 agreement among morphologists as to the phylogenetic series, from cyclo- 

 stomes to man, shown in the Figure. 



