634 COMPARATIVE ANATOMY 



they are metameric in the strict sense of the word metameric. The 

 reliability of neuromeres as criteria of metamerism rests upon the assump- 

 tion that vertebrates come from metameric ancestors like the annelids. 

 Those who hold this view explain the absence of neuromeres in amphioxus 

 as the result of degeneration. There is, however, a suspicion that we 

 have here another instance of reasoning in a circle. Until we have a more 

 general agreement among students of neuromerism in regard to the 

 existence of neuromeres in the trunk region, their presence in the hind- 

 brain region does little to strengthen the belief in the annelid ancestry of 

 vertebrates. 



It has generally been assumed by advocates of the annelid hypothesis 

 that annelids are derived from non-metameric ancestors like the flatworms 

 and actinians. The protonephridia of annelids resemble those of flat- 

 worms. In some ways the trochophore larva of annelids resembles a 

 flatworm. The mesenchyme of the larva is comparable with that of 

 the flatworm, and the coelomic cavities of the adult may be compared 

 with the gonadic sacs of nemerteans. The flatworms in turn may be 

 derived from the actinian by the partial union of the borders of its slit- 

 like mouth. By the persistence of the anterior and posterior portions 

 of the actinian mouth after the intermediate region had united, the 

 mouth and anus of the ancestor of annelids, it is assumed, were formed. 

 In this way, the nerve-ring which surrounded the mouth of the actinian 

 would be brought into the relations characteristic of annelids. 



Objections to the AnneUd Theory. One of the fundamental difficulties 

 which has turned some biologists away from the annelid theory is the 

 difference in the fate of the blastopore in annelids and chordates. Indeed, 

 had this difference been known when the annelid hypothesis was first 

 promulgated, it is doubtful whether the theory would have seemed so 

 plausible. It was Goette who, in 1895, first found the difference in the 

 fate of the blastopore an insuperable objection to the annelid theory. 

 Comparison of annelid and chordate is difficult, if not impossible, Goette 

 says, since in annelids the blastopore becomes, at least in part, the mouth, 

 while in chordates it becomes the anus, or lies near the anus. It was this 

 difference which led Grobben to classify animals into two great groups, 

 Protostomians and Deuterostomians. If this evidence has the weight 

 which it seems to have, the separation of annelids and chordates occurred 

 long before either of their adult types of organization had appeared, 

 possibly even before such an organism as a trochophore larva emerged 

 from a coelenterate ancestor. 



To meet this objection, supporters of the annelid hypothesis minimize 

 the importance of the fate of the blastopore and assert that even in 

 different genera of the same phylum the fate of the blastopore may differ, 

 so that its fate cannot have a phylogenetic significance. 



