THE ANCESTRY OF VERTEBRATES 



429 



more or less independently should not stretch credulity beyond the break- 

 ing point. For there is abundant evidence that this is exactly what has 

 occurred. 



BLASTOPORE OOUm-ANUS) Bl-ASTOPORECPiOJTH-ANUS) 



A coelenterate: bfi^tworm. canneud. 



Bl>STOPORECM0UTH) 

 a MOLLUSC 



BLASTOPORE CANUSJ 



BLASTOPORE (ANUS) 



CMeurenterk: canal) 



E ECHINOOERM, 



F HEMICHORDATE. 



G UROCHORDATE, 



H. CEPHALOCHORDATE. 



Fig. 375. — Diagrams of embryonic stages illustrating the contrast in the fate of the 

 blastopore in various groups of animals. The forms in which the embryonic blastopore 

 becomes the mouth were grouped together by Grobben as PROTEROSTOMIA. The 

 DEUTEROSTOMIA include those animals in which the blastopore becomes the anus 

 or lies in the anal region. The coelenterates, flatworms, annelids, and molluscs are 

 Proterostomians, while echinoderms and chordates are Deuterostomians. 



A somewhat similar view is that of Grobben who divides animals into 

 two phylogenetic series based upon differences in the fate of the primitive 

 mouth or blastopore. The forms in which the blastopore becomes the 

 adult mouth (or cardiopore) reach their evolutionary climax in arthropods 

 and molluscs. Those, on the other hand, in which the blastopore becomes 



APICAL PLATE 



CARDIOPORE 

 STOMODEXWi 



PBEORAL CIUATID 



ntOTOCPHRIDIUM 



* MUOOERM 



Fig. 376. — A diagram of a trochophore larva According to Delsman's theory of the 

 origin of vertebrates, the cardiopore (blastopore) of the trochophore larva is homologous 

 with the neurenteric canal of vertebrates. In other words, the relatively short stomo- 

 deum of the annelid is stretched to become the neural tvibe of vertebrates. (Redrawn 

 after Hatschek.) 



the anus or lies near the anus lead to the chordates and vertebrates. 



(I^'ig- 375) 



A third attempt to solve the phylogenetic problem on the basis of 

 embryological evidence is that of Delsman, who derives chordates, annelids 

 and molluscs from a form like that of the trochophore larva characteristic 



