300 



COMPARATIVE ANATOMY 





of the fact that many openings such as the gill-slits have made their 

 appearance in the course of phylogenesis. 



The fate of the original blastoporic mouth of Coelenterates, has been 

 mentioned in connection with the classification of animals. In most 

 invertebrates, the blastopore becomes the mouth, while in echinoderms 



and chordates it either becomes the 

 anus or lies in the anal region. Ani- 

 mals in which the coelenterate mouth 

 persists as the definitive mouth are 

 known as Protostomians, while those 

 in which a new mouth is formed are 

 called Deuterostomians. The two 

 main branches of the animal king- 

 dom shown in Fig. i are separated 

 on the basis of this distinction. 



According to the protostoma 

 theory of Adam Sedgwick (1884) 

 the oral surface of coelenterates 

 with its encircling nerve ring 

 becomes the ventral, and neural, 

 surface of non-chordates, while in chordates it forms the dorsal, 

 and neural surface. Thus the theory assumes that the non-chordates 

 are the descendents of coelenterates which moved with the oral 

 side down, while the chordates have come from coelenterates which 



ec 



Fig. 251. — Schematic section of the 

 hinder end of an amphibian embryo, show- 

 ing the relations of the neurenteric canal. 

 ac, alimentary canal; ec, ectoderm (black); 

 n, notochord; ne, neurenteric canal; nt, 

 neural tube; p, proctodeum; pa, post-anal 

 gut; y, yolk. (From Kingsley's "Com- 

 parative Anatomy of Vertebrates.") 



.BLASTOPORE 



NEUROPOREi 



CBLASTOPORE) 

 ,'NEURENTERIC CANAL 



'^'"\^^^' 



A B. C. 



Fig. 252. — A diagram illustrating the way in which according to Delsman the blasto- 

 poric mouth of coelenterates is in chordates converted into the neurenteric canal. 

 Delsman homologizes the chordate neural tube with the ectodermal foregut of annelids. 



moved around with the oral side up. Sedgwick holds that the two sides 

 of the elongated slit-like mouth of the coelenterate (such as an actinian) 

 have become the right and left sides of the bilaterally-symmetrical bodies 

 of higher animals. He finds that in the primitive worm-like arthropod 

 Peripatus the elongated gastrula mouth closes along the median line of 

 the embryo, but anterior and posterior dilatations persist as the definitive 

 mouth and anus. In this way the alimentary canal is formed with two 

 external openings. The concrescence of the right and left halves of the 

 vertebrate embryo is taken as a confirmation of the theory. It seems 



