ON THE ORIGIN OF VERTEBRATES FROM ARACHNIDS. 347 



extraordinary feature of the Vertebrate brain wLich has 

 always been justly regarded as the strongest evidence in favour 

 of the origin of Vertebrates from segmented animals. But if 

 we still attach the same significance to the cranial flexure that 

 many of the ablest zoologists have done in the past, then we 

 exclude the Annelids from any direct genetic connection 

 with the Vertebrates ; at the same time we strengthen the 

 Arachnid theory, because the Arachnid brain, not only in its 

 flexure, but also in its primitive continuity with the nerve- 

 cord, fulfils completely the condition demanded by a rational 

 theory of the origin of Vertebrates. 



There is another neural flexure of equal importance with 

 that mentioned above; 1 shall call it the somatic flexure. 

 At certain periods it is present in nearly all Arthropods as a 

 ventral flexure of the tail, producing in such forms as Scor- 

 pio and others an S-shaped embryo. Now, in many mam- 

 malian embryos there is a strong flexure just behind the 

 brain, the '^ cervical flexure" of His, and in many fish and 

 amphibian embryos there is at one period a strong upward 

 curve of the tail. In fact, the Vertebrate embryo is also some- 

 what S-shaped, a condition to be expected on the Arthropod 

 theory, but otherwise inexplicable. 



Beard attempts to explain how the distal portion of the 

 hypophysis cerebri may represent an Annelid oesophagus, but 

 he only succeeds in showing how the difficulties are thickening 

 around the Annelid theory. He does not recognise that if 

 there is no Annelid brain in Vertebrates there 

 should be no cranial flexure, and no reason what- 

 ever for regarding the present Vertebrate mouth 

 as secondary, or the hypophysis or any part of it 

 as the remnant of a former oesophagus. Moreover, 

 his view necessitates the assumption that the fore-brain repre- 

 sents a single segment, whereas its complex nature can hardly 

 be doubted. 



On the other hand, (1) the strong cranial flexure in Arach- 

 nids, (2) the primitive continuity of the fore-brain and ventral 

 cord, (3) the absence of the mesoblastic somites in the fore- 



