438 THE ORIGIN OF VERTEBRATES 



enormous variety of form during the great trilobite era — the forma- 

 tion of a mid-ventral canal out of this deep ventral groove is seen to 

 be not only easy to imagine, but most probable, provided that a 

 necessity arose for such a conversion. 



For what purpose might such a tube have been formed ? I would 

 suggest that it might have acted as an accessory food-channel, which 

 was of sufficient value at the time to give some advantage in the 

 struggle for existence to those members of the group who were thus 

 able to supplement their intake of food, but at the same time was 

 so inefficient that it was quickly superseded by the new alimentary 

 canal, and thus losing its temporary function, became solid, and was 

 utilized to form an axial supporting rod. 



There is a very considerable amount of evidence in favour of the 

 view that the notochord was originally a digestive tube ; in fact, as 

 far as I know, this conclusion is universally accepted. The evidence 

 is based essentially upon its development and upon its structure. It 

 is formed in the vertebrate from the same layer as the alimentary 

 canal, i.e. the hypoblast, and in Amphioxus it commences as a 

 groove in the dorsal wall of the future alimentary canal ; this groove 

 then closes to form the tube of the notochord, and separates from 

 the alimentary canal. Embryologically, then, the notochord is 

 looked upon as a tube formed directly from the alimentary canal. 



As regards its structure, its tissue is, as already stated, something 

 sui generis. Notochordal tissue lias no resemblance to bone or 

 cartilage, or any of the usual supporting tissues. Such a tissue is 

 not, however, entirely confined to the notochord of the vertebrates, 

 but tissue closely resembling it has been found not only in Amphioxus 

 and the Tunicata, but in certain other invertebrates, in the Entero- 

 pneusta (Balanoglossus, etc.), in Cephalodiscus, and in Actinotro- 

 cha. In all these latter cases, such a tissue is invariably found in 

 disused portions of the alimentary canal ; a diverticulum of the 

 alimentary canal becomes closed, vacuolation of its lining cells takes 

 place, and a tissue resembling notochordal tissue is formed. 



Owing to the notochord being invariably so striking and mys- 

 terious a feature of the lowest vertebrates, the term vertebrate, which 

 is inappropriate in the members of the group which do not yet possess 

 vertebras, has been largely superseded by the term chordate, with the 

 result of attributing an undue preponderance to this tissue in any 

 system of classification. Hence, wherever any animal has been found 



