504 EMBEYOLOGY OF THE LOWEE VEETEBEATES oh. 



problem from Palaeontology. The most ancient Vertebrates of which 

 fossil remains are known had probably already evolved to a far 

 greater distance from the original type of Vertebrate than that which 

 separates them from the existing Vertebrates of to-day. And the 

 probability is that the earliest Vertebrates went on existing and 

 evolving through long ages before they developed those complex 

 skeletal structures which are alone adapted for preservation as fossils 

 in the geological record. Comparative Anatomy fails us too — for up 

 to the present no existing type of animal has been discovered which 

 can justifiably be interpreted as an unmodified survivor of the 

 original Vertebrates. 



It is Embryology alone which yields us examples of Vertebrates 

 in the earliest stages of evolution but the data afforded by that 

 science do not carry us beyond the formulation of a few very broad 

 and general conclusions regarding the prevertebrate phases in the 

 evolutionary history of the phylum. 



(1) The fact that Vertebrates, like other Metazoa, commence 

 their existence as a unicellular zygote appears to justify us in postu- 

 lating a unicellular i.e. a Protozoan ancestral stage. 



(2) The fact that there occurs in the admittedly more primitive 

 Vertebrates a gastrula stage appears to justify us in postulating a 

 diploblastic or Coelenterate ancestral stage. 



(3) The facts which are united together in the Protostoma 

 hypothesis suggest that the coelenterate ancestor evolved along lines 

 somewhat similar to those of the modern Sea-anemones with their 

 elongated slit-like protostoma dilated at each end and surrounded by 

 a concentration of the ectodermal nerve-plexus. 



(■4) The facts that the coelome was probably originally segmented 

 (as indicated by Amphioxus), that the excretory organs are in the 

 form of nephridial tubes, that the vascular system consists funda- 

 mentally of longitudinal vessels on opposite sides of the alimentary 

 canal connected together by vascular arches, the blood passing 

 tail wards in the vessel on the neural side of the alimentary canal — 

 suggest that there intervened between the coelenterate phase and the 

 vertebrate phase a stage which possessed many features in common 

 with those animals which are grouped together to-day in the phylum 

 Annelida. We may suppose that this annelid-like creature became 

 evolved from an Anemone in which the body had become drawn out, 

 as in the genus Herpolitha or one of the brain corals, and which had 

 become actively motile. In the two diverging stems which gave rise 

 to Annelids and to Vertebrates respectively we may take it that a 

 difference existed in the normal position of the body — -the former 

 progressing with their neural, the latter with their abneural, surface 

 underneath. It is conceivable that this difference may have been 

 associated with the difference between a creeping mode of life in 

 which the chief sensory impressions were related to the solid sub- 

 stratum and a swimming mode of life in which they rather came 

 from above, 



