6 GENERAL SURVEY OF THE ANIMAL KINGDOM. 



Yet it is no longer possible to draw a boundary line between Verte- 

 brates and Invertebrates with that firmness of hand which characterised 

 the early or, indeed, the pre-Darwinian classifications. For we now 

 know ( I ) that Fishes and Cyclostomata do not form the base of the 

 Vertebrate series, for the lancelet and the Tunicates must also be in- 

 cluded in the Vertebrate alliance ; (2) that Balanoglossus and Cep halo- 

 discus have several Vertebrate-like characteristics ; (3) that some of the 

 Invertebrates, especially Chaetopods and Nemerteans, show some hints 

 of affinities with Vertebrates. The limits of the Vertebrate alliance 

 have been widened, and though the recognition of their characteristics 

 has become more definite, not less so, the apartness of the sub-kingdom 

 has disappeared. 



It does not matter much whether we retain the familiar title Verte- 

 brata, or adopt that of Chordata, provided that we recognise (i) that 

 it is among Fishes first that separate vertebral bodies appear in the 

 supporting dorsal axis of the body; (2) that, as a characteristic, the back- 

 bone is less important than the notochord, which precedes it in the 

 history alike of the race and of the individual. Nor need we object to 

 the popular title backboned, provided we recognise that the adjective 

 "bony" is first applicable among Fishes, and not even to all of them. 



The essential characters of Vertebrates may be summed up in the 

 following table, where they are contrasted, somewhat negatively, with 

 what is true of Invertebrates : 



" BACKBONKLES," INVERTEBRATE 

 OR NON-CHORDATE. 



The greater part of the nervous system is 



on the ventral surface. 

 No corresponding structure is known. 



No corresponding structures are known 

 with any certainty. 



The eye is usually derived directly from 



the skin. 

 The heart, if present, is dorsal. 



" BACKBONED," VERTEBRATE 

 OR CHORDATE. 



The central nervous system brain and 

 spinal cord is dorsal, and t^tbular. 



There is a dorsal supporting axis or noto- 

 chord, which is in most cases replaced 

 by a backbone. 



Gill-slits or visceral clefts open from the 

 sides of the pharynx to the exterior. 

 In fishes, and at least young amphi- 

 bians, they are associated with gills 

 and are useful in respiration ; in 

 higher forms they are transitory and 

 functionless, except when modified 

 into other structures. 



The essential parts of the eye are formed 

 by an outgrowth from the brain. 



The heart is ventral. 



INVERTEBRATES. 



Molluscs. 



This series of forms includes Bivalves, such as cockle and 

 mussel, oyster and clam ; Gasteropods, such as snail and 

 slug, periwinkle and buckie ; Cephalopods, such as octopus 



