8 GENERAL SURVEY OF THE ANIMAL KINGDOM. 



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 

 backbone 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, if we recognise that the adjective " bony " 

 is first applicable among Fishes, and not even to all of these. 



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 : — 



' Backboneless," 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 tubular. 



There is a dorsal supporting axis or 

 notochord, 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, or Backboneless Animals. 



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 and pearly nautilus. They may be placed 

 highest among Invertebrates, since many of them exhibit a 

 concentration of the nervous system greater than occurs 

 elsewhere. 



Unlike Vertebrates, and such Invertebrates as Insects 

 and Crustaceans, Molluscs are without segments and 

 without appendages. A muscular protrusion of the ventral 

 surface, known as the "foot," serves in the majority as an 

 organ of locomotion. In most cases a single or double 

 fold of skin, called the "mantle," makes a protective shell. 



