12 THE CLASSIFICATION 01 ANIMALS. 



tions, then, between the animals and plants in regard to their 

 chemical constituents. Perhaps the greatest differences are to 

 be found in the metabolism of organisms. We cannot feed an 

 animal on purely inorganic food, whilst, on the other hand, we 

 can so feed a plant. Both must have salts and water ; but 

 whilst plants can be nourished with the addition of carbon 

 dioxide and nitrates of ammonia, an animal must have, nitrogen- 

 ous and carbonaceous matter in some organic form and not in a 

 mineral form. An animal absorbs oxygen and gives out COg ; 

 a plant exhales oxygen which is derived from the absorbed CO2. 

 Thus we see that there are differences between the plant and 

 the animal, but that many of them do not invariably hold good. 

 There are forms of life which we may fairly say bridge over the 

 great hiatus that separates the horse from the grass upon which 

 it feeds : such intermediate species, interesting as they are, we 

 must only briefly refer to in this manual. 



The Classification of Animals. 



The old method of classifying animals was to divide them into 

 two sub-kingdoms, known as the Invertebrata and the Vertehrata, 

 — the absence or presence, respectively, of an internal skeleton 

 being the character upon which this division was based. 



Invertebrates are those animals which have no internal skel- 

 eton ; but, of greater importance still, they possess no structure 

 known as the notoehord. The notochord is a primitive axial 

 skeletal rod, found on the dorsal surface. In aU invertebrate 

 animals th6 nervous system is ventral — that is, it is always 

 present on the lower surface of the animal ; whilst, on the 

 other hand, the haemal or blood system is dorsal, the ali- 

 mentary canal or gut being situated between. Invertebrates 

 may possess a skeleton, but it is always external (exoskeleton). 



Vertebrates, on the other hand, always possess a notochord, 

 and nearly always an internal skeleton, composed of an axial rod, 

 the vertebral column, besides the cranium, and an appendicular 



