12 THE CLASSIFICATION OF ANIMALS. 



The Classification of Animals. 



The old method of classifying animals was to divide them 

 into two sub-kingdoms, known as the Invertehrata and the 

 Vcrfebrata, — the absence or presence, respectively, of an in- 

 ternal skeleton being the character upon which this division 

 was based. 



Invertebrates are those animals which have no internal skel- 

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

 known as the notochord. The notochord is a primitive axial 

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

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

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

 other hand, the htemal 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 

 skeleton — the limbs. The vertebral column — the backbone — 

 and the cranium enclose the central nervous system, which is 

 always dorsal, whilst the nervous system in invertebrates is 

 ventral. The haemal system— the heart — is placed ventrally, — 

 that is, in the reverse position to that in which it is found in 

 the former group. 



Just as there are intermediate forms between the animals and 

 plants, so are there connecting links between these two primary 

 groups of animals. A small fish, known as the lancelet 

 {AmjyJiio.riis lanceolatus), found in the sands of the Mediter- 

 ranean, has no proper internal skeleton at all, yet it has a noto- 

 chord and dorsal nervous system. The groups of Ascidiaiis, or 

 sea-squirts, are in their young stages distinctly vertebrates ; for 

 the young so-called Appeinlicidaria lavva has a dorsal nervous 

 system and an axial rod, but the adult Tunicate, as it is also 



