THE CLASSIFICATION OF ANIMALS. 13 



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 ventraUy, — 

 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 

 {AmpMoxus 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 Ascidians, or 

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

 the young so-called Appendicvlaria larva has a distinct dorsal 

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

 is also called, is distinctly an invertebrate animal with no noto- 

 chord and a ventral nervous system. 



How, then, can we distinguish these from true vertebrates 1 

 At no time do they possess a brain or cranium as we see in the 

 higher animals. They are called, therefore, Acrania, to dis- 

 tinguish them from all the other vertebrates, which are known 

 as the Craniota. This was the most generally adopted primary 

 grouping of animals, into Invertebrata and Vertebrata ; but for 

 many reasons a more recent classification has many advantages 

 over it. This latter is based on the cell-structure of the 

 animal. By it the whole animal kingdom is divided into 

 three primary groups, known as the Protozoa, Mesozoa, and 

 Metazoa. 



The Protozoa are those animals of extremely simple organisa- 

 tion, and whose bodies are composed of a single cell. 



The Mesozoa are a small group of parasitic animals whose 

 bodies are composed of a few cells only, the cells retaining more 

 or less their original form. These constitute a connecting 

 link between the single-celled Protozoa and the third group, 

 the Metazoa. 



