14 THE CLASSIFICATION OF ANIMALS. 



The Metazoa constitute a group that includes the majority of 

 animals. These are built up, not of one cell or a few cells, 

 but of countless numbers of cells, which form the complicated 

 animal tissues — muscular, nervous, connective, &c. This divi- 

 sion will be found to contain both invertebrates and vertebrates. 



The Protozoa form the first group of animals, the lowest 

 organisms, single-celled creatures, which are, nevertheless, im- 

 portant to us agriculturally, as many of them produce diseases, 

 such as diphtheritic roup in fowls, liver-rot in rabbits, malarial 

 fever in man, psorospermosis of the skin, &c., in man and many 

 domestic animals. 



The Mesozoa consist of one division only. They are small 

 parasitic germs, such as the Dicyema, which are found in 

 invertebrate animals. They are extremely rare, and as they 

 are of no agricultural importance whatever, can at once be dis- 

 missed with a word of remembrance that they constitute one of 

 those stepping-stones that fill up the great gap between the 

 Protozoa and the Metazoa. 



The multicellular animals, or Metazoa, are divided into the 

 following groups, called classes : — 



1. Gmlenterata, or Jellyfish, Polyps, Sea-anemones, Corals, &c. 



2. EcMnodermata, or Starfish, Sea-urchins, and the nearly 



extinct Sea-lilies. 



3. Vermes, or Worms. 



4. Mollusca, or Shells. 



5. Arthropoda, or the Jointed-limbed animals, as Insects, 



Spiders, Scorpions, and Crabs, &c. 

 The above are all Invertebrate Metazoa. The Spongidce, or 

 Sponges, may belong to this division ; but whether they are to 

 be looked upon as colonies of Protozoa, or Metazoa, there is 

 some diversity of opinion. They seem to present most affinities 

 to the Metazoa, and should doubtless be included in the Inver- 

 tebrate division of that group. 



6. The sixth class of Metazoa include the Ascidians, Tuni- 

 cates or Sea-squirts, the Amphioxus or Lancelet, and the worm- 



