THE ANIMAL KINGDOM 91 



X. Vertebrata. The most highly developed an- 

 imals, those above the simplest ones possessing ver- 

 tebrae, but this feature is not common to all. Ex- 

 cluding Tunicates and Amphioxus, five classes are 

 recognized: Fishes, Amphibia, Reptiles, Birds and 

 Mammals. The latter class including man. There 

 are about 30,000 living and a large number of fossil 

 species. 



Intermediate between the molluscs and the 

 vertebrates are sea-squirts, or Tunicates. A phylum 

 Chordata is frequently made to include the Tunicates, 

 other primitive forms, and the vertebrates because 

 all these animals are alike in possessing a notochord, a 

 dorsal nerve-cord and gill-slits. The Tunicates num- 

 ber about 300 species. 



There are several other groups of invertebrates of 

 uncertain position but which are often elevated into 

 the rank of phyla. If the Brachiopods are classed as 

 shelled worms, the most important of the remaining 

 forms are the Bryzoa. These are moss-like, incrusting 

 animals, inhabiting both salt and fresh water, and 

 numbering about 700 living and 1800 fossil species. 



For a more detailed analysis of the animal kingdom, 

 with sketches and consideration of the structure of 

 types, consult the various manuals and text-books. 



