THE CLASSIFICATION OF ANIMALS. 15 



like Balanoglossus These form the conneeting group between 

 the Invertebrate and Vertebrate Metazoa. 



The Vertebrate Metazoa are contained in five classes, 

 namely — 



7. Pisces, or Fish. 



8. Beptilia, or Snakes, Crocodiles, and Lizards. 



9. Amphibia, or Toads, Frogs, and Salamanders. 



10. Aves, or Birds. 



11. Mammalia, or Quadrupeds and Man. 



The above eleven classes of Metazoa may be grouped in two 

 divisions, according to the absence or presence of a notochord. 

 Those without a notochord are called Achordata, those with 

 a notochord Ghordata. The latter, again, are divided into 

 Aerania and Oraniota. 



The Aerania include, besides the Tunicates, the worm-like 

 creature called Balanoglossus and the quaint little fish-like 

 Amphioxus. These always have at some period of their life a 

 dorsal nervous system and a notochordal rod which extends 

 nearly the whole length of the body ; but the nervous system, 

 which develops as an open canal (another character common to 

 vertebrate animals), never expands anteriorly into a brain. In 

 fact, in general appearance Tunicates and Balanoglossus are in- 

 vertebrates, while Amphioxus forms another stage higher, con- 

 necting the lower animals with the Fish. Amphioxus has been 

 described by Couch and others as a fish. The Oraniota, on the 

 other hand, have the anterior end of the nervous cord enlarged 

 into a brain placed in a cartDaginous or bony box, the cranium, 

 and are supplied with an internal skeleton. 



The groups of animals, then, may be tabulated as foUows : — 



