14 THE CLASSIFICATION OF ANIMALS. 



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- 

 like Balanoglossus. These form the connecting group between 

 the Invertebrate and Vertebrate Metazoa. 



The Vertebrate Metazoa are contained in five classes, 

 namely — 



7. Pisces, or Fish. 



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



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



10. Aves, or Birds. 



11. Alamiiialia, 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 Achorduta, those with 

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

 Acrania and Craniota. 



The Acrania 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 Craniota, on the 

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

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

 and are supplied with an internal skeleton. 



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



