4 STATEMENT OF THE SUBJECT. 



writers designate at least the subordinate groups by a 

 combination of characteristics more or less different in 

 their nature. 



The number of species of animals is not known, but 

 may safely be estimated as high as a million, or even 

 more, of which the small and microscopic comprise an 

 immense majority. 



Cuvier has shown that the animal kingdom comprises 

 four great Branches or Types, Vertebrata, Articulata, 

 Mollusca, and Radiata.* 



The Branch of Vertebrata comprises all animals which 

 have an internal skeleton with a backbone for an axis. 

 Man and all the higher animals belong to this branch. 



The Branch of Articulata comprises all animals whose 

 bodies are made up of similar rings or segments, placed 

 transversely to the longitudinal axis ; and whose parts 

 which correspond to a skeleton are external. All In- 

 sects, Crabs, Lobsters, Shrimps, and Worms belong to 

 this branch. 



The Branch of Mollusca comprises soft-bodied animals, 

 such as Cuttle-fishes, Squids, Snails, Mussels, Oysters, 

 and Clams. 



The Branch of Radiata comprises animals whose parts 

 are more or less symmetrically arranged around a verti- 

 cal axis. Sea-urchins, Star-fishes, Crinoids, Jelly-fishes, 

 Coral animals, and Sea-anemones belong to this branch. 



Baer has shown that for each of these branches there 

 is a special mode of development in the egg. In Ver- 

 tebrates the germ divides into two folds, one turning 

 upward and the other downward. In Articulates, the 

 germ lies with its back portion upon the yolk, and ab- 

 sorbs the latter into that part of the body. In Mol- 

 lusks, the germ lies upon the yolk, and absorbs the latter 



* To these we must now, perhaps, add a fifth branch, the Protozoa 

 (** p. 533). 



