The animal kingdom is divided into the following sub-kingdoms, each of which is subdivided into classes. The following table shows these divisions in their proper order, beginning with the lowest: SUB-KINGDOM I—_Protozoa_. { Class I—Monera. { Class II—Gregarinida. { Class III—Rhizopoda. { Class IV—Infusoria. SUB-KINGDOM II—_Spongida_. SUB-KINGDOM III—_Cœlenterata_. { Class I—Hydrozoa. { Class II—Anthozoa. { Class III—Ctenophora. SUB-KINGDOM IV—_Echinodermata_. { Class I—Crinoidea. { Class II—Asteroidea. { Class III—Echinoidea. { Class IV—Holothuroidea. SUB-KINGDOM V—_Vermes_. { Class I—Flat Worms. { Class II—Round or Thread Worms. { Class III—Rotifera. { Class IV—Polyzoa. { Class V—Brachiopoda. { Class VI—Annelidæ. SUB-KINGDOM VI—_Mollusca_. { Class I—Lamellibranchiata. { Class II—Gasteropoda. { Class III—Cephalopoda. SUB-KINGDOM VII—_Articulata_. { Class I—Crustacea. { Class II—Arachnida. { Class III—Myriapoda. { Class IV—Insecta. SUB-KINGDOM VIII—_Tunicata_. SUB-KINGDOM IX—_Vertebrata_. { Class I—Pisces. { Class II—Reptilia. { Class III—Aves. { Class IV—Mammalia. SUB-KINGDOM I. _Protozoa_ (first animals). These earliest formed animals are distinguished for the simplicity of their structure. In some cases their animal nature was long ago in doubt, and they were, for a time, put down as probably belonging to the vegetable kingdom. The border line between the two has never been very definitely located. Biologists may fail to tell just what special quality distinguishes the minute animal from the microscopic plant. This is not wonderful, when it is remembered that myriads of animals, known to be such, are so small that it requires a lens of strong magnifying power to discover them. Three thousand of them, placed side by side, would make a line but little over an inch long.