180 GENERAL PRINCIPLES OF ZOOLOGY. 



which influence terrestrial forms; the distribution of the latter, 

 however, is cosmopolitan. The same infusorians and rhizopods, 

 copepods, fresh-water sponges and polyps which occur in America 

 seem to be distributed over the entire earth. This is connected 

 with the fact that all these animals have resting stages in which 

 they endure desiccation. The resting stage, be it as a hard-shelled 

 gg or as an encysted animal, may be borne about by the wind, or 

 may be carried with the mud by aquatic birds, and upon again 

 reaching the water resume its active state. 



VI. DISTRIBUTION OF ANIMALS IN TIME. 



It is the province of a special science, paleontology or paleo- 

 zoology, to treat of the character and distribution of animals in 

 the earlier periods of the earth's history, but since it is necessary 

 to draw upon paleontological facts to understand the living forms 

 an outline of the geological periods with the characteristic animals 

 may be given here. 



g 



I. Azoic OR ARCHEAX ERA. 



No organisms are certainly known from this age. The animal 

 nature of Eozoon canadense of the Laurentian beds, once referred 

 to the Foraininifera, is more than doubtful. 



II. PALEOZOIC ERA. 



1. Cambrian. 4. Carboniferous. 



2. Silurian. 5. Permian. 



3. Devonian. 



The oldest paleozoic period, the Cambrian, contains only 

 invertebrate fossils: trilobites, gigantostracn, cystoids, nautiloids, 

 gasteropods, and a few lamellibranchs. Trilobites, cystoids, 

 gigantostraca, and the blastoids and tetracoralla, which appear in 

 the Silurian, reach their culmination and become extinct in the 

 paleozoic. Fishes appear in the Silurian, and acquire a great 

 development in the Devonian. The earliest Amphibia come from 

 the carboniferous, the reptiles appear in the Permian. 



III. MESOZOIC ERA. 



1. Triassic. 2. Jurassic. 3. Cretaceous. 



The mesozoic era was the age of reptiles, which were repre- 

 sented by numerous forms, some of gigantic size; most of them 



