Aves, Amphibia, Pisces, Insecta and Vermes. The knowledge of this last class, which included all invertebrate animals except the arthropods, was in a very confused state, and one of the chief objects of the many able zoologists of the generation immediately following him was to remedy this condition. The men whose services were greatest in this direction were O. F. Müller, Lamarck and Cuvier. In 1794 Lamarck first distinguished the vertebrates from the invertebrates and subdivided the latter group into the five classes of Mollusca, Insecta, Vermes, Echinodermata and Polypi. Thus a long step was taken towards modernizing the system and this early effort of Lamarck may be said to be the first modern classification of animals. He, in his later works, further subdivided the invertebrate types until he had ten, the fundamental idea at the basis of his classification being that the various groups of animals constitute a single ascending series which begins with the lowest and ends with the highest. This principle of the unity of the type found a wide acceptance among the naturalists of that time and was based upon the law: Natura non facit saltum.
In 1812 Cuvier published his division of the animal kingdom into four branches or types and in 1817 his great work "Le Règne Animal" which established the second great reform of the system and was destined to exert an influence only second to that of Linnæus's "Systema Naturæ" upon the study of animals and the development of the system. In these works Cuvier controverted the principle of the unity of type. among animals and taught that instead of one four distinct and permanent types prevail. It was upon these four types that he based his four fundamental branches of the animal kingdom: Vertebrata, Articulata, Mollusca and Zoophyta or Radiata.
A comparison of this classification with that of Linnæus will show what a tremendous advance had been made in the development of the system in the half century separating them. The group of animals which had benefited most in this general advance was probably the Mollusca, which was Cuvier's special field of research. The lowest group in Cuvier's system, as that in Linnæus's, was the one about which the least was known, the Zoophyta or Radiata being made up of several distinct and heterogeneous groups of animals which bore no near relationships to one another.
This condition led to an active investigation during the generation immediately following of all the lower animals and a very large number of works of fundamental importance appeared. Rudolphi studied the parasite worms, Tiedemann and L. Agassiz the anatomy and Johannes Müller the development of echinoderms, Ehrenberg the microscopic animals, Eschscholtz, Sars and others jellyfish and polyps. The knowledge of these two latter groups was also very much extended as the result of various scientific expeditions which were sent out by