4 INTRODUCTION 



and 1835 with Darwin as naturalist, and the American expedition under 

 Captain Wilkes between 1838 and 1842 with James Dwight Dana as the 

 principal naturalist. 



The influence of all these investigations, and also that of the newly 

 established cellular theory of the structure of plants and animals, on the 

 development of the zoological system, led to the third great reform of the 

 latter. In 1845 von Siebold subdivided Cuvier*s fourth type, the Zoophyta 

 or Radiata, into three types or phyla, the Protozoa, Zoophyta, and Vermes, 

 confining thus the term Zoophyta to the truly radiate animals. He also 

 broke up Cuvier*s second type Articulata, removing the Annelida to the 

 new phylum Vermes and creating another new phylum for the Crustacea, 

 Arachnida, Myriapoda, and Insecta which he called the Arthropoda. Two 

 years later R. Leuckart broke up the phylum Zoophyta, subdividing it 

 into the phyla Echinodermata and Coelenterata, and emphasized the iso- 

 lated position of the Protozoa. Milne-Edwards also formed still another 

 new type or phylum, the Molluscoidea, in which he included the Bryozoa 

 and Tunicata. The animal kingdom was thus in 1850 subdivided into 

 eight phyla, the Protozoa, Ccelenterata, Echinodermata, Vermes, Arthro- 

 poda, Molluscoidea, Mollusca, and Vertebrata, an arrangement which is 

 still found in many textbooks. 



Darwin's Origin of Species was published in 1859 and the fourth 

 and last important reform of the zoological system of classification was the 

 direct consequence of the doctrines therein promulgated. The theory of 

 the common descent and blood relationship of all animals which Darwin 

 taught was at variance with Cuvier*s theory of fixed types and in harmony 

 with Lamarck's theory of the essential unity of the animal kingdom, 'and 

 was first employed by Haeckel as the basis of a system of classification. 

 In 1877 he called attention to the need of placing the entire system on an 

 evolutionary basis and at the same time subdivided the animal kingdom 

 into the two great groups of the Protozoa and the Metazoa, and the 

 latter into the two great groups of the Coslenterata and the Ccelomata. In 

 still more recent times other authors, notably Hatschek, following HaeckePs 

 lead, have carried the subdivision still further on the same basis. The old 

 idea of types, however, has a very tenacious life and is still the basis of 

 the classification of animals in most textbooks and probably rightly so. 

 For animals can, as a matter of fact, notwithstanding their ultimate 

 relationships with one another, be grouped in a number of distinct types 

 or phyla, each of which has a characteristic plan of structure. Cuvier's 

 belief, however, that these types are fixed and isolated creations has long 

 since been abandoned. 



Very important has been the formation in recent times of the phylum, 

 Chordonia or Chordata, which brings under the same subdivision all the 



