EVOLUTION OF THE ANNELIDA, ONYCHOPHORA, 

 AND ARTHROPODA 



By R. E. SNODGRASS 

 Bureau of Entomology and Plant Quarantine, U. S. Department of Agriculture 



CONTENTS 



PAGE 



I. The hypothetical annelid ancestors i 



II. The mesoderm and the beginning of metamerism 9 



III. Development of the annelid nervous system 21 



IV. The adult annelid 26 



The teloblastic, or postlarval, somites 26 



The prostomium and its appendages 32 



The body and its appendages 34 



The nervous system 39 



The eyes 45 



The nephridia and the genital ducts 45 



V. The Onychophora 50 



Early stages of development 52 



The nervous system 55 



The eyes 62 



Later history of the mesoderm and the coelomic sacs 62 



The somatic musculature 64 



The segmental appendages 67 



The respiratory organs 70 



The circulatory system 70 



The nephridia 72 



The organs of reproduction 74 



VI. The Arthropoda 76 



Early embryonic development 80 



Primary and secondary somites 82 



The cephalic segmentation and the development of the brain 89 



Evolution of the head 107 



Coelomic organs of adult arthropods 126 



The genital ducts 131 



VII. Phylogenetic conclusions 132 



References 149 



I. THE HYPOTHETICAL ANNELID ANCESTORS 



Among the simplest of the metazoic animals that lead an active, 

 free existence is the planula larva of the Coelenterata. The planula 

 develops into a polyp or a medusa because it is a young coelenterate, 

 but, so far as its structure goes, it contains the fundamental building 



Smithsonian Miscellaneous Collections, Vol. 97. No. 6 



