INVERTEBRATE ZOOLOGY 



CHAPTER I 

 INTRODUCTION 



This book deals with that portion of the field of zoology which 

 concerns all animals other than those included in the single 

 phylum Chordata. Its scope is thus expressed in negative 

 terms, for all animals lacking a backbone are invertebrates. 

 Early in the development of modern classification the term 

 Vertebrata was introduced as the name of a phylum or branch to 

 include the four highest classes of the Linnean system. Popu- 

 larly, then, all animals have become recognized as either verte- 

 brates or invertebrates. More recently, three small groups of 

 animals, including Amphioxus, the sea squirts, and Balano- 

 glossus, have been shown to possess characters which seem to 

 point to definite relationships with the Vertebrata, even though 

 they lack a vertebral column. One of these characters is the 

 presence of a notochord, a structure which in the embryology 

 of the vertebrates is the forerunner of the vertebral column. 

 As a consequence, the Leptocardii, the Tunicata, and the Enter- 

 opneusta as prochordates have been combined with the true 

 Vertebrata to form a phylum which bears the name Chordata. 



Technically, then, the terms chordate and non-chordate furnish 

 a more sound basis for distinguishing the highest phylum of 

 animals from all the lower phyla combined, but the widespread 

 and popular acceptance of the terms vertebrate and invertebrate 

 have operated against the general introduction of these terms. 

 Strictly speaking, the prochordates are invertebrates but they 

 are not discussed in this book. 



In the classification here adopted, twelve non-chordate phyla 

 are recognized, as follows: Protozoa, Porifera, Coelenterata, 

 Ctenophora, Plathelminthes, Nemathelminthes, Trochelminthes, 

 Coelhelminthes, MoUuscoidea, Echinoderma, Mollusca, and 

 Arthropoda. 



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