5G2 THE ANATOMY OF INVERTEBRATED ANIMALS. 



semble one another, and will differ from all other animals in 

 certain respects. Each such assemblage is, in fact, a " nat- 

 ural order " in the sense in which that word is used by bota- 

 nists ; and, although the number of these natural orders may- 

 be increased by the discover}'' of new forms, or diminished by 

 the ascertainment of closer bonds of union than are at present 

 known to exist between the orders already discriminated, 

 yet, the morphological types w^iich they represent will al- 

 ways remain ; and, therefore, the knowledge of their charac- 

 ters, once acquired, will be a permanent possession. 



It is not needful that these natural orders should be mor- 

 phologically, still less numerically, equivalent ; and, in form- 

 ing them, it is more important that similarities should not be 

 neglected, than that differences should be overlooked. Those 

 W'hich have been recognized in the preceding pages are enu- 

 merated in the following list, arranged in sections correspond- 

 ing with the chapters in which they are discussed. Under 

 the head of each section I shall proceed to make such obser- 

 vations as have been suggested to me by new information or 

 by further reflection, during the progress of this work. 



Section I. — Monera [Forami7iiferd\ [Heliozoa], JRadio- 

 laria, Protoplasta^ Gregarinidm^ Catallacta, Infusoria [ Ojyci- 

 Unina^ Ciliata^ Flagellata^ Tentaculiferd]. 



Section II. — Porifera, Hydrozoa^ Coralligena \Cteno- 

 ]L)horci\. 



Section III. — Turhellaria, Rotifer a [Nematorhynchci]^ 

 Trematoda, Cestoidea. 



Section IV. — Hlrudinea^ Oligochccta, Polychmta^ Gephy- 

 rea. 



Section V. — Crustacea, AracJmidn, [Pycnogonida, Tardi- 

 grada, Pentastomida\ 3Iyriapoda, Insecta. 



Section VI. — Polyzoa^ Brachiopoda, Lamellibranchiata, 

 Odontophora. 



Section VII. — Echinodermata. 



Section VIII. — Tunicata. 



Section IX. — Peripatidea, Mijzostomata, Enteropneusta, 

 Chcetognatha, Nematoidea, Physemaria, Acanthocephala, 

 Dicyemida. 



Section T. — In the commencement of Chapter II., I have 

 expressed a doubt as to the validity of the distinction of the 

 groups contained in this section by the presence or absence 



