562 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 discovery 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 which 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 

 which 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 \Foraruiniferc{\ \Heliozoa\ JRadio- 

 laria, Protoplasta, Gregarinida?, CataUacta, Infusoria [ Opa- 

 linina, GUiata, FlageUata, Tentaculiferd]. 



Section II. — Par if era, Hydrozoa, Goralligena [Cteno- 

 phora\ 



Section III. — Ticrbellaria, Rotifera [Nematorhyneha], 

 Trematoda, Cestoidea. 



Section IV. — Hirudinea, Oligochceta, Polychceta, Gephy- 

 rea. 



Section V. — Crustacea, Arachnida \Pycnogonida, Tardi- 

 grada, Pentastomidd], Myriapoda, Insecta. 



Section VI. — Polyzoa, Braehiopoda, Lamellibranchiata, 

 Odontophora, 



Section VII. — Eehin odermata. 



Section VIII. — Tunicata. 



Section IX. — Peripatidea, 3fyzostomata, Enter opneusta, 

 Chmtognatha, Nematoidea, Physemaria, Acanthocephala, 

 Dicyemida. 



Section I. — 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 



