INTRODUCTION. 15 



cal works. When organs in different animals agree with one 

 another in their plan of structure, they are said to be " homolo- 

 gous," no matter what may be the functions which they perform. 

 For example, the arm of a man, the fore-leg of a horse, the wing 

 of a bird, and the swimming-paddle of a dolphin or whale, are 

 all composed essentially of the same structural elements, and 

 they are therefore said to be homologous, though they are 

 fitted for altogether different functions. 



On the other hand, when organs in different animals per- 

 form the same functions, they are said to be " analogous," 

 whatever their fundamental structure may be. Thus the 

 wing of a bat, the wing of a bird, and the wing of an insect, 

 all serve for flight, and they are therefore " analogous " organs. 

 They are all, however, constructed upon different plans, and 

 they are, therefore, not "homologous." At the same time, 

 however, it is to be remembered that there are plenty of cases 

 in which organs in different animals are not only constructed 

 upon the same plan, but also perform the same function, so 

 that they are both homologous and analogous. 



GENERAL DIVISIONS OF THE ANIMAL KINGDOM. 



As already stated, the entire animal kingdom may be di- 

 vided into some half-dozen primary plans of structure or mor- 

 phological types, to one or other of which every known animal 

 is referable. These primary types are known to naturalists as 

 the sub-kingdoms, under the following names : Protozoa, Coe- 

 lenterata, Annuloida, Annulosa, Mollusca, and Vertebrata. The 

 characters and minor subdivisions of these sub-kingdoms form 

 the subject of the remainder of this work. In the mean while, 

 it is sufficient to state that Ihe first five of these are often 

 grouped together under the collective name of the Inverte- 

 brata, or " invertebrate animals." The Invertebrata, compris- 

 ing the Protozoa, Ccelenterata, Annuloida, Annulosa, and 

 Mollusca, are collectively distinguished by the following 

 points among others : The body, if divided transversely, or 

 cut in two, shows only a single tube containing all the vital 

 organs (Fig. 1, A). These organs, in the higher Invertebrata, 

 consist of an alimentary or digestive cavity, a circulatory or 

 " haemal " system, and a nervous or " neural " system. The 

 side of the body on which the " haemal " or blood-vasculai 

 system is placed is called the " haemal aspect ; '' while the side 

 of the body on which the main masses of the nervous system 

 are situated is called the "neural aspect." When there is 



