INTRODUCTION. 15 
logical and physiological differences between animals, a short 
explanation may be given of the meaning of the terms Homo- 
logy and Analogy, which are in constant use in zoological 
works. When organs in different animals agree with one 
another in their plan of structure, they are said to be “homo- 
logous,” no matter what may be the functions which they per- 
form. 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 ele- 
ments, 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 perform 
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, there- 
fore, 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 dot homo- 
logous and analogous. 
GENERAL DIVISIONS OF THE ANIMAL KINGDOM. 
As already stated, the entire animal kingdom may be divided 
into some half-dozen primary plans of structure or morphologi- 
cal types, to one or other of which every known animal is refer- 
able. These primary types are known to naturalists as the szé- 
kingdoms, under the following names: Protozoa, Celenterata, 
Annuloida, Annulosa, Mollusca, and Vertebrata. The charac- 
ters and minor subdivisions of these sub-kingdoms form’ the 
subject of the remainder of this work. In the meanwhile, it is 
sufficient to state that the first five of these are often grouped 
together under the collective name of the /uvertebrata, or “in- 
vertebrate animals.” The /zvertebrata, comprising the Protozoa, 
Caelenterata, Annuloida, Annulosa, and Mollusca, are collec- 
tively distinguished by the following points amongst 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 /zvertebrata, consist of an alimentary or 
digestive cavity, a circulatory or ‘‘hamal” system, and a ner- 
