14 MANUAL OF ZOOLOGY. 



complexity of the organism thus necessarily increasing part 

 passu with the complexity of the function. This gradual sub- 

 division and elaboration is carried out equally with the other 

 two physiological functions, viz. reproduction and correlation, 

 and it constitutes what is technically called the "specialisa- 

 tion of functions," though it has been more happily termed by 

 Milne-Edwards " the principle of the physiological division of 

 labour." It is needless, however, to remark that in the higher 

 animals it is the functions of correlation which become most 

 highly specialised disproportionately so, indeed, when com- 

 pared with the development of the nutritive and reproductive 

 functions. 



b. Morphological Type. The first point in which one 

 animal may differ from another is the degree to which the 

 principle of the physiological division of labour is carried. 

 The second point in which one animal may differ from another 

 is in its "morphological type;" that is to say, in the funda- 

 mental plan upon which it is constructed. By one not specially 

 acquainted with the subject it might be readily imagined that 

 each species or kind of animal was constructed upon a plan 

 peculiar to itself and not shared by any other. This, how- 

 ever, is far from being the case ; and it is now universally 

 recognised that all the varied species of animals however 

 great the apparent amount of diversity amongst them may be 

 arranged under no more than half-a-dozen primary morpho- 

 logical types or plans of structure. Upon one or other of 

 these five or six plans every known animal, whether living or 

 extinct, is constructed. It follows from the limited number of 

 primitive types or patterns, that great numbers of animals 

 must agree with one another in their morphological type. It 

 follows also that all so agreeing can differ from one another 

 only in the sole remaining element of the question namely, 

 by the amount of specialisation of function which they exhibit. 

 Every animal, therefore, as Professor Huxley has well expressed 

 it, is the resultant of two tendencies, the one morphological, 

 the other physiological. 



The six types or plans of structure, upon one or other of 

 which all known animals have been constructed, are techni- 

 cally called "sub-kingdoms," and are known by the names 

 Protozoa, Ccelenterata, Annuloida, Annulosa, Mollusca, and 

 Vertebrata. We have, then, to remember that every member 

 of each of these primary divisions of the animal kingdom 

 agrees with every other member of the same division in being 

 formed upon a certain definite plan or type of structure, and 

 differs from every other simply in the grade of its organisation, 



