34 ELEMENTS OF BIOLOGY. 



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 agree- 

 ing can differ from one another only in the sole remaining 

 element of the question namely, by the amount of speciali- 

 sation of functions which they exhibit. Every animal, there- 

 fore, as Professor Huxley has well expressed it, is the resul- 

 tant 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, Coelenterata, Annuloida, Annulosa, Mollusca, and 

 Vertebrata. We have, then, to remember that every mem- 

 ber 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 struc- 

 ture, and differs from every other simply in the grade of its 

 organisation ; or, in other words, in the degree to which it 

 exhibits specialisation of functions. 



It is to be remembered, also, that whilst all naturalists 

 recognise distinct plans of structure or " morphological 

 types" in both the animal and vegetable kingdoms, all 

 are not agreed as to the number of these types. In other 

 words, all zoologists are not yet agreed as to the characters 

 which should be regarded as constituting a distinct " mor- 

 phological type." The result of this is that different authori- 

 ties divide the animal kingdom into a different number of 

 "sub-kingdoms." Most modern naturalists, however, are 

 agreed as to the morphological distinctness of the Protozoa, 

 Ccelenterata, Annulosa, Mollusca, and Vertebrata. There 

 thus remains the sub -kingdom of the Annuloida alone, about 

 which any serious divergence of opinion is entertained. 

 Following Huxley, this division is here regarded as having 



