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 fundamental 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 morphological types or plans of structure. Upon one or other of these six (or perhaps seven) 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 technically called "sub-kingdoms," and are known by the names Protozoa, Coelenterata, Echinodermata, 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, or, in other words, in the degree to which it exhibits specialisation of function.