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circulation, are also well protected, and kept separate from each other, and from 
the alimentary or assimilating system; so that no two of these can interfere with 
each other, unless by such an injury as would be fatal to the animal. Then, as 
the whole of them are within the mechanical system, none of them can receive any 
displacement by the natural action of that system. It is thus evident that, in such 
animals, the greatest care is taken both of the compound system which carries on 
the vital functions, and of that which is understood to be more immediately con¬ 
cerned in the function of sensation. 
It is very different with invertebrated animals, in all their classes, which are 
far more numerous and varied than those of the vertebrated ones. The whole of 
their structures, vital, motive, and sensal, are lodged within the same cavity; and 
thus, if we except the motive one, which gains from the arrangement some mecha¬ 
nical advantages afterwards to be noticed, they cannot have the same freedom or 
action as in the vertebrated animals, which have them apart. Accordingly, the 
organs of assimilation, of respiration, and of circulation, are far less perfect than 
they are in the vertebrated animals. No single description can be made applicable 
to all the differences which are found among them; but it may, in general, be 
stated that there is much less distinctness in the stomach and its auxiliary organs, 
though this is probably the most important part of them, because every animal 
must receive matter for its growth, and also for its nourishment; consequently, 
this part is the most complete. In the circulation there is no distinct heart, for, 
in many of them at least, the assimilated blood goes directly to the growth or 
nourishment of the parts ; and they are provided with a sort of breathing tubes 
generally distributed through the cavity of the body, which perform the necessary 
process of aeration upon the nourishing fluid in its progress to the different parts. 
The system of sensation is, however, the least perfectly developed of the whole. 
There is not, in any invertebrated animal, any organ which can be positively said 
to be a true brain ; and, generally speaking, the central parts of the nervous sys¬ 
tem are placed near the system of nourishment, the most conspicuous ganglion, or 
enlargement, being situated on the gullet, and the others in the continuation of 
the cavity of the body. In the orders which are most humble in their organiza¬ 
tion, the radiata, there are no symmetrical organs, the counterparts of each other 
upon opposite sides, as we find in all vertebrated animals, and in the higher orders 
of the invertebrated ones. The whole proceeds, as it were, from a centre, and, 
in very many instances, almost any point is capable of becoming a centre; for if 
the body is divided, the parts, in time, become entire animals. 
It should seem, therefore, that the invertebrated animals are founded upon the 
system of assimilation, or nourishment, and that their predominant function is 
that of growth. They all do, indeed, possess sensation in some degree or other, 
higher in one class, and lower in another ; but this part of their general system is 
always very inferior in its structure, and very subordinate in its power, to the 
