ON THE DIFFERENCES BETWEEN VERTEBRATED AND 
INVERTEBRATED ANIMALS. 
By Robert Mudie. 
The differences between the two grand divisions of the animal kingdom— 
those which have a vertebrated back-bone and internal skeleton on which all their 
organs of motion are inserted, and those which have no such skeleton, but have 
their organs of motion inserted in an external crust, or integument, of some de¬ 
scription or other—offer many important lessons to the student of nature ; and, in 
as far as the mechanical action of the animals is concerned, they furnish a count¬ 
less number of examples, the proper understanding of which is very essential in 
the mechanical arts. These are the two grand objects which we ought always to 
have in view when we study nature : because the first is at once the source and 
the gratification of mental inquiry, and the other enables us to turn our know¬ 
ledge to practical use, in a world where the labours and the enjoyments of society 
must keep pace with each other. 
But though the more solid parts which sustain the immediate organs of mo¬ 
tion in the vertebrated animals are internal of those organs, and the sustaining 
parts in the invertebrated animals are external, it must not be supposed that the 
two grand divisions are reverses of each other ; for there are in the bodies of all 
animals many other structures than sustaining parts, and muscles to put those 
parts in motion, producing the external actions of the animal, varying according 
to the place which it occupies in the great system of nature. 
There are four other essential systems possessed, in a greater or less degree, 
by animals of all kinds, though their general perfection or development, as it is 
called, and also their relative development in proportion to each other, are exceed¬ 
ingly varied in the different races. These four systems are, an assimilating sys¬ 
tem, a circulating system, a breathing system, and a nervous system ; which last 
is understood to be that upon which sensation, the grand characteristic of animals, 
depends, though upon this subject we cannot come to any very positive conclusion. 
The reason is, the animal cannot live without the joint action of all these systems ; 
and the dead animal, though it can shew us the anatomical structure, or number, 
form, and arrangement of the parts of the animal, can tell us nothing whatever 
about life. Hence we know life only as a phenomenon of the compound, and, 
consequently, we cannot refer it to any of the component parts separately from 
the rest. 
We have countless instances of the effect of such unions when we examine 
compound substances, and the elements into which we can resolve them, or by the 
union of which we can reproduce them. Water, for instance, is exceedingly 
refreshing to animals and to plants, when applied to them in substance ; but nei- 
