Invertebrate Animals. 279 
is one of the most prominent animal characteristics, this ap- 
pears also to be a very essential distinction. In the Linnean 
worms the progression of the body is accomplished in various 
ways, but where it is not by means of cilia, as in the infusoria and 
in the young of many radiata, the alternate dilatation and con- 
traction of the whole body perform the principal part. The 
lateral cirri and bristles of the majority of the Annelides are 
of themselves not in the condition to move the bodies, but at 
most serve as points of support, just as the Gasteropods do 
not glide forwards by means of the lower surface of the sto- 
mach, but do so upon it by means of extensions and contrac- 
tions of the whole body. The arms of the Cephalopods, like 
those of the polypi intended for the same purpose, serve to 
seize their prey and to convey it to their mouths, and are only 
employed occasionally for dragging their bodies forwards. 
What are termed the feet of the Echinodermata also are not 
limbs, but belong entirely to the skin. In insects, on the 
other hand, the organs of motion form not only a particular 
system of limbs, which this whole division of animals possesses 
in a constant fundamental type, though one which exhibits 
various modifications, but they form a system which also gives 
rise to a peculiar external skeleton-structure, and stands in 
close connection with a further articulation of the body. We 
may assume that in the articulated insect each joint or seg- 
ment had originally a pair of articulated legs, as we find in 
the Myriapods, for example in the Scolopendride, which 
apparently exhibit a passage to the Annelides and first of all 
to the Nereide. Nevertheless, the fundamental type of the 
structure of insects is quite different, and we perceive, when 
we glance at the whole series of this large division of the ani- 
mal kingdom, with reference to the history of development, 
that with the appearance of limbs which take upon themselves 
the function of the progressive movement of the body, there 
are united other relations of the articulation of the body which 
are of essential importance for the organisation. 
The various functions of life of the Linnzan insects are more 
‘or less centralized in different segments of the body. First of 
all, there is separated that portion which contains the organs 
of the vegetative functions, or, in other words, the organs of 
