INTRODUCTION. 
19 
SCORPION AND CENTIPEDE : DIVISION ARTICULATA. 
different principles. In the first and lowest, it may almost be said to be amorphous. The organs, 
such as they are, follow no particular arrangement ; and in many cases it is impossible even to fix 
their relative position. In the second, however, a certain symmetry is observable ; and this is 
the case also with the remaining groups, the characters of which we have yet to lay before the 
reader. But this symmetry is of a very different kind : in the Radiata^ the parts of the body 
are all grouped around a common axis, every organ being merely a repetition of its fellows ; while 
in those which must now pass under consideration, the organs of the body are arranged more or 
less distinctly in pairs on each side of the body, so as to produce what has been termed by zoolo- 
gists a bilateral symmetry. In none do we find this mode of construction so completely exhibited 
as in the animals forming the third primary division of the animal kingdom, to which we must 
now direct attention. 
The most striking peculiarity of these animals, by which — although the division contains an 
almost infinite variety — insects of all kinds, crabs, lobsters, centipedes, &c. — they may generally 
be distinguished at the first glance from all other creatures, is, that their bodies and limbs are 
composed more or less distinctly of segments or rings. From this, which is their most prominent 
character, they have been denominated articulated animals. They are also sometimes called 
annulose or ringed animals. These constitute the division Articulata. 
The joints or segments of which their bodies are composed are formed essentially by a series 
of transverse folds in the integument of the animal. In many of the lower forms, the skin still 
remains perfectly soft and flexible ; but in by far the greater number these folds become trans- 
formed into a series of horny or crustaceous rings, united to each other by a softer portion of the 
integument, so as to permit a greater or less degree of flexibility. The limbs, as well as the body, 
are constructed of rings of various forms ; and these, taken together, may be regarded, to a certain 
extent, as a sort of external skeleton, fulfilling, as they do, most of the purposes of the skeleton in 
man and the animals most related to him. Like this, it gives support to all the soft parts of the 
body, and furnishes points of attachment for the muscles ; which again, by their action on the 
movable pieces composing it, give rise to the various movements of the creature. In many cases, 
all the segments composing the body, with the exception, perhaps, of those at the two extremities, 
are exactly similar, — each presenting the same form and bearing the same organs as its neighbor. 
An instance of this may be seen in the centipede, figured above ; and it is still more strikingly 
exemplified in many marine worms. Generally, however, the segments present marked differences 
of form and comparative size, and in the structure of their appendages : this is very distinctly 
observable in the insects and crabs. 
