INTRODUCTION. 



10 



SCORPION AND CENTIPEDE I DIVISION AUTICUEATA. 



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 ease 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 

 nnnulose 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 

 exe-nplified 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. 



