ARTICULATED TYPE OF STRUCTURE. 69 



with a jointed envelope. Hence the character of this series is 

 derived from the inclosure of the body and limbs in a jointed 

 envelope ; and from the perfect resemblance of the two halves to 

 each other. The repetition of parts is found to exist in great 



FIG. 29. CENTIPEDE. 



degree in the internal as well as in the external organs. Thus in 

 the Centipede, every segment of the body is provided with its 

 own air-cavity for respiration, and with its own nervous ganglia ; 

 and the dorsal vessel, which represents the heart in these animals, 

 is also divided by partitions, into a series of chambers corre- 

 sponding to the segments. Further, the conformity between 

 the two halves of the body is extremely perfect ; for not only 

 are the exterior of the trunk and limbs perfectly symmetri- 

 cal, but the equality extends also to the organs of digestion, 

 circulation, and respiration, which, with very few exceptions, 

 would be divided into two equal halves, by a line passing down 

 the middle of the body. Now this perfect symmetry, which 

 is seen nowhere else than in the Articulata, is evidently connected 

 with the high development of their powers of locomotion. A 

 little consideration will show, that it is impossible for an animal 

 to move with great energy and rapidity, unless the two sides of 

 its body are equal in power and weight. Every one knows that 

 a Bird, if one of its wings be clipped, cannot fly straight ; and 

 that a Man, having a burthen to carry, will accomplish it much 

 better by dividing it into two portions, and raising one with each 

 hand, than by attempting to bear it all on one side. We shall 

 hereafter see that, in rapidity and energy of movement, there 

 are no animals which, when their size is taken into account, can 

 be compared with a large number of the Articulata. The sen- 

 sory organs, too, are highly developed ; and in the construction 

 of the eyes we again meet with the repetition, which is so charac- 

 teristic of the whole organisation; each of the bodies that project 



