146 



LIFE, IN ITS INTERMEDIATE FORMS. 



Air now becomes the medium of respiration, and, ac- 

 cordingly, this function is carried on by a new set of 

 organs, called trachea. These are pipes which ramify 

 throughout the whole body, communicating with the at- 

 mosphere by certain minute orifices {spiracles), situated 

 one on each side of every segment. 



There is considerable diversity between the animals of 

 this class in organic development. The feeble inert Julus 

 is but little elevated above the Worm ; its body being 

 divided into forty or fifty segments, each of which carries 

 two pairs of minute and powerless feet. Its mouth is 

 furnished with a pair of horny plates, with toothed edges, 

 which are brought into contact by a movement from right 

 to left. The head bears a pair of thread-like horns, evi- 

 dently organs of sense ; these are shadowed out in the 

 appendages of the heads of many Annelida \ but being 

 now, for the first time, distinctly jointed, a new name is 

 given them, — that of antenna. These organs henceforth 

 occupy an important place in the economy of the Articu- 



LATA. 



In the Centipede (Scolopendra), we have a much more 

 vigorous and formidable creature. The most obvious 

 change from the Julus is the concentration of its parts ; 

 the segments are greatly reduced in number, but propor- 

 tionally developed in size ; they are furnished with more 

 powerful muscles, and each bears but a single pair of 

 limbs, which are longer, more distinctly jointed, and en- 

 dowed with greater powers of motion. Besides the cutting 

 blades, with which the mouth is armed in common with 

 the Julus, the Scolopendra is endowed with peculiar 

 weapons of offence in the form of a pair of stout curved 



