146 LIFE, IN ITS INTERMEDIATE FORMS. 
Air now becomes tlio 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 tho whole body, communicating with the at- 
mosphere by certain minute orifices ( spiracles ), situated 
one on each side of every segment. 
Thero is considerable diversity between the animals of 
this class in organic development. The feeble inert Julus 
is but little elevated above tho 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 boars 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 antennal. 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 
