

AKTHROPODA 



183 



The excretory organs take the form of Malpighian 

 tubules which vary in number from two to a hundred. 

 They open into the intestine. 



The circulatory organs are poorly developed, but they 

 are compensated for by the trachea?. The number of 

 tracheal openings on the side of the thorax and abdomen 

 varies from three to ten, but there are never more than 

 one for each segment. They branch 

 internally again and again until the 

 fine branches permeate every part of 

 the body. Sometimes they are en- 

 larged into air sacs which make the 

 body light. Air is drawn in by a 

 rhythmical enlargement of the abdo- 

 men and reaches all the tissues of the 

 body. It is expelled again by the 

 contraction of the abdomen. 



The alimentary canal passes be- 

 tween the two halves of the nerve 

 cord which unite to form a dorsal 

 ganglion, or brain, and a ventral 

 cord. The eyes, simple and com- 

 pound, are on the head. The organs 

 for hearing may be on the base of the abdomen, on the 

 legs or on the antennas. Taste is on the lower lip and 

 smell on the antennas. 



When the young leave the egg they are very small; 

 they may have the adult appearance, or they may be like 

 the adult except that the wings are small and later in- 

 crease in size with every moult, or they may be entirely 

 different from the adult, in which case they undergo a 



Fig. 101.— Milliped. 



