ARTHROPODA 181 



surface. As in the earthworm it has a ganglion and a 

 pair of nerves for every segment. At the anterior end 

 it passes around the alimentary canal in a ring. On the 

 dorsal and ventral sides the juncture of the two parts 

 forming the ring is marked by a concentration of gan- 

 glia. The number of ganglia involved may be found by 

 counting the number of nerves given off. The total num- 

 ber of ganglia corresponds with the number of pairs of 

 appendages and with the number of segments, so that the 

 internal evidences of segmentation are in harmony with 

 the external sign. It will be found exclusive of the eyes 

 to be twenty. \ 



Acerata. — The acerata lack antennae. The body is 

 divided into two parts, the cephalothorax which bears 

 six pairs of appendages, and the abdomen which is with- 

 out appendages. They breathe through gills, lungs or 

 trachea, and the reproductive ducts open near the mid- 

 dle of the body. The best known representatives of the 

 group are the scorpions, spiders, daddy-long-legs, ticks, 

 mites, etc., that are such pests. In the spiders the cepha- 

 lothorax and the abdomen are unsegmented externally 

 but they are sharply separated. In front are the poison 

 jaws and on the tip of the lower surface of the abdomen 

 are two or three pairs of spinnerettes. These give off 

 a fluid which hardens when exposed to the air. This is 

 used to protect the eggs and to form the web that is used 

 as a home and as a trap for prey. 



Myriapoda. — To this group belong the centipeds and 

 the millipeds. In it there is no distinction between the 

 thorax and abdomen. The head is well-defined and 

 is succeeded by a long series of segments which bear one 



