128 FIRST BOOK OF ZOOLOGY. 



attains adult size, when it may possess over a hundred seg- 

 ments. 



Like true insects, the young myriapod makes its appear- 

 ance from the egg with three pairs of legs. The body, how- 

 ever, is never divided into a thoracic portion, and an abdomi- 

 nal portion, as in the true insects, or into two regions as in 

 the spiders, but after the head there succeeds a continuous 

 row of similar segments to the tail. 



118. In studying the insects, spiders, and centipedes, or 

 myrmpods, the pupils have learned something about three 

 groups of animals which have in common a body. composed 

 of segments, arid possessing jointed legs. They all breathe 

 air through holes in the side of the body, called spiracles, 

 the air, thus breathed, finding its way through various parts 

 of the body by means of little tubes called tracheae, except in 

 the spiders, where little sacs, called pulmonary sacs, take the 

 place of tracheae. 



In the true insects the segments of the body are gathered 

 into three regions, called respectively the head, thorax, and 

 abdomen. In the spiders the segments of the body are gath- 

 ered into two regions, called respectively the cephalo-thorax, 

 and abdomen, the head being merged in the thorax. In the 

 myriapods the head is again distinct as in the true insects, 

 but the remaining segments of the body are distinct and are 

 not grouped into regions. 



The true insects have three pairs of legs. The spiders 

 have four pairs of legs, while the myriapods have no definite 

 number of legs. In some species there are nearly two hundred 



