IN SEC 7' S AND THEIR NEAR RELATIVES. 45 



mites developed in the gall can escape, in order to start new 

 galls. In addition to the swelling of both surfaces of the 

 leaf its internal structure is seen to be modified. In some 

 parts there is a great multiplication of the cells, and in 

 others a large part of the cells have been destroyed. Two 

 eggs of mites are represented in this gall. As the season 

 advances, and the galls become dry and brownish or black, 

 the thickening of the leaf becomes less marked. In fact, in 

 some cases there is a shrinkage of the parts affected. Fig- 



FiG. 54 —Section of leaf showing- structure of gall in autumn : ^, gall ; «, «, 

 uninjured part of leaf ; o, opening of gall. 



ure 54 represents a section through a leaf collected and 

 studied in October. 



Among the scavenger mites there are some that infest 

 food products. Thus mites are sometimes found in cheese^ 

 in sugar, and in preserved meats. 



Class Myriapoda (Myr-i ap'o-da). 

 The Centipedes and the Millipedes, 



The mefnbers of this class are air-breathifig Arthropods, in 

 which the head is distinct from the thorax, and the thorax 

 and abdomen form a continuous region, with from six to two 

 Jmndred segments, each bearing a pair of legs. The head 

 bears a single pair of antennce. 



The thousand-legged worms, as they are commonly 

 called, are well-known and generally feared creatures. But 

 few students find them attractive subjects of study ; never- 

 theless it is well to know something about them, for some of 

 them are dangerous animals, and some are harmless. A few 

 species are injurious to agriculture, while others are to be 



