WATER-MITES AND THE WATER-BEAR. 283 



more very small dark spots never bordered by a plate. 

 These are the external openings of the tracheae or air- 

 tubes, which extend through the body and supply it 

 with oxygen. As the mites are not known to come to 

 the surface for a supply of air, as so many aquatic 

 animals do, the tracheae are supposed to be able to ab- 

 sorb it directly from the water. The tracheal open- 

 ings are not an important aid in ascertaining the iden- 

 tity of the creature, but the beginner must not mis- 

 take them for the aperture bordered or covered by the 

 ventral plates. In some mites they are not well 

 marked, and may be overlooked, but there is still an- 

 other dark spot usually present near the posterior part 

 of the ventral surface which must not be confounded 

 with the ventral opening because it is in the median 

 line. This is the external opening of the intestine. 

 It is never bordered by plates, and is always behind 

 the ventral orifice, but not always conspicuous. 



Equally important to the student are certain eleva- 

 tions of the ventral surface which appear to cover the 

 attached ends of the legs. These are called the coxce, 

 the plural of coxa, a Latin word meaning the thigh. 

 They are variously shaped and arranged, one coxa 

 seeming to cover the end of each leg, or appearing to 

 be the thigh belonging to that leg. They are motion- 

 less, however, and are really only elevations of the 

 cuticle, beneath which the muscles of the legs may be 

 seen in action. In some mites the coxee on each side 

 of the body are arranged in groups of two each, the 

 borders of the two which form the group being in con- 

 tact either by their whole length, as in Figs. 195, 196, 

 and 197, or only at some single point, as in the poste- 

 rior group shown in Fig. 194. In Figs. 194, 195, 196, 



