THE WATER-MITES ^HYDRACARINA) 



855 



short free existence becomes a parasite either on an aquatic insect 

 which remains habitually in the water or on one which leaves the 

 water and becomes aerial. Other species place the eggs singly in 

 the tissues of fresh water mussels, or in masses between the gills, 



FIG. 1320. Various types of hydrachnid larvae (each figure showing the legs of one side only). 

 a, Diplodontus; b, Hygrobates; c, Arrhenurus; d, Hydrachna. (Modified from Piersig.) 



and still others in the substance of fresh-water sponges or in the 

 gelatinous matrix of a colonial protozoan. In these cases the larva 

 does not become free but remains in the body of the mollusk or 

 other animal in which the eggs were laid. During this parasitic 



