186 



but on the moist polished glass progression is slow. The differ- 

 ence in the rate of locomotion on the various supporting- surfaces 

 is due to the fact that the setae, which afford important aid in loco- 

 motion, easily or with difficulty find temporary hold on the irregular- 

 ities of the surface. The crawling consists of an extension of the 

 anterior region and a drawing of the posterior region after it. Al- 

 though this procedure is the normal and usual one it is, under some 

 circumstances, reversed and the result is a temporary backward move- 

 ment. 



There is no evidence whatever of an ability to progress by swim- 

 ming. In a series of tests made by placing worms in test tubes full 

 of sewage, they sank, in every case, to the bottom, and were never 

 able to leave it. If placed in water whose depth was less than their 

 length they could often get to the surface by crawling up the side of 

 the dish; but if the water was deeper than their length it was im- 

 possible for them to get away from the bottom. They often ex- 

 hibited random wriggling movements in water, but such motions were 

 ineffectual so far as locomotion was concerned. 



RELATION TO UGHT 



Specimens of this species make a decidedly negative response to 

 light. They occur normally below the surface of the filter bed, where 

 practically all light is excluded. No worms were found on the well- 

 lighted surface of the filter bed. In the spring and early summer, 

 when the maximum abundance occurs, the worms often make their 

 appearance in considerable quantity in the effluents of the sprinkling 

 filters, regularly accumulating on the shady side of the secondary 

 settling basins. Filter-bed rock was frequently put into battery jars 

 with worms and placed before a large laboratory window. In such 

 situation the worms invariably crawled away from the light side to- 

 wards the dark side, and then, if undisturbed, went into the central 

 region of the mass of rock. Sudden exposure to light calls forth a 

 response in the form of active crawling movements which cease when 

 the worms find themselves in a position where the light is distinctly 

 less intense; and exposure to direct sunlight produces an immediate, 

 active, negative response. 



RESISTANCE TO DESICCATION 



It was noticed that when masses of filter rock which contained 

 worms were brought into the laboratory and exposed to the air, so 

 that evaporation could take place, the gradual dr^-ing of the surfaces 



