The Caddis-Worm 



unskilled oarsman to fresh latitudes. It be- 

 comes a voyage on the right seas when the 

 crossing measures a hand's-breadth. 



However, tacking on the surface of the 

 water affords the Caddis-worm no pleasure. 

 It prefers to twitter in one spot, to remain 

 stationary in flotillas. When the time comes 

 to return to the quiet of the mud-bed at the 

 bottom, the animal, having had enough of 

 the sun, draws itself wholly into its sheath 

 again and, with a piston-stroke, expels the air 

 from the back-room. The normal density is 

 restored and it sinks slowly to the bottom. 



We see, therefore, that the Caddis-worm 

 has not to trouble about hydrostatics when 

 building its scabbard. In spite of the incon- 

 gruity of its work, in which the bulky and less 

 dense portions seem to balance the more solid, 

 concentrated part, it is not called upon to con- 

 trive an equipoise between the light and the 

 heavy. It has other artifices whereby to rise 

 to the surface, to float and to dive down again. 

 The ascent is made by the ladder of the water- 

 weeds. The average density of the sheath is 

 of no importance, so long as the burden to be 

 dragged is not beyond the animal's strength. 

 Besides, the weight of the load is greatly re- 

 duced when moved in the water. 



209 



