GLANDS 



149 



manently by a secretion which acts like a cement. Leathery 

 tubes are formed by many worms by a secretion which hardens 



FIG. 73. Oven-shaped shelters of the caddice-worm, composed of pebbles 

 fastened together with silk fibres spun from the mouth. 



FIG. 74. Another type of shelter tubes of caddice-worms, composed of sticks 

 and pebbles. The caddice-worm is the aquatic larva of the caddice-flv 

 (Trichoptera). 



in the water into an exceedingly tough fibre. There is often 

 considerable lime mingled with these secretions and sometimes 



