GLANDS 



149 



manently by a secretion which acts Hke 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. 



^y'^^-S^^'^'' "^ 



f i* f. I 







.' ^it 



Fig. 74. — Another type of shelter tubes of caddicc- worms, com|)osc'd of slicks 

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

 (Trichoptera). 



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

 considerable Hme mingled with these secretions and sometimes 



