Corals, Worms, etc. 



these it builds its little shelter, slowly, laboriously, yet with 

 infinite patience. The sand is taken up, grain by grain, in 

 the tentacles and passed to the animal's mouth, where it is 

 covered with saliva ; then the tentacles transfer the sand 

 grain to the margin of the tube and place it in position. 

 The sand-mason is a little builder using sand grains in 

 place of bricks, saliva in place of mortar. 



A closely related species uses fragments of broken 

 shell for its home ; others make shells so closely resembling 

 those of certain snails that there is every excuse for 

 mistaking the worms for molluscs. There are hosts of 

 other worms flat-worms, tape-worms, ribbon and round 

 worms. Most of them show life histories of the greatest 

 interest ; many of them are parasitic. But, though interest- 

 ing, their doings hardly form pleasant reading, and of signs 

 of ingenuity they display not a particle, unless the fact 

 that many of the parasitic worms require two hosts to 

 complete their life cycle. Thus the tape-worm of the 

 mouse can only complete its growth within the intestines 

 of the cat ; that of the rabbit must pass to the dog for 

 complete development ; a snail parasite would die out 

 entirely were it not swallowed by the thrush, and so on. 

 Interesting but nauseating. 



295 



