236 ANIMAL LIFE 



species form movable cases of the most varied materials. 

 Others bind portions of the river-bed together into a 

 burrow or fixed case, which they quit at pleasure. 

 Some are adapted to still water, others to rapids. 

 A few inhabit brackish water or are marine, and at least 

 one is terrestrial and lives in moss at the base of trees. 



The caddis-worm or larva may be easily exposed 

 by passing a pin through the harder end of its case, 

 and it is then seen to be a fleshy creature provided 

 with a head, a thorax bearing six legs and four tufts 

 of bristles, and an abdomen of nine segments, the first 

 and last of which carry peculiar processes, whilst the 

 intermediate segments bear filamentous gills. The 

 last pair of processes are hooked, and grip the sheath 

 or case so firmly as to remind one of the hermit crab 

 which clings to its shell by the hooked tip of its tail. 

 The first abdominal processes are three in number, 

 though not all alike, since the middle one can be 

 protruded or withdrawn whilst the others are fixed 

 by minute hooks into the sides of the case. The use 

 of these processes has been determined by extracting 

 larvae and giving them a transparent substance (mica) 

 out of which to form a new case. When this is done 

 the abdomen is seen to be fixed in front and to un- 

 dulate gently for the greater part of its length. It is, 

 in fact, drawing in a current of water that flows over 

 its body and through the case, thus aerating the gills. 



The manufacture of the case is a process full of 

 interest and of possibilities of experiment. In its 

 shape, materials, and construction the different species 



