MATERIALS WHICH FORM THEIR CASES. 239 



external configuration. Some are formed of nu- 

 merous little pieces of grass, and stems of aquatic 

 plants, cut into suitable lengths, and placed cross- 

 ways, forming a rude polygonal figure ; others are 

 constructed of bits of stick, or grains of sand or 

 gravel, cemented strongly together ; and others, 

 again, are composed of fresh-water shells, each con- 

 taining its own proper inhabitant, — " a covering," 

 as Kirby and Spence remark, " as singular, as if a 

 savage, instead of clothing himself with squirrel 

 skins, should sew together into a coat the animals 

 themselves." 



Some of these cases exhibit shells of three or 

 four different kinds, and are hence interesting to the 

 Conchologist, as well as to the Entomologist. Three, 

 now in my cabinet, present specimens, altogether, of 

 seven fresh-water shells, namely : — Planorbis mar- 

 ginatus, P. contortus, P. vortex, P. glaber (?), Cyclas 

 cornea, Valvata obtusa, and Paludina impura. 



To protect themselves from the attacks of their 

 enemies, and, at the same time, to give admission to 

 the supply of water essential to their existence, the 

 caddis-worms, before assuming the torpidity of 

 their pupa state, adopt an ingenious expedient. They 

 construct a kind of grating, which they fix across 

 each extremity of their domicile, and thus pro^-ide 

 at the same time for respiration and defence. It is 



