216 



Aquatic Organisms 



builds by adding pieces one by one at the end of the 

 tube, bedding each one in this secretion, which hardens 

 on contact with the water and holds fast. Small snails 



and mussel shells are 

 sometimes added to the 

 exterior with striking 

 ornamental effect, and 

 sometimes these are 

 added while the protes- 

 ting molluscs are yet 

 living in them. 



Some of the micro- 

 caddis-flies (family Hy- 

 droptilidae) fashion 

 " parchment" cases of 

 the silk secretion alone. 

 These are brownish in 

 color and translucent. 

 They are usually com- 

 pressed in form and 

 are carried about on 



FIG. 124. Cylindric sand cases of edge. AgTaylea decor- 



Leptoceridae ' (en - ates the parchment 

 with filaments of Spiro- 

 gyra, arranged concentrically over the sides in a single 

 external layer. 



Some caddis-worms build no portable cases at all, but 

 merely barricade themselves in the crevices between 

 stones, attaching pebbles by means of their silk secre- 

 tion, and thus building themselves a walled chamber 

 which they line with silk. In this they live, and out of 

 the door of the chamber they extend themselves half 

 their length in foraging. Other caddisworms construct 

 fixed tubes among the stones, and at the end of the tube 

 that opens facing the current they spin fine-meshed 

 funnel-shaped nets of silk. These are open up stream, 



