THE NET-MAKING CADDIS WORM 



bed of the stream, the foot of the bank, and even the 

 channel are affected. Little islets appear, and these 

 slowly increase. Thus the changes go on, those minute 

 and ceaseless changes by which, in the slow movement 

 of ages, the face of nature is varied and renewed. One 

 always must count upon the value of seemingly trifling 

 forces and phenomena in the processes of world-building. 



As one looks at these rude dens, he naturally asks, 

 how were the pebbles that compose them assembled? 

 Does the little cairn-builder collect, select, and arrange 

 them? Are they chance accmnulations ? It may be 

 that many of the pieces drift before the current when 

 detached from the bed of the brook by the action of 

 the stream, or by the movements of the multitude of 

 water denizens, such as small fish, frogs, water snakes, 

 crabs, and sundry insect larvae, and become entangled 

 in the sticky threads which caddises fasten to their 

 lodging-rock. Thus their great variety of form may be 

 partly due to the chance action of the riffle. 



But the builders certainly have control over the posi- 

 tion of the several pieces. This appears from the gen- 

 eral design of the structures, which, with all their ir- 

 regularity, are plainly intended for dwelling-places, and 

 admirably serve their end. The pebbles are so adjusted 

 that the silken tubes, above referred to as occupying 

 the cavities formed within the heaps, have sufficient 

 room, with free points of entrance and exit. 



However, one likes to bring personal observation to 

 determine such a point. To lie or crouch for hours 

 upon the sloping bank of a brook and watch the move- 

 ments of water larva) is not just now possible to the 

 author, although he has spent many pleasant hours in 

 such studies in earlier years. Let us see what can be 



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