340 



Aquatic Societies 



(i). Bottom sprawlers that lie exposed, or only 

 covered over with adherent silt. These are character- 

 ized by a marked resemblance to their environment. 

 Such crustaceans as the crawfish and Asellus, such 

 insects as Ephemerella, Casnis and other mayfly nymphs 



Libellula, Didymops, 

 Celithemis (fig. 200) 

 and other dragonfly 

 nymphs, and certain 

 snails and flatworms 

 belong here. 



(2). Bo torn dw 1- 

 lers that descend more 

 or less deeply into the 

 mud or sand, by the 

 various means already 

 discussed in the pre- 

 ceding chapter. 

 Among the shallow 

 burrowers are many 

 shell-bearing molluscs, 

 both mussels and 

 snails; a few mayfly 

 and dragonfly nym- 

 phs. Descending 

 more deeply in muddy 

 beds are some true worms and horsefly larvae. 



The shelter-building forms of the bottom may be 

 grouped as: 



(i). Forms making portable shelters. These are 

 mainly caddis-worms that construct cases of pieces of 

 wood or grains of sand. 



(2). Forms making fixed shelters These are 

 such caddis- worms as Polycentropus, such worms as 

 Tubifex (see fig. 83 on p. 174) and such midges as 

 Chironomus (see fig. 134 on p. 220). 



FIG. 200. A bottom spra \\ier: nymph of 

 of the dragonfly, Celithemis eponina. 



