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, Ceenis and other mayfly nymphs 



Libellula, Didymops, 

 : ;,-■;, Celithemis (fig. 200) 



and other dragonfly 

 nymphs, and certain 

 snails and flatworms 

 belong here. 



(2). Bottom 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, 

 7 both mussels and 

 snails; a few mayfly 

 and dragonfly mmi- 

 phs. Descending 

 more deeply in mudd}^ 

 beds are some true worms and horsefly larvae. 



The shelter-building forms of the bottom ma}' 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 flxed 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 sprawler: nymph of 

 of the dragonfly, Celithemis eponina. 



