Shelter-building Forms 371 



but on a wholly different plan. The gills are not 

 involved in the disc, but instead the body itself is 

 flattened and shaped to an oval form underneath, and 

 fringed with close set hairs. 



There is in the mayflies a rather close correlation 

 between the degree of flattening of the body and the 

 rate of flow of the water inhabited. It is well illus- 

 trated by the allies of Heptagenia; also by those of 

 Ephemerella, among which occur swift -water forms. 

 Epeorus, Iron and Rithrogena form an adaptive series. 

 Among the Parnid beetles, Elmis (fig. 2i4b), Dryops 

 and Psephenus (fig. 160) form a parallel series. 



There are snails that dwell in the rapids. The most 

 limpet-shaped of these is Ancylus (fig. 160 on page 260) 

 whose widely open and flaring shell has in it only a 

 suggestion of a spiral. Certain other snails (such as 

 Goniobasis livescens) are of the ordinary form and are 

 able to maintain themselves on the stones by means of 

 a very stout muscular closely-adherent foot. Simi- 

 larly, a number of flat worms, that 

 adhere closely are found creeping 

 in the rapids. 



Shelter -building foragers are num- 

 erous in individuals but few in 

 kinds. One tube-dweller, Hy- 

 dropsy che, is a plancton gatherer 

 and has been already discussed. 

 There are other shelter building 

 caddis -worms living among stones 

 in running water. Ryacophila FIG. 220. Two pupal 

 builds at close of larval life a barri- cases of the caddis-fly 



i r .. , .. ~ Ryacophila, removed 



cade or stones as shown in the fig. f rom the stones. 

 125 on page 217, and shuts itself in 

 and spins about itself a brownish parchment-like cocoon 

 of the form shown in the accompanying figure. Heli- 

 copsyche constructs a spirally coiled case that is 



