MEANS OF DISPERSAL. 35 



found upon the cases of caddis-worms : on some of those 

 fromthe ponds at Finchley, before mentioned, were several 

 living specimens of Sphceriimt corneum ; indeed, with 

 Limnephilus flavicornis^ which makes most of the shell- 

 cases so common in our fresh waters, as Mr. McLachlan 

 has observed, it matters little whether the shells are 

 empty or still contain the living animal/ After the fly 

 has emerged, the empty cases, in all probability, are 

 sometimes floated to great distances, but the molluscs 

 may not always survive until set free by the rotting 

 of the binding materials. In March, 1892, I saw a 

 number of empty cases, bearing shells of Sp/icerium, 

 Pisidzuni, Bythinia^ Valvata, Planorbis^ Limncea^ 

 AncyluSy and Succinea, floating amongst debris at the 

 margin of an overflow by the Lea, but I could not 

 ascertain that any of the shells contained living animals. 

 It is notorious how often shells and shell-bearing caddis- 

 cases are found adhering to the under-sides of floating 

 trunks and branches of trees, or hidden away in their cre- 

 vices,and in such positions the creatures are often likely to 

 be carried to considerable distances, sometimes, perhaps, 

 for hundreds of miles. Those molluscs which habitually 

 float, foot uppermost, on the under-surface of the water 

 are sometimes drifted along by steady currents. Mr. 

 George Roberts has remarked that Lhnncea peregra 

 " frequently allows itself to drift on the surface of run- 



^ R. McLachlan, '"Science Gossip" for 1868, pp. 152-3; and 

 see "Science Gossip^' for 1866, pp. 95 and no; George 

 Roberts, "Topography and Nat, Hist, of Lofthouse," 1883 

 p. 238. 



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