48 SINGULAR HABITAT — UNDERGROUND SNAILS chap. 



could not have been washed into the streets from any adjacent 

 river or pond, and their true origin was probably indicated 

 when it was found that the funnel-shaped cloud which burst 

 over the town had passed across the one piece of water 

 near Paderborn, which was known to contain the Anodonta in 

 abundance. 



Cases of Singular Habitat. — Mollusca sometimes accus- 

 tom themselves to living in very strange localities, besides the 

 extremes of heat and cold mentioned above (pp. 23-24). In 

 the year 1852, when some large waterpipes in the City Road, 

 near St. Luke's Hospital, were being taken up for repairs, they 

 were found to be inhabited in considerable numbers by Neritina 

 fluviatilis and a species of Limnaea?- Dreissensia polymorpha 

 has been found in a similar situation in Oxford Street, and also 

 in Hamburg, and has even been known to block the pipes and 

 cisterns of private houses. In an engine cistern at Burnley, 

 60 feet above the canal from which the water was pumped 

 into the cistern, were found the following species : Sphaerium 

 corneum^ S. lacustre ; Valvata piscinalis^ Bithynia tentaeulata ; 

 Limnaea peregra^ very like Succinea in form and texture ; 

 Planorhis alhus^ P. corneus, P. nitidus, P. glaher^ and thousands 

 of P. dilatatus^ much larger than the forms in the canal below, 

 a fact probably due to the equable temperature of the water in 

 the cistern all the year round.^ In certain parts of southern 

 Algeria the fresh-water genera Melania and Melanopsis inhabit 

 abundantly waters so surcharged with salt that the marine 

 Cardium edule has actually become extinct from excess of 

 brine. The common Mytilus edulis is sometimes found within 

 the branchial chamber and attached to the abdomen of crabs 

 (^Carcinus maenas)^ which are obliged to carry about a burden 

 of which they are powerless to rid themselves (see p. 78). A 

 variety of the common Limnaea peregra lives in the hot water 

 of some of the geysers of Iceland, and has accordingly been 

 named geisericola. 



Underground Snails. — Not only do many of the land Mol- 

 lusca aestivate, or hibernate, as the case may be, beneath the 

 surface of the soil, but a certain number of species live perma- 

 nently underground, like the mole, and scarcely ever appear 

 in the light of day. Our own little Caecilianella acicula lives 

 1 Zoologist, X. p. 3430. 2 Science Gossip, 1888, p. 281. 



