THE CONCHOLOGIST. 



25 



ADVENTITIOUS PROTECTION IN FRESH- 

 WATER MOLLUSCA. 



By C. Clare Fryer. 



THE persistence of the " primitive " horn 

 colour in the Limnceidce, and the simple 

 dull greens and purples of the Paludinidcz 

 and U)iionida. are doubtless due in great 

 measure to the less competition that exists 

 between freshwater forms and the consequent 

 slower and less efficient action of natural 

 selection on a more constant environment, 

 but such colours are at the same time the 

 best protection, assimilating to that of the 

 muddy surroundings, for the prevalence of 

 dull green and brown colours, doubtless due 

 to the same cause, may be noticed in 

 freshwater insects. 



Concealment is further enhanced by coat- 

 ings or stains of mud, lime, etc., or growths 

 of weed. 



The Uniohida: vary greatly in colour, and 

 very generally assimilate to that their sur- 

 roundings — thus those living in clear streams 

 and rivers are light green and often 

 " rayed " — whilst those in muddy ponds are 

 darker (see Dr. Gould in Bost. Soc. Nat. 

 Hist., vol. xiv, Ap. 5, 1871). Specimens from 

 a stagnant pond near the Arch on Hampstead 

 Heath, are of a very deep, almost black, 

 olive green and live in rich black mud, at 

 the upper end of the pond an iron-charged 

 stream tinges the water and mud red, and 

 the mollusca are similarly coated with red 

 oxide of iron. 



Splncrium corneum varies in colour in 

 different waters. On the caked mud of a 

 dry ditch at Childs Hill were numerous 

 Pisidium pusi/iian, but very hard to dis- 

 tinguish owing to their coating of mud. 



Paludina vivipara from the Thames at 

 Marlow, living in comparatively clear water 

 were light green and often bandless. Speci- 

 mens from the Leg of Mutton pond at 

 Hampstead are darker, have a greater 

 development of purple, and frequently 

 covered with conferv^e. Dr. Gray notices 

 of Neritina fltiviatilis that " the shells are 

 often covered with calcareous incrustations 

 deposited by the waters, which makes them 

 look like pieces of dirt, and thus escape 

 being seized on by the fish," while in clear 

 waters its chequered shell and spotted hind 

 part of the foot may aid it in concealment 

 among the fine pebbly sand of swift streams. 



Linmcea stagnalis — a fine and old specimen 

 living in a patch of conferva in a quiet 

 bay of the Brent, at Whembley, was {|uite 

 obscured by a particularly fine growth of 

 the same weed, extending some inches 

 beyond the shell, while it is noteable that 

 my other specimens from this stream are 

 very thin, of small size and light in colour: 

 two being the variety albida. 



Limncea peregra varies greatly in colour 

 and texture, and always seems most admir- 

 ably adapted to the surrounding conditions. 

 When the shell is thin and light coloured, 

 the dark bluish animal variegated with 

 yellow spots and patches is visible through 

 the body whorl, and owing to the irregular 

 distribution of colours obscures the outline 

 and assimilates to the muddy bottom. 



This species sometimes dots the mud 

 banks in great numbers from which the 

 water has receded, and is in such a plight 

 often coated with mud, and so to some 

 extent protected. 



A drain at the narrow end of the Leg 

 of Mutton Pond is highly charged with 

 oxide of iron which forms a deposit on 

 everything in it including the few plants- 

 there is no sheltering pond weed, and 



