SCIENCE-GOSSIP. 



PROTECTIVE COLOURATION IN BRITISH CLAUSILIAS. 



By Wilfred Mark Webb, F.L.S. 



'THE notes that are made from time to time on 

 -*- the colouration of British non-marine molluscs 

 are few enough and sparse enough, to be sure, but 

 even these as a general rule, deal only with the 

 tints or markings themselves, and not with their 

 significance. Indeed the difficulty of ascertaining 

 the conditions of existence to which the Mollusca in 

 general have to conform, makes the knowledge on 

 questions of colouring in the group exceedingly 

 limited, and the literature on the subject exceedingly 

 scattered. For these reasons it may be of interest 

 to chronicle a resemblance to its surroundings, 

 presumably protective, in the case of Clausilia 

 biplicata, Mont., recalled to mind by a futile 

 attempt to visit a well-known locality for this 

 rare snail. The fact that experienced collectors 

 are often taken in — and in such instances, there 

 is a secondary, if not a primary pleasure when 

 the deception is discovered — gives considerable 

 weight to the " tit - for - tat " argument that the 

 mollusc will in turn be mistaken for the object 

 which in the first place was mistaken for it. 



At various times the writer has collected some 

 hundreds of specimens of Clausilia biplicata on the 

 southern banks of the River Thames, in a spot 

 once a swamp, now " improved " by the contents 

 of many dust-heaps, fenced in and made useful, 

 and from first to last he was often deceived by 

 the shrivelled bud -scales presumably of the old 

 osiers at the bases of whose stems the sought-for 

 molluscs lived. In connection with this fact, one 

 must mention a case in which the bloom-sheaths 

 of the black poplar were taken for two slugs Avion 

 hortensis and Agriolimax agrestis. (G. Sherriff Tye : 

 " On a case of Protective Resemblance among 

 Slugs." — "Journal of Malacology," vol. iii., 1894, 

 page 21.) Mr. Tye, who fancied he saw a number 

 of these slugs in his garden, says, "I thought 

 this a fair opportunity for giving our pet thrush 

 and blackbird a treat, so I ' went ' for those slugs, 

 but was surprised to find that I had been deceived 

 — they were poplar budsheaths." 



The fascination which the discovery of protective 

 resemblances has for the biologist, makes it 

 difficult for him to abstain from jumping to con- 

 clusions or from formulating theories based upon 

 insufficient facts, but one may make the suggestion 

 that Clausilia biplicata gains some advantage from 

 its likeness to the bud-scales, and that there is 

 here a case of protective resemblance as strong as 

 that given by Mr. Tye. Mr. Sheriff Tye is not, 

 however, quite technically correct in using the term, 

 " protective mimicry," mimicry being restricted to 

 cases where an animal derives a. benefit from 



resembling another which possesses some special 

 life-preserving quality. At the same time, the 

 preference which the British Clausilias have 

 for the neighbourhood of tree-trunks must be 

 remembered, and following up the suggestion 

 thrown out, it may be noted that the shell of 

 Clausilia laminata, Mont., for instance, is com- 

 paratively smooth and generally resembles the 

 bud-scales of the beech, a tree under which it 

 is commonly to be found. With regard to Clausilia 

 rolphii, Gray, there is another feature which it may 

 not be out of place to mention here. The shell in 

 this species, which at first is dark-coloured and 

 glossy, rapidly takes on, when it has reached its 

 full size, a dull and weather-beaten appearance : 

 indeed, out of some sixty specimens collected by 

 writer (Wilfred Mark Webb : " Mollusca at Limps- 

 field, Surrey, April, 1887." — Science - Gossip, 

 June, 1887), only six retained their youthful 

 brilliancy. Here there can be no doubt that through 

 this weathering, which harmonized completely with 

 the dead alder-twigs among which the colony lived, 

 the same end is attained as by the covering of the 

 shell with mud by Buliminus obscurus, Mull., and 

 other molluscs. 



It is always being brought home to those who 

 work for the advancement of biology, that what 

 appears to them to be the easy and pleasant work 

 has been already done, while the difficult subjects 

 only lie before them for investigation, but if the 

 question of the significance of colour be complex, it 

 has at least an interest attached to it which cannot 

 easily be surpassed. 



Since the above lines have been put into type, 

 the writer's attention has been called to a note read 

 by his friend, Mr. Miller Christy, before the Essex 

 Field Club, in 1882 (Proc. Essex Field Club, 

 vol. iii., pages xcii.-xciii.). The following extract 

 may be given: "Of the habits of C. biplicata in 

 this country I know nothing, but on the Continent 

 it lives upon wet rocks. Of the remaining three 

 species, C. rugosa may be described as very com- 

 mon, C. laminatata as common, and C. rolphii as 

 rare and local. It is a well-known fact that all 

 three frequently live upon or under beech-trees. 

 I have collected them all in such situations, the 

 first two abundantly, and the last more sparingly, 

 among the numerous clumps of beeches which 

 fringe the Northern edge of the South Downs. 

 Now, the bud -cases of beech-trees are in shape 

 long and pointed, and in colour hazel-brown, 

 thereby resembling very closely, in these parti- 

 culars, as well as in size, the shells of the Clausilias, 

 so that, at a glance, the difference between the two 



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