2l6 



REMARKS ON THE CAUSE OF ABNORMALITY IN 

 PLANORBIS SPIRORBIS. 



By R. STANDEN. 



(Read before the Society, Nov. 9th, 1898). 



Mr. A. G. Stubbs, in his interesting paper {vide p. 106-108 anted) on 

 the remarkable assemblage of monstrous shells from a ditch at Tenby, 

 makes a suggestion as to the possible cause of this phenomenon, which 

 distinctly merits consideration. But it has occurred to me that the 

 same cause would equally apply to other ditches, and yet I do not 

 know of any similar abnormalities occurring in such profusion else- 

 where, although individual cases of distortion in Planorbis spirorbis, 

 etc., are by no means uncommon, as the experience of most collectors 

 will show. The statement that they only occur in a restricted area of 

 this particular ditch, made me curious to investigate the general fauna 

 of that area more fully, and on communicating this desire to Mr. 

 Stubbs, he very kindly sent me a number of living P. spirorbis, both 

 normal and abnormal, in all stages of growth, together with samples 

 of the aquatic vegetation. 



On placing these in a tank, I soon noticed that all the shells were 

 more or less covered with a flocculent growth, which a pocket lens 

 resolved into luxuriant colonies of Epistylis anastatica, one of the 

 rigid-stalked Vorticellidae. Closer microscopic examination showed 

 that the needle-like stalks of this organism form dense spiky tufts, and 

 each stalk is furthermore studded with numbers of diatoms, which add 

 to the rigidity of the mass. On carefully examining the points where 

 the whorls of the shells first show a tendency to scalarity, more espe- 

 cially in young examples, I was quite satisfied that this growth of 

 Epistylis is the primary cause of abnormality in the Tenby specimens. 

 During the resting periods which intervene between the successive 

 " growth stages " of the shell, the Vorticellce increase rapidly, and 

 cover every portion ; and should a vigorous colony chance to locate 

 itself in the suture immediately at the junction of the mouth and 

 whorl, especially when the shell is quite young, the animal, on com- 

 mencing to add a new portion to its shell, is confronted by a prickly 

 living bush — which, doubtless, possesses some irritant or stinging 

 power as well — and is obliged to carry on the construction of its 

 shell at a tangent, in order to avoid the interfering parasite. In this 

 way the regular coiling of the shell is interrupted, and it twists off 

 into all manner of eccentric forms, as is so admirably shown in the 

 plate accompanying Mr. Stubbs' paper. 



These observations were further confirmed on receiving another 

 large consignment of shells taken, at my suggestion, from different 



