SYEES : VARIATION IN RECENT MOLLUSCA. 263 



In confinement Limncea is said ' to vary in rate of growth, either 

 with the nature of the water or the space the animals have in which 

 to move ; L. megasoma has been found to have narrower upper 

 whorls in subsequent generations ; the long and short spired races of 

 L. peregra have been produced at will by changing the nature of the 

 water ; while it has even been asserted ^ that L. involuta, kept in 

 confinement, has, in a few generations, lost its involute spire and 

 become L. peregra. 



Since the year 1767, when Geoffroy first noticed them, cases 

 of remarkable distortion of the shell of Planorbis have been frequently 

 recorded ; sometimes isolated specimens were found, at others whole 

 colonies appeared. Often other species occurring with them were 

 found to be similarly distorted, but not invariably so. Similar 

 variations have been observed in other species, but Planorhis has 

 furnished the most striking instances and is here taken as a typical 

 case. These forms exist in widely scattered localities, being described 

 from England, France, Belgium, Germany, Switzerland, etc. This 

 form of distortion is not confined to recent molluscs, a similar specimen 

 of the Tertiary Plmiorbis euomphaliis, J. Sby., having been observed. 

 The appearance of these shells is very remarkable. They may be 

 turreted, reversed, scalariform, and the direction of the whorl may 

 vary from time to time. Pire ^ described and figured a long series of 

 Planorhis complanatus from a pond in which he notes the presence 

 of abundance of Lemna and Conferva. Van den Broeck* in discussing 

 this paper suggested, and adduced experiments to prove, that scalariform 

 shells moved more easily through the thick coating of weed than the 

 normal ones ; he further pointed out that being air-breathers they 

 would desire to come to the surface, hinting also that we might be 

 observing stages in the evolution or progress of Planorhis. Clessin, 

 however, insisted^ that these variations were separately formed and 

 not transmitted. 



Ruff ord,'' noticing some abnormal specimens, found, on investigation, 

 that in a number of them the worm Chcetogaster limnm was attached 

 to the animal between the head and tentacles, and was inclined to 

 attribute the results to its presence, though he very frankly stated 

 that he had seen a normal specimen with a worm so attached. 



Stubbs ' illustrated a long series of forms of Planorhis spirorhis, and 

 stated that the other species found in the same ditch near Tenby 

 were also occasionally distorted or abnormal, and cites as a possible 

 cause, quoting Taylor,® that the efforts to force their way through the 

 mud in which they are sometimes left partially embedded, owing to 



1 Semper: " Animal Life." Hazay : Mai. Blatt.,3er. ii, vol. iv, p, 220. Varigny: 

 Journ. Anat. Phj'siol., vol. xxs, p. 147. 



* More, Zoologfist, 1889, p. 154; cf. Williams, t.c, p. 235. 

 3 Ann. Soc. Mai. Belg., vol. vi, p. 23. 



* Ibid., vol. vii, pp. x-xx. 



6 Mai. Blatt., vol. xx, p. 68. 



8 Zoologist, 1898. pp. 191-192. 



'' Jom-n. Conch., vol. ix, pp. 106-108, pi. iv. 



^ Monogr. Brit. Moll., vol. i, p. 118 ; cf. Journ. Conch., vol. viii, pp. 382-384. 



