VANSTOIJE : SINISTRAL CnAEACTER OF TLANORBIS. 255 



et respiratoire a droite, bien que I'auteur affirme, dans I'explicatioii des 

 planches, qii'ils sont places a gauche." As the result of his investiga- 

 tion, he found that an embryo 0-9 mm. in diameter was provided with 

 a globular shell having a distinctly left-handed spiral. He therefore 

 concluded that " les Planorbes out une coquillc spirale senestre chez 

 I'embryon, et discoide senestre chez I'adulte, 1' animal restant toujours 

 senestre." 



I. and II. Embryos of riamrhis corneus (after Fisclier) showing sinistral 



spiral shell. 

 III. Embryo of Linuusa, sp. ? (after Fischer) showing dextral spiral shell. 



Some two years ago, whilst studying this question, I examined 

 a number of the eggs of P. corneus (not being aware at the time 

 of the existence of Fischer's paper), and found the young shells 

 were undoubtedly sinistral. I also examined an older specimen about 

 4 mm. in diameter, where the flat discoidal stage had been reached ; 

 in this the sides of the pei'istome were equally developed, thus 

 showing no bias in either direction. Three weeks later, when about 

 6 mm. in diameter, the shell had begun to assume the more adult 

 appearance, having one side of the mouth more dilated. 



It has long been known that monstrosities occur in this genus, 

 where the normal discoidal form is replaced by a shell having a raised 

 spire. Such " heterostrophic " forms may be either sinistral, as in 

 P. leucostoma, or dextral, as in P. complanata. According to Von 

 Jhering, P. leucostoma may be regarded as atavistic, as I'epresenting 

 the sinistral raised type froni which the normal discoidal Planorbis 

 has no doubt descended. The other forms, such as P. complanalus 

 and P. multiformis^ may be considered as stages in advance of the 

 discoidal form, and so according to Von Jhering are " ultra-sinistral," 

 a better term perhaps being pseudo-dextral. A similar view has 

 been taken by Arnold Lang with regard to the genus Ampullaria, 

 where we have raised dextral forms passing through discoidal into 

 raised pseudo-sinistral forms, the spires having emerged on the 

 umbilical sides of the Planorbis-Yikk'^ shells. 



With regard to the apparently dextral character of the peristome 

 of the adult shell of Planorbis and the dextral position of the shell 

 in the living animal, these may be the first indications of a tendency 

 to pass over into a pseudo-dextral form, which has been realised by 

 P. complanata. 



