No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 427 



age is known to be reversed.^ Planorbis is not usually described, 

 however, as having a sinistral shell ; in fact, the shell may be 

 markedly dextral. In the celebrated series of Planorbis shells 

 found in the deposits at Stannheim there is every gradation 

 between shells which are coiled in one plane and shells which 

 are as markedly dextral as those of Littorina or Paludina. Many 

 recent species, P . albus, co^nplanatus, and nitidus, for example, 

 have a shell with a more or less decided dextral coil. Never- 

 theless Planorbis is a reversed form, whatever the direction of 

 the coil of its shell may be. The anal and genital orifices and 

 the opening of the mantle cavity lie on the left instead of the 

 right side. This is the essential point ; the direction of the 

 coil of the shell is a secondary matter. The shell of Planorbis 

 belongs to the type which Lang calls " pseudo-dextral," and has 

 no necessary connection with the essential features of the asym- 

 metry of the animal. 



The fact that in five cases a reversed asymmetry of the ani- 

 mal is associated with a reversal of cleavage, while in all dextral 

 forms whose cleavage is known with any degree of accuracy 

 the cleavage is dexiotropic or unreversed, certainly affords a 

 strong presumption in favor of the view that there is some 

 causal relation between the nature of the asymmetry of the 

 body and the type of cleavage of the egg. And there are facts 

 which render this conclusion more or less probable a priori. 

 Conklin found that the beginning of asymmetry in Crepidula 

 could be traced back to the cleavage of a single entodermic cell. 

 The time and direction of the cleavage of this cell were found 

 to give the initial bending of the entodermic area in a direction 

 which determined the direction of the coil of the embryo. If 

 we suppose a reversal of cleavage to occur in Crepidula, leading 

 to a corresponding division on the other side of the body, it is 

 not improbable that there would result a reversal of the asym- 

 metry of the animal. 



An interesting case in relation to this problem is afforded by 

 the "inverse embryos" of Ascaris, described by Zur Strassen 

 ('96). In the normal embryos of Ascaris certain cells are 



^ I have recently found reversed cleavage in another sinistral gasteropod, 

 Ancyhts rivularis Say. (See the American Nahiralist, November, 1S99.) 



