RELATING TO THE ASYMMETRY OF THE GASTROPODA. 147 



mass led, not only to the shiftings we have mentioned, but also to 

 the degeneration of single organs. Lang, in this way, traces back 

 the absence of the organs originally forming the left part of the 

 pallia! complex (the left gill and the left renal aperture, etc.) which 

 is to be noticed in various ( Gastropods («..'/., the Monotocardia among 

 the Prosobranchia and the Opisthobranchia) to the fact that the left 

 side was exposed to specially strong pressure, through which these 

 organs were prevented from functioning and degenerated. In other 

 cases (Haliotis) the right (originally the left) gill is said to be smaller 

 than the left (originally the right), and there is also an inequality in 

 the kidneys of those Gastropods (Haliotis, Patella) in which the 

 excretory organ is paired.* 



The inclination of the visceral sac naturally led to its becoming 

 coiled. Lang rightly traces this to the fact that, in order to avoid 

 distortion, the upper side has to grow more than the lower. This 

 unequal growth gives rise finally to the spiral coiling of the sac, which 

 is followed in its shape by the shell. In those shells that are inclined 

 to the left, further room for extension is given on this side, especially 

 when the shell and visceral sac are directed backward. This unequal 

 growth determines the formation of the so-called dextrally twisted 

 shell. An original inclination to the right must be assumed for the 

 shell that shows the sinistral twist. In other respects the process is 

 the same in the two cases. The causes that lead to the inclination 

 to one side or the other ai*e difficult to determine, indeed, at the 

 present time, they are hardly known. -j- 



Sonie of the sinistrally twisted Gastropods have their inner organs 

 arranged in the same way as the ordinary dextrally twisted forms. 

 In such cases we have a false coiling which, it has been assumed, 

 arose through the flattening of a dextrally twisted shell to such an 

 extent that it became coiled in one plane. In this case the spiral 

 might again assert itself on the side opposite to that on which the 

 umbilicus originally lay, and in this way a false spiral might form 

 on the umbilical side and a false umbilicus on the spiral side (Sim- 

 ROTH, v. Jhering, Lang, No. 61). An indication of such a process 



*[This unequal development of the gills is very marked in Pleurotomaria, 

 the right (originally left) gill being much the shorter of the two ; this is the 

 gill which is suppressed in the Monotocardia. Curiously enough the kidneys 

 in some Diotocardia {e.g., Haliotis, Patella) show exactly the reverse condition 

 to that seen in the gills, i.e., the right (primary left) kidney is much larger 

 than the left (primary right) ; nevertheless, it is apparently the latter 

 nephridium which persists in the Monotocardia.— Ed.] 



t [See footnote, p. 108, on the cleavage of the egg of sinistral Gastropods. — 

 Ed.] 



