vn MOLLUSCA—THE ASYMMETRY OF THE GASTROPODA 157 



(which in a Prosohranch would lie to the right in the mantle cavity near the anus) 

 which is wanting. The anus is no longer the centre of the pallial group of organs, but 

 lies outermost on one side. While in the Pi-osobranchia, for example, the original 

 left half of the pallial complex (which would now lie on the right) has disappeared, 

 those organs of the complex (the original right) which are retained, shift from 

 the left to occupy the empty space. Consequently, we find the anus no longer 

 anteriorly in the middle line, but on the right side, close to the extreme right of the 

 mantle cavity. 



But what is the reason of the disajspearance of the left half of the pallial complex 

 in the MonotocarcUa, Opisthohranchia, and Pulmonata ? 



In answering this question we must refer back to paragraph 3, where it was seen 

 that if the spire-like shell assumes the only possible lateral inclination, the mantle 

 cavity and the pallial complex within it are subjected to unequal pressure. If the 

 shell is inclined to the left, the side of the })osterior mantle cavity subjected to the 

 greatest pressure is the left, and the pressure continually decreases towards the right. 

 These variations of pressure are also retained during tlie whole time in which the 

 backward displacement of the shell and the forward displacement of the pallial 

 complex takes place. In other words, i.e. described in terms of our theory, from the 

 very commencement of the development of the Gastropod organisation, the original 

 left organs of the pallial comjilex were subjected to unfavourable conditions. In 

 this left -sided compression of the mantle cavity the ctenidium especially would 

 necessarily be reduced in size and become rudimentary, and might entirely disapjiear. 



As a matter of fact, the original left half of the pallial complex (which would 

 now lie on the right) has entirely disappeared in many Diotocardia (the so-called Azy- 

 gobranchia), in all Monotocardia, and in the Opisthobranchia. The fact that the 

 original right gill, the only one remaining, has also disappeared in the Pulmonata 

 is accounted for by the change to aerial respiration. It is an interesting fact that 

 in the Basommatophora the original right osphradium is retained. 



If, however, the original left gill did not quite disapj^ear, but only became 

 smaller, we should have to expect that in such Diotocardia as still possess two gills, 

 the original left (now the right) would be the smaller. This would be the case at 

 least in the more primitive forms with shells still twisted. Haliotis and FissureUa 

 are the only Molluscs to which this ajiplies. In Haliotis, whose shell is still 

 twisted, the right (originally left) gill is in reality the smaller. But in FissureUa 

 and Subemarginula, where the asymmetry of the mantle cavity has been secondarily 

 lost, the inequality in the size of the gills has also disappeared. 



12. 



Another imsolved problem remains. Why does the shell continue to grow 

 asymmetrically coiled with a dextral twist, after the cause of this asymmetry, viz. 

 the change from the incline to the left to the backward incline of the shell, simultane- 

 ously with the shifting forward of the mantle cavity and pallial complex, has ceased 

 to act, i.e. when the shell has definitely assumed the posterior, and the pallial 

 complex the anterior, i^osition ? The explanation of this lies in the asymmetry so 

 early apparent in the mantle cavity, which from the beginning is more spacious 

 to the right (now left) than to the left, the consequence being that the left half of 

 the pallial complex atrophied. This as3anmetry of the pallial comjilex and mantle 

 cavity remained after the displacements of shell and ^jallial complex had been 

 definitely accomplished in the Prosobranchia, i.e. the asymmetrical growth, and 

 therefore the continuous coiling of the visceral dome and shell in a spiral twist, 

 continued. 



In altogether exceptional conditions, which rendered a ilat cup-shaped shell 



