146 GASTROPODA. 



The cause of thi* asymmetry is to be sought in the manner of life of 

 the Gastropoda, i.e., in the development of the foot as a massive creeping 

 organ and in the simultaneous development of the shelly covering of the 

 body. At first the visceral mass was fairly equally distributed over 

 the body, which was covered only by a flattish shell. These original 

 forms no doubt most nearly resembled the Chitones, apart from the 

 segmentation of the shell found in the latter. So as to give greater 

 freedom to the head which carried the sensory organs and the mouth, 

 to allow the foot to grow larger and also to make it independent of 

 the rest of the body, this organ became restricted to a smaller part 

 of the body. This led to the formation of the high visceral sac, to 

 which, as the part specially needing protection, the shell also became 

 restricted, although the head and foot might still be drawn in under 

 the latter, which consequently had to be of larger size than would be 

 necessary in a merely protective covering. The animal was thus 

 obliged to carry not only the high visceral dome, but a calcareous 

 .shell capable of accommodating the whole body. If this heavy 

 mass became too high, it would be in a state of unstable equilibrium 

 and would naturally become inclined, the best inclination being- 

 backward, as hindering the animal least in creeping. But since the 

 mantle-cavity with its important organs (the gills, the apertures of 

 the intestinal canal, the kidneys and the genital organs) lay at the 

 posterior end of the body, such a backward inclination of the visceral 

 mass would be so unfavourable as to be at first impossible and the 

 only inclination which seems possible would be to the side. This 

 lateral projection of the sac, however, too greatly impeded locomotion, 

 and in spite of the disadvantages mentioned above, the visceral dome 

 tended to incline backward. If we assume that the visceral dome 

 inclined to the left side, the great pressure from the left would tend 

 to squeeze the pallial complex towards the right. Herein, therefore, 

 lay the cause of that displacement to the right and then forward 

 which has been described above (Fig. 60). Ontogenetically, this pro- 

 cess takes the form of more active growth of the posterior part of the 

 body on the left side, which leads to the bulging of the visceral sac, 

 and the forward displacement of the anus then follows (cf. p. 138). 



It would not be surprising if the pressure of the inclined visceral 



for, from the study of other points in their anatomy it has long been concluded 

 that the Opisthobranchs and Pulmonates (i.e., the Euthyneura) are to be 

 derived from the Prosobranchia after the latter attained the streptoneurous 

 condition. If this is the case, we must regard the condition met with in the 

 Euthyneura as a retrogressive one and not as an arrested stage in the rotation 

 of the pallial complex. — Ed.] 



