STRUCTURAL MODIFICATIONS 137 



Clymenididae, Ammonitidae), and among some primitive 

 Gasteropods (Bellcrophon) ; among others it is wound not 

 in a flat spiral but in a helix, thus becoming dissymmetrical, 

 as has also happened in the case of some of the later 

 Ammonites (Turrilites). What relation is there between all 

 these facts ? 



It cannot be supposed, as we said above, that the dorsal 

 cone of the Cephalopods and Gasteropods was able to grow 

 upright on the creature's back in spite of its weight, which must 

 then have flattened it. We may rather suppose that these 

 organisms originally either floated or swam on their backs 

 in the water, the ventral surface uppermost, and were able 

 to maintain this position by some means or other of 

 suspension and locomotion. In this case their dorsal surface 

 would inevitably have yielded to the pressure of the viscera 

 under the influence of gravitation and to the pull of the 

 calcareous protective shield, when there was one on this side. 

 A pendent dorsal cone must thus have been formed in the water, 

 and it is only thus that the posture of the Cephalopods and the 

 primitive straight-shelled Gasteropods can be conceived. On 

 the other hand, it is impossible not to notice that all the present 

 swimming Molluscs swim on their backs with their ventral 

 side uppermost (species of Nautihdae, Ianthinidae, Carinaridse, 

 Pterotracheidae, Pteropoda), and that all the marine larvae of the 

 Gasteropods provided with a shell swim in the same fashion. 

 If, as w r e have explained, ancestral forms are recapitulated 

 embryogenetically, here is a definite indication that the 

 ancestors of the present Molluscs were swimming organisms, 

 and swam in the inverted position still maintained in larvae, 

 and re-adopted by adults when they revert to life in the ocean. 



The causes that have produced the coiling of the shell are 

 not in the least mysterious ; they were in part suggested long 

 ago by Arnold Lang. The gills of the Cephalopods are situated 

 in a cavity, within which the anus also opens, and which there- 

 fore corresponds to the posterior region of the body. The 

 organism is able to respire freely only if the opening of this 

 cavity is uncovered by a forward inclination of the point of 

 the dorsal cone. This point is then pushed back and upward, 

 as a result of the resistance of the water to the mollusc's 

 weight, but the mollusc is propelled forward by suddenly 

 expelling the water contained in the branchial cavity ; the 



