SEGMENTAL BILATERAL SYMMETRY 227 



the present-day Fish's anatomy. Apparently the digestive 

 load was strongly attracted to the earth, and the alimentary 

 tube, instead of being suspended in the general somatic 

 cavity, sank down to be as near the earth as possible, where 

 with its contents it became so much " ballast " in the body 

 of the Fish. And as alimentary development proceeded, 

 with the elongation of intestines and the formation of 

 associated organs such as the liver, the digestible parts would 

 form a bulky and heavy mass. The fact that the cross- 

 section of the Fish is not circular, in spite of even friction 

 forces having played during development on all parts of 

 the circumference, but shaped as below, 



*i*5* 



toalL— 



can hardly be attributed to anything else than the pull of 

 Gravity on the digestive tract. That is, the region where 

 this tract is situated, termed the ventral region, came 

 habitually to face the earth through the Force of Gravity, 

 and the density of the digestive organs and contents was so 

 relatively high as to resist the forces making for a circular 

 cross-section and gave the Fish its ventral fulness. 



It may be remarked that in all vertebrates the ventral 

 surface is invariably that which is habitually turned towards 

 the earth, and this inclines us to believe that the digestive 

 viscera and their contents have always tended to be of 

 greater specific gravity, or to be more strongly attracted 

 towards the earth, than the nervous, muscular, and supporting 

 tissues of animals. Exceptional cases, such as those of the 

 sloth, or of man, only reflect the late adoption of peculiar 

 posture. 



The viscera of the developing primitive Fish would thus 

 act as so much ballast, which, by exerting pressure down- 

 wards and outwards at the call of Gravity, would give the 

 Fish a pear-shaped instead of a circular cross-section, and 



