GASTEROPODS 329 



go, man is then a symmetrical creature ; likewise the vertebrates 

 in general. Cases of actual symmetry are found in the lobsters 

 and crabs and in the segmented worms. It is usual in describing 

 mollusks broadly and in general terms to call them symmetrical 

 animals, yet here is the largest class within the phylum, whose 

 representatives are none of them symmetrical. In the Gasteropoda 

 the mouth is anteriorly placed in the head, just as in the ideal 

 form, but the digestive tract, after traversing the visceral cavity 

 in the usual fashion, suddenly turns forward and terminates 

 in an excretory opening either on the right or on the left 

 side of the animal, just back of the head. The various other ori- 

 fices for the genital and renal glands are also placed in this 

 unexpected anterior position. The gills, or branchiae, are to be 

 found upon one side only, forward of the heart, and differ 

 somewhat in appearance from the plume-like processes of the 

 AmpTiineura. 



What, then, has become of the corresponding gill we should ex- 

 pect to find on the other side? The symmetrical shape of the 

 foot is not in the least altered, but what has happened to cause 

 this distortion of the visceral portion of the animal? And, 

 finally, will this torsion of the body of the snail perhaps account 

 for the spiral nature of the shell ? 



An evolutionary process is supposed to have taken place in 

 mollusks, which, if real, would no doubt account for these curious 

 conditions of torsion in the Gasteropoda; but the theory upon 

 which this process is based is altogether speculative, and is not 

 fully sustained by the facts in the case. Originally all mollusks 

 are presumed to have been symmetrical, and are assumed to have 

 resembled very much in form the schematic creature we have 

 already described. Now, for some reason, certain of the primitive 

 mollusks, but not all of them, began to develop a larger visceral 

 mass, which, continuing to enlarge throughout many generations, 

 began finally to protrude above and form a hump on the dorsal 

 side of the animal. This hump, containing the liver, a portion of 

 the intestines, and the generative glands, as it increased in bulk 

 became so much elevated that it could no longer maintain itself 

 in an erect position over the body, but, impelled by its own 



