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 branchie, are to be 
found upon one side only, forward of the heart, and differ 
somewhat in appearance from the plume-like processes of the 
Amphineura. 
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 
