ORGANIC DEPENDENCE AND DISEASE 
17 
many angles of approach. The common clam is the perfect 
adjustment; buried in the mud and fortified by its coat of 
mail it is difficult to find a creature better adapted and pro- 
tected. It is a natural sequence, then, that the race of clam 
has abounded in all the seas since almost the earliest ages. 
Again, the pea crab hides himself in the living oyster, and 
the hermit crab backs himself into an empty conch-shell or 
periwinkle, hiding away his soft degenerate abdominal 
joints and tail and using the mouth of his bombproof for of- 
fensive as well as defensive purposes. Neither of these in- 
quilines comes out; neither would dare to expose his soft- 
ened mature body outside; but his adjustment is competent 
notwithstanding the fact that he is a degenerate whose an- 
cestors were hard-shelled and who, succumbing to the out- 
side struggle, found this protection inside the shells of the 
mollusks. The paleontologist Ruedemann lias beautifully 
shown that far back in Ordovician time or earlier, the acorn 
barnacles, whose hard-shelled descendants of today coat 
the submerged reefs of the sea and the hulls of befouled 
ships, were derived from the free-swimming crustaceans of 
the pliyllopod type, through attachment by their backs ; a 
process which seems to have started first as a partial burial 
of the carapace, leaving the food-grasping organs and 
gills exposed above the mud ; eventually becoming an actual 
solid fixation because of the distinct advantage in protec- 
tion and ease of feeding which the animal had discovered. 
Lateral stresses, Ruedemann thinks, the play of the cur- 
rents against the carapace and the strains against its side 
walls, developed the sutures which divide the peculiar shell 
of the Acorn barnacle. The other great class of barnacles, 
Lepas, or the commonly known Goose barnacles, whose clus- 
ters are found today in places where the other barnacles 
grow, seem to have had a like origin at a like period of 
earth history, through a cementation, not by the back of the 
pliyllopod ancestor, but rather by its head. These are most 
venerable degenerates of most adequate adjustment. They 
