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 Euedemann has 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 phyllopod 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 w r hich 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 

 phyllopod ancestor, but rather by its head. These are most 

 venerable degenerates of most adequate adjustment. They 



