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THE AMERICAN NATUEALIST [Vol. LIII 



III. The coloration of an Onchidium is the result of its 

 intensity of pigmentation, the texture of its mantle, and 

 the activity of its mantle glands. An adult measures, on 

 the average, 17 mm. long by 12 mm. broad by 6 mm. high 

 when resting undisturbed, being then oval in outline ; but 

 it becomes more elongated during creeping, measuring 

 then about 24 by 8 mm., and also less strongly arched, 

 being about 4 mm. high. Wlien first observed creeping 

 out of its nest for an airing," it usually happens that 

 the Onchidium bears upon its dorsal surface a thin pel- 

 licle of slime completely investing the mantle. This 

 slime-layer is tough, somewhat thickened about the pe- 

 riphery, and can be peeled off intact. Frequently it is so 

 removed, mechanically, during the snail's efforts to creep 

 out of its nest, for the passage-way is commonly so ob- 

 structed by the growth of Modiolus that the Onchidia 

 must slowly insinuate themselves through slit-like aper- 

 tures not more than 2 mm. wide. Occasionally Onchidia 

 are seen creeping about with their slime-coat but par- 

 tially removed. 



The slime-pellicle is encrusted with the muddy fecal 

 matter .wiiich the Onchidia deposit. This calcareous 

 mud is swallowed when feeding on the algas over which 

 they creep in the open ; it represents the ordinary silt of 

 the shore-line, and may in part be chemically abstracted 

 from the thin layer of sea-w^ater covering the algas as the 

 tide recedes, owing to photo synthetic removal of CO2, 

 especially as accelerated by increasing temperature and 

 by isolation as the sea-weeds become more exposed. 



When the slime-coat is removed, an ordinary 0. f ori- 

 danum seems at first sight jet black; the mantle is wet 

 and shiny; closer examination shows that the color is 

 really a very dark and smoky olive. The mantle-glands 

 continue their slime secretion, however, and by the action 

 of wind and sun the surface of the mollusk quickly as- 

 sumes a dry blue-black aspect, its numerous papillas glis- 

 tening in the light; the appearance of the mantle, with 

 a slight "bloom" on it, then resembles that of the tiny 



