ON CERTAIN PARASITES, COMMENSALS, AND DOMICILI- 

 ARS IN THE TEARL OYSTERS 3IELEAGBINu3^\ 



Bj' RoREKT E. C. Stearns. 



Tlie presence of nodules or tubercles on the interior surface of the 

 shells or valves of lamellibranch (bivalve) mollusks is of frequent oc- 

 currence. . These excrescences are nacreous or otherwise, according to 

 the character, in this respect, of the shell in which or upon which they 

 occur. They are found alike in fresh- water and marine species. lu the 

 pond and river mussels ( Unionidw), they are chiefly due to interior 

 causes; in marine forms, like the cockles [Cardium), mussels {Mytihis), 

 the scallops [Fecten), etc., these formations are generally traceable to ex- 

 terior causes. It is often the case that specimens of the large scallop of 

 the New England coast (P. tenuicostatus), are so burrowed into by a spe- 

 cies of sponge {Cliona snlphurea) that nearly the entire inside surface of 

 the valves will be roughened with sharp, thickly-set i)ustulie. 



So, too, with the beautiful pecten of the west coast, P. Jiastatus, com- 

 mon in certain localities in Puget Sound. Fully one-half of the speci- 

 mens obtained by the dredge are so defaced by the ravages of a similar 

 species of sponge as to be of no value. 



We sometimes meet with these noduhi? in the shells of marine gas- 

 tropods, notably the Haliotidcv, popularly known as Abalones, or ear- 

 shells, lu all of the marine s[)ecics in which these nodules occur it 

 will usually be found that the substance of the shell has been bored 

 into from the outside by either a species of i)holad or lithodomus. 



Neither of these forms are, properly speaking, either parasites or com- 

 mensals. 



They are, more definitely, domiciliares, and excavate their burrows, 

 not for the purpose of getting at the softer parts of the mollusk upon 

 whose shell they have ''squatted" iu order to use said softer parts as 

 food, after the manner of the predaceous Katieas and Purpuras, but 

 solely for the i)urp()se of a residence or domicile. 



Tlie lithodomi, especially, burrow into many species of shells, and the 

 })Iujlad, so often found in the heavy shells of old individuals of the 

 Haliotidiv, I am inclined to believe as a difterentiated and dwarfed va- 

 riety of a widely distributed rocJc-horer. A related form {Martc.sia 

 cu7iei/ormis Gray) is common on the Atlantic coast of North America, 

 and may often be seen in situ in the shells of the common oyster (0. 

 virginica). 



The burrows of these shell-boring pholads and lithodomi are at first 

 quite small, increasing in size in the same ratio as the burrower in- 



