60 
Tank No. 22. 
little value. The fable of the crab, Pinnotheres, acting as watchman 
to its host the Pinna, has been believed from the most ancient times up 
to the present day; similar crabs are known to frequent many other 
Mollusks, Ascidians and Sponges. It is doubtful whether in any case 
the host derives benefit from its lodger. 
Avieula (Fig. 101) is remarkable as a very near relation of Melea- 
grina margaritfera , the Pearl-oyster, which produces the finest pearls. 
Fig. 100. Two specimens of Fig. 101. Four specimens of Avieula hirundo 
Pinna nobilis partly buried in attached to a stone, 1/2 nat. see¬ 
the sand, */* nat. size. 
These are nothing more than secretions of carbonate of lime with which 
the animal covers up extraneous bodies (grains of sand etc.), which irritate 
its body. Each pearl contains at its centre some such object, and in 
this way man can cause the production of pearls, a fact of which the 
Chinese have taken advantage to make pearls of various forms. 
A shell of interesting habits is Lithodomus (Fig. 102), which is 
always found in holes in rocks or coral reefs. The animal is a favorite 
delicacy and often appears in the markets. It is not yet understood how 
it works its way into the stone; its shell is perfectly smooth and so it 
cannot file its way in, as the Piddock, Pholas (Fig. 103), does with its 
rough shell armed with hard ridges; probably Lithodomus secretes a fluid 
which acts upon the limestone and dissolves it. The holes they make are 
quite smooth inside. 
The Temple of Serapis at Pozzuoli has made these shells of in¬ 
terest, as its columns have a zone 6 feet high marked with the holes 
