Mollusea. 83 



come from the lake Lucrinus at Bajae. Brindisi, too, was one of the 

 principal localities, as it is even at the present day. for the culture of 

 the Oyster. 



The Edible Mussel, Mytilus edulis (Fig. 25), 'has the well known 

 almost triangular, blue-black shell. The animal has a so-called byssus- 

 gland with which it produces long horny threads, which fasten it to 

 rocks and woodwork. If it wants to leave its home, it produces a new 

 byssus with its finger-like foot and then it tears the old byssus away; 

 by repeatedly doing this it moves very slowly onwards. It flourishes 

 best in northern seas, where it is extensively collected and cultivated; 

 for this purpose trees are let down into the water and from time to time 

 drawn up, and the Mussels adhering are removed. 



Pinna (Fig. 128) is a large, thin-walled, club-shaped shell, which 

 is fixed in the mud by its narrow end. It also possesses a byssus which, 

 however, is much longer and composed of finer threads than that of the 

 Mussel. These silken threads were formerly used for the manufacture 

 of glo\ es, stockings and even entire garments ; in the 1 S th century there 

 existed at Naples and Sicily large works for spinning them. The shells 

 of Pinna also yield occasionally pearls, which, however, are of very 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 some other Mol- 

 lusks. Ascidians and Sponges. It is doubtful whether in any case the 

 host derives benefit from its lodger. 



Avicula (Fig. 129) is remarkable as a very near relation of Melea- 

 (jrina margaritifera, the Pearl-oyster, which produces the finest pearls. 

 These are nothing more than secretions of carbonate of lime with which 

 the animal covers up extraneous bodies (grains of sand, etc.), which irri- 

 tate 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 Lithe domus (Fig. 27), 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 r 

 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. 26), 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 

 formed by these shells. It is hence concluded that the temple must 

 have sunk under the level of the sea for a time and then have been 

 raised again. 



Deeply buried in the sand may be found the Razor-shells, Solen 

 Fig. 172) and Solecurtus Fig. 171), which are sought after as delicacies. 

 They are sold on the market with other edible shells as Fruits of the 

 Sea (Frutti di mare, and are eaten raw. 



While all the above named bivalves afford examples of the still-life 



6* 



