Fholadina.- 



-MOLLUSCA. Teiikdinisa. 



367 



substance of the shell in the Phulades is hard, anil, 

 according to the observations of M. Necker, has a small 

 qnantily of a very hard mineral in its composition, called 

 arragonite. The front surface is rough, with rasp-like 

 imbrications. It is more than probable then, that 

 with this combination of favourable circumstances, the 

 method of boring of these animals is chiefly, at least, 

 mechanical. The Pholades do not appear to perforate 

 substances harder than themselves. Professor Owen 

 attributes part of the process, to the action of the 

 foot, which is sucker-like, and enables the animal to 

 fix itself to the substance which it wishes to perforate. 

 The softness of this body offers no obstacle, for " it 

 is certain," says the Professor, "that the perpetual 

 renewal of a softer substance will render it capable 

 of wearing away a harder one, subject to the friction of 

 a softer surface, and not like it susceptible of being 

 repaired." There lies the whole mystery, exclaims 

 Mr. Lewis; "the soft muscular disc is perpetually 

 renewed, and the hard limestone has no self-renovating 

 power; and thus, just as falling water wears away 

 granite bj' the incessant repetition of gentle blows, so 

 do these mollusks excavate rock or wood by the 

 incessant repetition of muscular friction." By many 

 naturalists, however, the rotatory action of the rasiiing 

 shell, which has been demonstrated, is considered suffi- 

 cient to produce the excavations; and an experiment, 

 as a sort of test, was made by M. Caillaiul, who imitated 

 as nearly as possible the conditions of the mollusk, and 

 produced a perforation in limestone, by carefullyrotating 

 the valves of a Pholas under water. The cavities made 

 by the Pholades, if carefully examined, show transverse 

 groovings, or circular striae, such as could only have 

 been produced by the rotatory action of the valves. The 

 family is represented in Plate 10, fig. 9, by Pholas 

 ilactylus. 



A remarkable property of the animals of the Pholades, 

 and which has long attracted notice, is their phospho- 

 rescence or himinousness in the dark. The light is 

 bhiish-white, and is stronger as the animal is lively, 

 fresh, and sui>plied with its fluids ; and more powerful 

 in summer, and at the period of propagation, than at 

 other times. Reaumur ascertained that the Pholades 

 secrete a fluid in considerable abundance, a kind of 

 mucus, which is thro\vn off into the surrounding wafer, 

 anil produces this luminous appearance. Some of the 

 species, especially a West Indian one {Pholan costata), 

 are used as an article of food, and regularly sold in the 

 markets of Ilavannah. In this country none are used 

 by man; but the comiuon piddock, Pholas dadylus 

 (Plate 10, fig. 9), is used at Salcomb in Devonshire as 

 a successful bait for fish. 



This group consists of several genera, such as Pholas 

 (= Darnia), Dactylina, Xylophaga, Jouannetia, Pho- 

 ladidea, and Zirphaa. 



Genus Xylophaga. — IMost of tlie species of these 

 genera perforate rocks ; but those belonging to this 

 genus are found living in floating timber. They bore 

 about an inch deep, and invariably across the grain of 

 the wood, which is always submerged ; the burrows are 

 oval, and lined with shell. 



TERF.niNlNA. — The Ship-worms differ from the 

 Pholades in their shells being lodged at the inner extre- 



mity of a burrow partly or entirely lined with shelly 

 matter. The shell is globular, open in front and 

 behind, and the valves are trilobate, concentrically 

 striated, and divided by a transverse furrow. The 

 hinge margin is reflexed in front, and the cavity under 

 tlie beaks, internally, is furnished with a long curved 

 muscular process. The animal is worm-like, and the 

 foot is formed like a sucker, and possesses a foliaceous 

 border. As there are no plates or accessory valves to 

 protect the dorsal margin, the animal, which always 

 lives in wood, continues to bore deeper and deeper, and 

 lines the holes as it proceeds with a shelly tube for its 

 protection. The siphons are very long, united nearly 

 to the end, with fringed orifices ; and about the place 

 where the two separate, they are provided with small 

 calcareous bodies, called palettes or styles, which close 

 the mouth of the tube. The species are not numerous. 

 The}' are found in the seas of almost every clime, hving 

 in wood, which they perforate, and which, when broken 

 up, may be caiTied floating about to immense distances. 

 The burrows which they thus form are usually tortuous, 

 and always in the direction of the grain of the wood, 

 unless the animal meets another Teredo, or a knot in 

 the timber. The way in which these worms accom- 

 plish their perforations is still the subject of dispute. 

 JI. Deshayes maintains that the Teredo bores by means 

 of a solvent — a special solvent secreted by the foot. 

 The animal adheres to the wood, he says, by the foot, 

 and by it macerates the surface and renders it friable. 

 Mr. Hancock, on the other hand, says that the exca- 

 vating instrument of Teredo is formed of the anterior 

 portion of the animal, in the surface of which are 

 imbedded siliceous particles which, penetrating the skin, 

 " give to it much the character of rasping paper." The 

 whole forms a rubbing surface, which being applied 

 closely to the bottom of the cavity l:>y the adhesion of 

 the foot, enables the animal to rub down and penetrate 

 the wood. According to Mr. Osier, however, and some 

 other naturalists, the Teredo, like the Pholas, works iLs 

 way into and through the wood by mechanical means; 

 the shell in it, as in the other, being the eflicient instru- 

 ment. However this may be, the devastation and 

 destruction produced by this worm is immense. The 

 damage formerly done to ships by its boring powers 

 was so notorious in the days of Linna;us that that 

 celebrated man termed it the " Calamitasnavium;" and 

 its English name of "Ship- worm" testifies to the 

 estimation in which it was held by our forefathers. It 

 is equally destructive to piers and bulwarks, and in the 

 years 1731 and 1732 it had made such inroads upon 

 the piles in Holland as to cause the greatest alarm. 

 The piles which support the banks of Zealand and 

 Fricsland were threatened with total destruction, so 

 that it was feared this worm would " reclaim from man 

 what he had with unexampled labour wrested from the 

 ocean."— (./o^jirfon.) A great many remedies have 

 been tried to prevent the attacks of the Ship-worm, but 

 the most effectual plan hitherto devised is that now 

 adopted of covering the timbers exposed to their assaults, 

 with short, broad-headed nails, " which in salt water 

 soon invests the whole with a strong coating of rust 

 impenetrable by their augers." Notwithstanding the 

 bad character they possess, these little worms are often 



