Pholadina. MOLLUSCA. Teuedinina. 
substance of the shell in the Pholacles is hard, and, 
according to the observations of M. Necker, has a small 
quantity of a very hard mineral in its composition, called 
arragonite. Tlie 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 by 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 rasping 
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. Caillaud, who imitated 
as nearly as possible the conditions of the molhisk, and 
produced a perforation in limestone, by carefully rotating 
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 
dactylus. 
A remarkable property of the animals of the Pholades, 
and which has long attracted notice, is their phospho- 
rescence or luminousness in the dark. The light is 
bluish-white, and is stronger as the animal is lively, 
fresh, and supplied with its fluids ; and more powerful 
in summer, and at the period of propagation, than at 
other times. Eeaumur aseertained that the Pholades 
secrete a fluid in considerable abundance, a kind of 
mucus, which is thrown off into the surrounding water, 
and produces this luminous appearance. Some of the 
species, especially a West Indian one {Pholas costata), 
are used as an article of food, and regularly sold in the 
markets of Havannah. In this country none are used 
by man; but the common piddock, Pholas dactylus 
(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 
(= Barnia), Dactylina, Xylophaga, Jouannetia, Pho- 
ladidea, and Zirphcea. 
Genus Xylophaga. — Most of the 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. 
Teredinina. — The Ship-worms differ from the 
Pholades in their shells being lodged at the inner extre- 
367 
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 
the 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. 
They are found in the seas of almost every clime, living 
in wood, which they perforate, and which, when broken 
up, may be carried floating about to immense distances. 
The burrows which the}' 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. 
M. 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, hy 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 by 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 its 
way into and through the wood by mechanical means; 
the shell in it, as in the other, being the efficient 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 Linnmus that that 
celebrated man termed it the “ Calamitas navium ;” and 
its English name of “Ship-worm” testifies to the 
estimation in which it was held by mir 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 
Friesland were threatened with total destruction, so 
that it was feared this w'orm would “ reclaim from man 
what he had with unexampled labour wrested from the 
ocean.” — {Johnston.) 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 
