Shell equivalve, oblong-ovate, gaping chiefly anteriorly, inequilateral, thick, white exteriorly and inside polished; exterior covered with longitudinal furrows and concentric striae, with sharp radiating spines; no hinge; beaks hidden with callosities; a flattened spoon-shaped tooth, which curves forward, in each valve; accessory valves four in number.
The perforating powers of the Pholas have for a length of time been a subject of discussion amongst naturalists, and appear likely to continue so. Some thought that by means of its foot it perforated the soft clay or stone which hardened round it; and a Dutch philosopher named Sellius, nearly 130 years ago, published an account of the Teredo, wherein he showed that its shell could not be the instrument of perforation, and asked how it was possible that the extremely tender shell of the young Teredo could make a hole in solid oak - a material ten times harder than itself. He also observed that the form of the tube is evidently not the result of an auger-like instrument, because it is broader at the bottom than at the top and sides.
Pholas Dactylus. Piddock or Clam.
del __G. B. Sowerby, lith. Vincent Brooks,Imp.
Dr. J. G. Jeffreys, who quotes the above in his 'British Conchology,' agrees with Sellius that the foot or muscular disk, and not the shell, is "the sole instrument of perforation by the mollusca of stone, wood, and other substances, which is closely applied to the concave end of the hole, and is constantly supplied with moisture through the glandular tissues of the body". He adds, "By this simple, yet gradual process, the fibres of wood or grains of sand-stone may easily be detached or disintegrated, time and patience being allowed for the operation". Some naturalists believe that it is accomplished by means of an acid contained in the fish, by which it dissolves the calcareous rocks; while others maintain that the Pholas bores by using its shell as a rasp. This mechanical process is fully described by "Astur," who, from his own observations, has endeavoured to solve the problem, and who, to quote the late Mr. Buckland's words, is apparently the only person "who has ever seen the Pholas at work". In the 'Field,' "Astur" published some time since an interesting description of the method by which this mollusk bores its habitation. He says, "Having procured several of these mollusks in pieces of timber, I extracted one, and placed it loose in my aquarium, in the vague hope that it would perforate some sandstone on which I placed it. It possessed the powers of locomotion, but made no attempt to bore. I then cut a piece of wood from the timber in which it had been found, and placed the Pholas in a hole a little more than an inch deep. Its shell being about two inches long, this arrangement left about an inch and three-quarters exposed. After a short time the animal attached its foot to the bottom of the hole, and commenced swaying itself from side to side, until the hole was sufficiently deep to allow it to proceed in the following manner. It inflated itself with water apparently to its fullest extent, raising its shell upwards from the hole; then, holding by its muscular foot, it drew its shell gradually downwards. This would have produced a perpendicular and very inefficient action, but for a wise provision of nature. The edges of the valves are not joined close together, but are connected by a membrane; and, instead of being joined at the hinge, like ordinary bivalves, they possess an extra plate attached to each valve of the shell, which is necessary for the following part of the operation. In the action of boring, this mollusk, having expanded itself with water, draws down its shell within the hole, gradually closing the lower anterior edges, until they almost touch. It then raises its shell upwards, gradually opening the lower anterior edges and closing the upper, thus boring both upwards and downwards. The spines (points) on the shells are placed in rows, like the teeth of a saw; those toward the lower part being sharp and pointed, whilst those above, being-useless, are not renewed. So far for the operation of boring; but how to account for the holes fitting the shape of the animal inhabiting them? To this I fearlessly answer, that this is only the case when the Pholas is found in the rock which it entered when small. This mollusk evidently bores merely to protect its fragile shell, and not from any love of boring; and in this opinion I am borne out by my own specimens. The young Pholas, having found a substance suitable for a habitation, ceases to bore immediately that it has buried its shell below the surface of the rock, etc. It remains quiescent until its increased growth requires a renewal of its labours. It thus continues working deeper and deeper, and, should the substance fail or decay, it has no alternative but to bore through, and seek some fresh spot where it may find a more secure retreat".
At Amroth, near Tenby, is a submerged forest, the trees of which are completely perforated by the Pholas; and at spring-tides fine specimens may be collected. Montagu remarks that, whilst it is the general habit of shipworms (Teredo navalis, or Teredo norvegica) to bore parallel with the grain, the Pholas perforates the wood across the grain.*
Dr. J. G. Jeffreys mentions that Redi, in a letter to his friend Megalotti, describes the Teredo as being not only eatable, but excelling all shellfish, the oyster not excepted, in its exquisite flavour. Nardo also praises it, and wonders why the Venetians, who call it Bisse del legno, do not eat it.†
The German name for the Pholas is very appropriate, viz., die Bohrrrmschel, Steinbohrer, or pierce-stone; in France it is called le Dail commun, Gite, or Pitau; in Spain, Folado, Almeixa-bravas; in Minorca, Pens de cabra and Datil del mar; and in Sicily, Dattoli di mari.
An old fisherman told me that the Pudworm, as he called it, was a very delicate fish; and he had often noticed on the Hampshire coast, that at low springtides in the winter, when sharp frosts set in, and when that part of the shore where these mollusks bury themselves, is left exposed by the tide, they are all killed. He was in the habit of collecting the Pholas dactylus as bait for white fish, digging them out of the clay or shale; and he added that if he kept them a day or so before using them, they changed colour, and shone like glowworms, even shone quite brightly in the water, some distance below the surface, when put on the hooks for bait. This reminds me of the following quaint lines in Breton's 'Ourania,' quoted in Daniel's 'RuralSports':-
* Forbes and Hanley, ' British Mollusca.' † 'British Conchology,' vol. iii. p. 159.
" The glowworme shining in a frosty night Is an admirable thing in Shepheard's sight. Twentie of these wormes put in a small glasse, Stopped so close that no issue doe passe, Hang'd in a Bow-net and suncke to the ground Of a poole or lake, broad and profound, Will take such plentie of excellent fish As well may furnish an Emperor's dish,"
The luminosity of the Pholas after death is referred to by Pliny, who says, "The onyches shine in the dark like fire, and in the mouth even while they are eaten;"* and, "that it is the property of the dacytlus (a fish so called from its strong resemblance to the human nail) to shine brightly in the dark, when all other lights are removed, and the more moisture it has the brighter is the light emitted. In the mouth, even while they are eaten, they give forth their light, and the same, too, when in the hands; the very drops, in fact, that fall from them on the ground, or on the clothes, are of the same luminous nature".†
* Pliny, 'Nat. Hist.' vol. ii. bk. ix. c. 51. Throughout this volume I have used the translations of Pliny and Athenaeus in Bonn's Series of Classical Authors.
† Idem. vol. ii. bk. ix. c. 87.
Costa, as quoted by Dr. J. G. Jeffreys in his 'British Conchology,' says that it is so phosphorescent, that if the flesh is chewed and kept in the mouth, the breath becomes luminous and looks like a real flame.
Dr. Coldstream states that "the phosphorescent light of this mollusk is given out most strongly by the internal surfaces of the respiratory tubes, and that it is strongest in summer; and Professor John Muller has observed, that when Pholades are placed in a vacuum, the light disappears, but reappears on the admission of air; also, that when dried, they recover their luminous property on being rubbed or moistened.*
Many others have also made experiments with the Pholas, and have studied its phosphorescence, viz., Reaumur, Beccaria, Marsilius, Galeatus, and Montius. The two first mentioned endeavoured to render this "luminosity permanent, and the best result was obtained by placing the dead mollusk in honey, by which its property of emitting light lasted more than a year. Whenever it was plunged into warm water, the body of the Pholas gave as much light as ever".†
Beccaria also found that a single Pholas "rendered seven ounces of milk so luminous that the faces of persons might be distinguished by it, and it looked as if transparent".‡
Pholas dactylus, or the long oyster, as it is called at Weymouth, is not often eaten in England, but is generally used for bait. A Newhaven fisherman, however, told me they sometimes collect some for eating from the chalk boulders, between Newhaven and Brighton; that they were much more pleasant to the taste than whelks; and they only scald or boil them for a few minutes.
* Forbes and Hanly, vol. i. p. 107.
† 'Phosphorescence,' by T. L. Phipson, Ph.D., F.C.S., p. 105.
‡ Ibid. p. 104.
In France, in the neighbourhood of Dieppe, a great many women and children, each provided with an iron pick, are employed in collecting them either for sale in the market or for bait.*
I find from Mr. Morton, of St. Clement's, Jersey, who kindly sent me much information respecting the shell-fishes used as food in the Channel Islands, that in Jersey the Pholas is plentiful, and is sold in the market boiled ready for eating. In Spain it is considered as next best to oysters, and is sometimes eaten raw. All the Pholades are edible, and a large West Indian species, Pholas costata, is much prized, and is regularly sold in the markets of Havana, as we are informed by Forbes and Hanley. Athensaeus recommends these shellfish, as they are very nutritious, but he adds that they have a disagreeable smell.†
The Normandy method of cooking the Pholas (le dail commun) is to dress them with herbs and breadcrumbs, or pickle them with vinegar.‡
Large quantities of this fish are sold in the markets of La Rochelle, and Captain Bedford says that the Pholas crispata is eaten by the poor of Oban.§
* 'British Conchology,' vol. iii. p. 102.
† 'Deipnosophists,' vol. i. bk. iii. c. 35, p. 146.
‡ 'Cottage Gardener,' vol. i. p. 382.
§ ' British Conchology,' vol. iii. p. 114.