The Piddocks - Family Pholadidie

THE PIDDOCKS - FAMILY PHOLADIDIE. Shell bivalve, gaping at both ends; valves thin, white, hard, brittle, with rasp-like, overlapping laminations in front; hinge plate reflected over the umbones and a long curved, spoon-like process under each, for the attachment of muscles; accessory valves often present, external to the original shell; pallial sinus deep. Animal club-shaped; foot truncated in front; mantle closed, except pedal orifice; siphons large, long, united except near ciliated ends; branchial siphon (containing gills) closed throughout; hinge ligament strong, elastic, external.

A family of several genera of mollusks, living and fossil, which perforate rocks, clay or wood with vertical burrows, symmetrical and rarely in contact.

Genus PHOLAS, Linn.

Shell gaping,

cylindrical; dorsal margin protected by two accessory valves, one anterior, one posterior; beaks covered; combined siphons form a large cylindrical tube, ciliated at end. Foot short, large, The property of shining in the dark is common to the whole genus.

These burrowing shell-fish have a brief infancy of freedom, then settle down for life in a cell dug by themselves in rock or clay, or wood. Granite is not too hard for some species. The burrow is made by constantly turning the shell about in its close quarters, so that the hard rasp-like surface grinds off the inner wall, the foot clasping the support by suction. At first, the en trance is made by rubbing the rock surface with sand particles grasped by the foot. There is no moving out for the piddock. He is safe and comfortable; food and oxygen are within reach of his long siphons; his chief enemies are starfish and crustaceans which tear or nip off the ends of his siphons before they can be withdrawn.

315 The Piddocks Piddocks are eaten, pickled in vinegar,on the Norniandycoast ; they are also cooked with fine herbs and bread crumbs. They are collected for food and for bait near Dieppe by women and children who use a special iron pick.

The Angel's Wings (P. costata, Linn.) are found in colonies, ten inches to a foot deep in sandy mud in Florida; also in wood and rocks. They follow the coast northward • to Cape Cod, but are rare above Cape Hatteras. The white valves conform strikingly in outline, colour and sculpture to the conventional representation of angels' wings. They meet only at a point near the tips. They are seven to eight inches long. The spoon-shaped processes inside the umbones serve as attachments for the visceral and pedal muscles.

This mollusk is a staple article of food in the markets of Havana. Its other name is the Ribbed Pholas.

The Truncated Piddock

(P. truncata, Say) is short, pointed sharply at the posterior end of the shell, squared abruptly at the other. It is less than half the length of P. costata. The shell is rasp-like only at the pointed end. It burrows in mud or peat banks or in harder substances. Common on the whole east coast, companion of P. costata to its southernmost station; it is found

also on the west coast of South America.

The California Piddock (P. Californica, Conr., P. Pacifica, Strns.) has three types of sculpturing on the three triangular areas of its surface; sufficient roughness to burrow in mud and stiff clay. The valves are white, thin and delicate, but partially covered with a horny epidermis. This piddock is cylindrical, and swollen to considerable width, especially in front. The pos terior end is narrower and truncated abruptly. Length, 2i to 5 inches.

Habitat.— California.

Genus ZIRPH./EA, Leach Shell oval, without accessory valves, beaks protected by a membrane; epidermis thin; anterior end gapes widely.

The Rough Piddock (Z. crispata, Linn.) has an oval outline, ending in a point at the anterior end. A furrow from the beak to the margin divides each valve into a smooth posterior and a ribbed and toothed anterior half. With this rasp-like surface the 316 The Piddocks mollusk grinds or scrapes its way into the stiffest clay, its fa vourite burrow. It is two to four inches long. It is found in California, New England and Europe.

Genus PHOLADIDEA, Turt.

Shell globose-oblong, with cross furrow, anterior gape large, closed, when mollusk is full grown, with callous plate; accessory plates minute. Animal with fringed disk at tip of siphons, and horny cup at their base.

The Paper Piddock (P. papyracea, Sol.) of Europe is the type. Thin as paper, yet it burrows into sandstone, hard clay, peat and buried wood along the Irish and Channel coasts. The basal cup seems to be a shield for the retracted siphon.

P. penita,

Conr., occurs in soft rocks on the California and Oregon coasts. Its back is broad, the umbones covered with a round shield, in front of which lie the dorsal plates side by side. Two fin-like appendages proceed from and protect the base of the siphons. It is described as excellent eating. Length, 2 inches.

Some pholads burrow into oyster shells, some in hard timber, one in floating cakes of wax on the Cuban coast; an Australian species is found in resin; a fossil species resembles Teredo in lining its burrow with shell. A fresh water species works in wood sev eral miles above the mouth of a Bornean river.

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Genus XYLOPHAGA, Turt.

Shell globular, with cross furrows; gapes in front; is closed behind; burrow oval, with shelly lining; animal withdraws within shell, except siphons; foot thick, extensible.

The Wood Piddock (X. dorsalis, Turt.) of Europe and America, is a little nut-like mollusk, boring wood to the depth of an inch, and apparently eating the sawdust it makes. It always works across the grain. The boring instrument is the long elastic foot. The shell is covered with epidermis. Length, I inch.

This wood borer is in a sense a connecting link between Pholas and Teredo, at least as far as choice of material for bur rowing goes.

Habitat.— North Atlantic Ocean.

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