PELAYO, ((lied 7371. The first Christian King in Asturias. after the conquest of Spain by the Arabs. Practically nothing is known about his life. Be was. strictly speaking, nothing hut a robber chieftain; still by his de fense of Coy:1(1611ga. a mountain fastness. in 718 against the Moslems. he kindled the national spirit and became the founder of the Spanish monarchy. The legend states that at Covadonga he and thirty followers destroyed an Arab army four hundred thousand strong. Consult Burke, A History of Spain, vol. i. (London, 1S95). PEL'ECYP'ODA (Neo-Lat. nom. pl.. from pelays, axe :our, pons, foot ), or LAMELLIBRANCIIIATA, or BIVALVES. A class of bilaterally symmetrical acephalons mollusks, all of milk]: have bivalve shells, and gills in the form of vascular plates of membrane attached to the inner surface of the mantle. The body is very strongly compressed, the dorso-ventral diameter being nn ell greater than the lateral. The adduc tor muscle, which closes the shell, is single in some, double in the greater number. shell is naturally more or less open. owing to the peculiar elastic hinge which tends to keep it open. But the adductor muscles continually tend to close it. Consequently when a live molliksk is irritated the shell closes tight, but when the ani mal is dead the muscles relax and the shell gapes open. Important differences exit in the powers of locomotion. Oysters are fixed to one spot by one of the valves of the shell: but most of the peleeypods have the power of moving by swim ming, leaping, or burrowing in sand, some times in more than one of these ways, being pro vided for this purpose with a fleshy .muscular organ called the fool. l?ome, as mussels, nix themselves when young by a cable of ehitinous threads. tlw byssu.v. The mouth is jawless and toothless, but on each side are two flaps, the labial palpi. All seem to depend for their food on the currents of water continually brought by ciliary action into the mouth. The edges of the mantles are sometimes entirely separate from each other, but are often more or less fused. In the latter case two openings are left in the pos terior edge. through the lower of which water is brought into the mantle cavity, where, having been deprived of its oxygen by flowing over the gills, and of its nutritive material in the stomach. it passes (tilt through the upper opening, carrying away the waste. and, at the proper season. the
reproductive elements. Various modifications of this inhalent and exhalent apparatus exist ; and in many forms the orifices become elongated into two tubes called 'siphons.' which, as in the clam. may often lie extended several inches beyond the shell. Bivalves, with such siphons, live buried in mud or sand with only their siphons reaehing the surface. The edges of the mantle often hear tentacles, papilla-, glands, and more or less com plicated eyes. The two halves of the shell are usually of about the same size, but in some forms one valve is very much larger than the other. Generally the shell covers the whole animal, and when the shell is closed the animal is completely shut in. but in the ship-worm (q.v.1 the two halves of the shell are very small, and inclose only an insignificant part of the greatly elon gated body.
The ear or otocyst is situated in the 'foot.' Tint heart is three-chambered, there being two auricles and one ventricle. The sexes exist in different in dividuals. The number of t„.- is usually enor mous, the oyster prodneing about two millions ea eh seastol. The young peleeypod passes through a free-swimming veliger stage. and some bivalves get their growth in a single year. The fresh water mussels live from ten to twelve years, while the giant clam (Tridaena 1 is supposed to attain an age of from sixty to one hundred years. Alany bivalves are of great nse to man as food. notably oysters. clams, and scallops. Others, as the ship-worm (q.v.), are injurious. The mol lusks of this class vary in size from a small frac tion of an inch up to the giant clam (qs.) of the holies, which readies three feet in kngth. In many species the shells are very beautiful, the interior being lined with nacre, or mother-of-pearl. Pearls (q.v.) are formed in especially in the pearl oyster and fresh water mussels. Bivalves inhabit all parts of the morld in both salt and fresh water, and form a highly important part of the of many val u able fishes, especially those of the cod family, as well as of other marine animals. about 14,000 species are known, 8000 or 9000 of which are fossils: and they are generally grouped in five orders. distinguished by the character of the gills.
See.Slouusea; MOLLUSK and authorities cited thereunder; Colored Plate of ('LAMS AND EDIBLE .NICsSELS and Plate of ABALONE, ETC.