The Encyclopedia Americana (1920)/Bivalves
BIVALVES, those mollusks of the class Pelecypoda, whose coverings consist of two concave shell plates or valves.
Bivalves (for example, the clam), are entirely protected by the valves, which are connected by a hinge, consisting of a large tooth or teeth (usually three), and a ligament. In the clam both valves are alike, in the scallop the hinge margin is eared, and the shell is marked with radiating ridges, while in most bivalves there are simple lines of growth. On the interior, which is usually lined with mother-of-pearl, are either one (in oysters and scallops) or two (clams, etc.) roundish muscular impressions made by the single or the two adductor muscles by which the valves are closed. The shell is often covered by an epidermis. The hinge is situated directly over the heart, and is therefore dorsal or “hæmal.” The shell is secreted by the thickened edge of the mantle or body-walls. There is in bivalves in distinction from snails (Gastropoda) no head, and the mouth is not armed with teeth or a lingual ribbon, present in snails. The mouth is small with soft lips, and in each side is a pair of labial palpi. The short œsophagus opens into a small stomach which receives the contents of the liver. The long intestine is coiled in the visceral mass, the solid disc-like portion of the body in the clam and oyster; the intestine also usually passes through the ventricle of the heart, and then ends opposite the upper division of the siphon. This heart is three-chambered, consisting of a ventricle and two auricles. The siphon forms the so-called head of the clam, though it is situated at the posterior end of the body; it forms a double tube, ending in an excurrent and incurrent orifice surrounded by a circle of tentacles which are sensitive to the touch. The siphon is very long in the clam (Mya) and other bivalves which burrow in the sand or mud and live in deep holes. Locomotion is effected by the so-called “foot,” which is a wedge-shaped or hatchet-shaped fleshy tongue-like mass situated at the front end under the mouth. Its hatchet-shape gives the name Pelecypoda to the class. This foot is enormous in the razor-fish, which burrows with extreme rapidity in the sand. In fixed bivalves, such as the oyster and mussel, the foot and siphon are reduced by atrophy or are entirely wanting. There being no head, there are usually no eyes, except in the scallops, where they are numerous, large and situated on the thickened edge of the mantle. Bivalves breathe by one pair, more usually two pairs, of leaf-like gills, situated on each side of the visceral mass. The individuals are usually bisexual, each being male or female. The nervous system consists of three pairs of ganglia, connected by a nerve-thread. The supraœsophagal ganglion is the so-called “brain,” being situated over the mouth; the pedal ganglion is in the centre of the foot, while the visceral ganglion is near the middle of the body. Most bivalves possess an organ of the chemical sense and an organ of hearing and of equilibration, a very minute otocyst situated in the centre of the foot, and connected by a nerve with the pedal ganglion. The ovaries are yellowish, voluminous glands forming the larger part of the visceral mass. These mollusks are very prolific, the oyster laying about 2,000,000 eggs.
In the oyster (Ostrea) or in Anomia the shell is inequilateral, one valve, usually the left and lower one, being fixed to some object, and the intestine does not pass through the ventricle; in Arca the ventricle is double. In Lucina and Corbis there is but one gill on each side, and in Pecten, Spondylus and Trigonia the gills are reduced to comb-like processes. Contrary to the habits of most bivalves, the scallop can swim and even skip out of the water by violently opening and shutting its shell. Trigonia is also capable of leaping a short distance, while Lima is an active swimmer. The foot varies much in form; in the mussel, Pinna Cyclocardia, and the pearl-oyster it is finger-shaped and grooved, with a gland for secreting a bundle of threads, the byssus, by means of which it is anchored to the bottom. The foot in the quohog Venus, is large, these mollusks being very active in their movements. In Glycimeris the fringe is toothless, much as in the oyster. In Mactra the middle tooth of the hinge is large, the corresponding cavity large and triangular. In Saxicava and Panopæa, the pallial line is represented by a row of dots. In Macoma the siphons are very long.
Lithodomus, the date-shell, one of the mussels, bores into corals, oyster shells, etc.; the common Saxicava excavates holes in mud and soft limestone, as does Gastrochæna, Pholas and Petricola. Certain boring lamellibranchs, such as Pholas, are luminous.
A very aberrant form of bivalve mollusk is Clavagella, in which the shell is oblong, with flat valves, the left cemented,to the sides of a deep burrow. The tube is cylindrical, fringed above, and ending below in a disc, with a minute central fissure, and bordered with branching tubules. In Aspergillum, the watering-pot shell, the small bivalve shell is cemented to the lower end of a long shelly tube, closed below by a perforated disc like the nose of a watering-pot.
Bivalves, in growing, usually pass through a pre-swimming larval stage called a “trochosphere,” resembling a top, and moved by a circlet or zone of ciliæ. After a while two flaps (vela) arise on each side of the mouth, forming veliger stage; meanwhile the shells arise, and as they become larger and heavier, the young bivalve sinks to the bottom, and begins to use its “foot” for burrowing. Some bivalves, such as the fresh-water mussel Unio, go through a parasitic stage in their development.
Some bivalves arrive at maturity in a single year. The fresh-water mussels live from 10 to 12 years, while the giant clam (Tridacna gigas) probably lives from 60 years to a century.
The bivalves began to appear in the Cambrian Period; they became more frequent in the Ordovician and Silurian, but they did not abound until toward the Mesozoic Age, since the seas during the Palæozoic Age were crowded with brachipods. Oysters date from the beginning of the Mesozoic. The genus Mucula and its allies are very primitive forms, and nearly allied to the earliest known bivalves. Of about 15,000 known species of bivalves, two-thirds (10,000) are fossil.
The class Pelecypoda (or Lamellibranchiata) is divided by the gill characters (consult Parker and Haswell's ‘Zoology’) into five orders, namely: (1) Protobranchia, (2) Filibranchia, (3) Pseudo-lamellibranchia, (4) Eulamellibranchia, (5) Septibranchia; and by Dall, from the hinge-characters, into three ordinal groups: Prionodesmacea, Anomalodesmacea and Teleodesmacea. In Neumayr's group Palæoconcha, now forming a part of the Prionodesmacea, are included certain primitive types which appear to have given origin to certain more modern groups. For further information and the literature of the subject see Mollusca.