Old Earth Ministries Online Earth History Curriculum
Presented by Old Earth Ministries (We Believe in an Old Earth...and God!)
This curriculum is presented free of charge for use by homeschooling families.
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Chapter 5 - The Silurian Period
Lesson 29: Bivalves
are marine and freshwater
molluscs belonging to the class Bivalvia.
Other names for the class include Acephala, Bivalva, Pelecypoda, and
Lamellibranchia. The class contains 30,000 species, including
Chapter 5: The Silurian Period
The shell is composed of two valves which are hinged together. The shells of marine bivalves commonly wash up on beaches (often as separate valves) and the shells of freshwater species can sometimes be found along the flood plains of rivers, and other freshwater habitats.
Bivalves typically have two-part shells, two valves, that are joined by a ligament. The two valves usually articulate with one another using structures known as "teeth" which are situated along the hinge line. In many (but by no means all) bivalve shells, the two valves are symmetrical along the hinge line.
This exoskeleton serves not only for muscle attachment, but also for protection from predators and from mechanical damage. The shell has several layers, and is typically made of calcium carbonate precipitated out into an organic matrix. It is secreted by a part of the molluscan body known as the mantle.
Bivalve shells are collected by professional and amateur conchologists, and are sometimes harvested for commercial sale (the international shell trade), occasionally to the detriment of the local ecology.
Shell Anatomy, Structure and Composition
The mantle itself is attached to the shell by numerous small mantle retractor muscles, which are arranged in a narrow line along the length of the interior of the shell. The position of this line is often quite clearly visible on the inside of each valve of a bivalve shell, as a shiny line, the pallial line, which runs along a small distance in from the outer edge of each valve, usually joining the anterior adductor muscle scar to the posterior adductor muscle scar. The two adductor muscles are what allow the bivalve to close the shell tightly.
In some bivalves the mantle edges fuse to form siphons, which take in and expel water during suspension feeding. Species which live buried in sediment usually have long siphons, and when the bivalve needs to close its shell, these siphons retract into a pocket-like space in the mantle. This feature of the internal anatomy of a bivalve is clearly indicated on the interior of the shell surface as a pallial sinus, an indentation in the pallial line.
The valves of the shell are made of either calcite (as with, e.g. oysters) or both calcite and aragonite, usually with the aragonite forming an inner layer, as is the case with the Pterioida which have this layer in the form of nacre or mother of pearl. The outermost layer of the shell is known as the periostracum, which is composed of a horny organic substance. This forms a yellowish or brownish "skin" on the outside of the shell. The periostracum may start to peel off of a shell when it is allowed to dry out for long periods.
The shell is added to, and increases in size, in two ways - by increments added to the open edge of the shell, and by a gradual thickening throughout the animal's life.
The radical structure of the bivalves reflects their behaviour in several ways. The most significant is the use of the closely-fitting valves as a defence against predation and, in intertidal species, against desiccation. The entire animal can be contained within the shell, which is held shut by the powerful adductor muscles. This defence is difficult to overcome except by specialist predators such as sea stars and oystercatchers.
Most bivalves are filter feeders although some have taken up scavenging and predation. Nephridia remove the waste material. Buried bivalves feed by extending a siphon to the surface (indicated by the presence of a pallial sinus, the size of which is proportional to the burrowing depth, and represented by their hinge teeth).
There are four feeding types, defined by their gill structure:
Razor shells can dig themselves into the sand with great speed to escape predation. Scallops, and file clams can swim to escape a predator, clapping their valves together to create a jet of water. Cockles can use their foot to leap from danger. However these methods can quickly exhaust the animal. In the razor shells the siphons can break off only to grow back later.
Comparison with Brachiopods
Bivalves are superficially similar to brachiopods, but the construction of the shell is completely different in the two groups. In brachiopods, the two valves are on the dorsal and ventral surfaces of the body, while in bivalves, they are on the left and right sides.
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