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A sea shell is the most universally identifiable part of a creature known as a mollusk. Mollusks (Sea Shells) are invertebrate animals (think of a snail) with an un-segmented, basically symmetrical body, generally consisting of head, foot, visceral hump and mantle.

Mollusks (Sea Shells) are descendants from primitive wormlike creatures inched around in the ooze of primeval seas millions of years ago. As dissolving land masses began to feed salts and chemicals into oceans, the first Mollusks (Sea Shells) digested them and eventually used them to build durable shelters.

Mollusk (Sea Shell)

Mollusk (Sea Shell), common name for members of a phylum of soft-bodied animals are usually with a hard external shell. Familiar Mollusks (Sea Shells) include the clam, oyster, snail, slug, octopus, and squid. The mollusk phylum is the second largest in the animal kingdom, after the arthropods. Earlier estimates of the number of mollusk species sometimes exceeded 100,000, but more recently this figure has been reduced to less than 50,000; the new estimates are incorporated here.

Mollusks (Sea Shells) are highly successful in terms of ecology and adaptation, with representatives in virtually all habitats, but they are most diverse in the sea. Among them are some advanced animals, such as the octopus and squid. Giant squid are also the largest invertebrates, weighing up to 900 kg (1980 lb). Most Mollusks (Sea Shells), however, are about 1 to 20 cm (about 0.4 to 8 in) long, and some are scarcely visible.

The first Mollusk (Sea Shell) fossils appear in early Cambrian rocks, about 600 million years old. Seven of the phylum's classes have living representatives: the wormlike, shell-less aplacophorans, with 250 species; the chitons, with 600 species; the monoplacophorans, with 10 species; the bivalves, such as clams, with 7500 species; the scaphopods, or tusk shells, with 350 species; the gastropods, such as snails and slugs, with 37,500 species; and the cephalopods, such as octopuses and squid, with 600 species. Several fossil classes and thousands of fossil species are also known.also gastropod, soft-bodied type of mollusk that is protected by a very hard shell known as Queen Conch shell.

Seashell - Mollusk - General Characteristics
Although few features are common to all Mollusks (Sea Shells), the animals are not readily mistaken for anything else, and all may be treated as variants on a common theme (not to be confused with a common ancestor). A theoretical, idealized Mollusk (Sea Shell) would crawl on a single flat, muscular foot, and the body would have at least a suggestion of a head at one end and an anus at the other. Above the body would be an external shell mounted on a visceral hump containing internal organs.

This shell, secreted by a sheet of tissue called the mantle, is complicated in Mollusks (Sea Shells), being made up of calcium carbonate and other minerals in an organic matrix produced in layers by the mantle at the edge of the shell and under it. An outer layer without minerals, called the periostracum, generally covers it. The shell may be multiple, as in chitons, or paired, as in bivalves. In various Mollusks (Sea Shells) the shell is reduced in size and is sometimes lost completely; in aplacophorans there is no direct evidence that a shell ever existed.

At the posterior end of the idealized Mollusk (Sea Shell) would be a groove or depression called the mantle cavity, with gills to each side of the anus, and openings to the kidneys and reproductive structures. A single pair of gills is common, but many gastropods have only one gill. The cephalopod nautilus has two pairs, and monoplacophorans and chitons have several to many pairs.

Generally the molluscan gut is equipped with jaws and a tongue like structure, called a radula, with teeth on it. Also present are a stomach and a pair of digestive glands. The nervous system consists of a ring of nerves around the anterior part of the gut, with one pair of nerve trunks to the foot and another to the viscera. Ganglia around the gut usually are developed into a brain with various sense organs; the nervous system of cephalopods is as complex and as highly organized as that of fishes. The heart is located at the posterior end of the body; it sends blood into an open system that forms the main body cavity. Associated with the heart is a complex of organs that includes the kidneys and gonads and sometimes other reproductive structures.

Seashell - Mollusk - Scientific classification
Mollusks (Sea Shells) make up the phylum Mollusca. In the class Aplacophora, the body is wormlike. No shell exists, only a tough mantle, and the foot has virtually been lost. The three orders of the class Polyplacophora (chitons) have a series of eight shell plates (valves) in a row and are well adapted to clinging on rocks. The mainly fossil Monoplacophora is now known to have one living genus, Neopilina, discovered in deep water in 1952. The animal has a single flat shell and multiple gills. The class Bivalvia has a shell divided into two valves, and they feed with their gills. As a consequence the head is poorly developed. Members of the class Scaphopoda (tusk shells) have a long, tapered, slightly curved shell and live on sandy bottoms. Members of the class Gastropoda (snails and slugs) are asymmetrical and have only one shell or, as in slugs, are shell-less. The three subclasses of the Gastropoda are the Prosobranchia (mostly marine snails, with three orders), Opisthobranchia (sea slugs and their allies, with eight orders), and Pulmonata (lunged mollusks, largely freshwater and terrestrial, with two orders). The class Cephalopoda is modified by reduction of the foot and shell and the development of arms around the mouth. The two subclasses are Nautiloidea (Nautilus, with four gills and other archaic traits such as an external shell) and Coleoidea (octopuses, squid, and cuttlefish, with two gills and other advanced traits).

Classification of Mollusks
Mollusks are normally classified into nine or ten classes in which one of them is known only from fossils, they are :

  • Aplacophora
    There are around 250 species in this class and are deep-sea worm like creatures.
  • Caudofoveata
    This class consists of 70 known species and this includes deep-sea wormlike creatures.
  • Polyplacophora
    It consists of 600 species, mainly animals that live on rocks on marine shorelines.
  • Monoplacophora
    These are deep-sea limpet-like creatures that consist of 11 living species.
  • Bivalvia (also known as Pelecypoda)
    There are 8,000 species in this class, which has shell with two valves and a muscular foot, for example clams, oysters, scallops, mussels, etc.
  • Scaphopoda
    This consists of 350 marine species.
  • Gastropoda
    The estimated species in this class is around (75,000 to 150,000). Some of the examples are nudibranchs, snails and slugs, limpets, conches, etc.
  • Cephalopoda
    This class consists of 786 marine species, which has a large well-developed head and foot with many tentacles.
  • Rostroconchia
    Only the fossil remains of this class with more than 1,000 species.
  • Helcionelloida
    This class can be also seen in fossils only.

Seashell - Mollusk - Behavior
Although vision is poor in most Mollusks (Sea Shells), cephalopods such as squid have eyes with lenses, retinas, and other features remarkably like those of vertebrates. Some gastropods have a well-developed sense of smell and can locate food in the water at a considerable distance. Predators may similarly be detected by the chemical senses and are sometimes evaded by leaping or swimming. Some Mollusks (Sea Shells) exhibit complicated courtship behavior. Advanced cephalopods possess considerable ability to learn from experience.

Seashell - Mollusk - Reproduction
The basic Mollusk (Sea Shell) pattern is to have separate sexes, with sperm and eggs spawned into the water, where fertilization and early development occur. In most Mollusks (Sea Shells) a larval stage follows, in which the larvae swim about for a while and then settle on the bottom and mature; this stage is often modified or absent, however. Fertilization may also be internal, with glands secreting protective coverings around the eggs. Slow-moving creatures such as Snail often evolve into hermaphrodites (both male and female), because this doubles the number of appropriate mates. Sometimes the mother protects the developing eggs. Some oysters are remarkable in caring for the young inside the mantle cavity and switching back and forth from being males to being females.

Seashell - Mollusk - Ecology and Importance

Mollusks (Sea Shells) are abundant and hence important in food chains in many habitats. A large number are herbivores or grazers, especially the chitons and many gastropods. Tusk shells and some other Mollusks like Quahog (Sea Shells) feed on matter deposited on the bottom, whereas most bivalves filter suspended materials from the water. Many gastropods are carnivorous, most of them preying on slow-moving or attached animals. Cephalopods are active predators on larger animals such as crabs. Numerous Mollusks (Sea Shells) are important food sources for humans, but some gastropods damage crops, and others harbor disease-causing parasites.