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Mollusca,
Intro
After the Arthropods the Molluscs are the most successful of the animal phyla in terms of numbers of species. There are about 110,000 species known to science most of which are marine. They occupy a vast range of habitats however both aquatic and terrestrial, from the arctic seas to small tropical streams and from valleys to mountainsides 7,000 metres high, there are a few adapted to live in deserts and some are parasitic. They also exhibit an enormous range in size, from species which are almost microscopic to the largest of all invertebrates the giant squid which can weighs 270 kg and measures up to 12 metres long in the body, with tentacles as much as another 50 metres in length. Many species are common and many more a beautiful. Most species secrete a shell of some sort, these shells are long lasting and have been collected by human beings for thousands of years, some of these shells, and the pearls which come from oysters, which are also molluscs may be among the earliest forms of money.
Nervous System
Mollusks have a relatively complex nervous system, which varies from species to species reaching the height of complexity at the octopus. The octopus is thought to be among the most intelligent of all invertebrates, with a mental capacity likened to that of a domestic cat. Sensory ability in some mollusks (notably the cephalopods) is considerable, with a variety of organ systems, as well as large, complex eyes. The eyes of the giant squid are the largest in the animal kingdom, approaching the size of dinner plates. It has recently been demonstrated that squid can successfully locate and capture transparent prey in the water by means of a specialized polarization vision.
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The gastropods
Includes the snails, whelks, periwinkles, abalone, and slugs, are the largest group of mollusks (at least 37,500 living species). They have either a single shell or no shell. Gastropods are common in both salt and fresh water and on land. Class Gastropoda includes herbivores, omnivores, a wide variety of specialized carnivores, scavengers, and even some parasites.
Includes the snails, whelks, periwinkles, abalone, and slugs, are the largest group of mollusks (at least 37,500 living species). They have either a single shell or no shell. Gastropods are common in both salt and fresh water and on land. Class Gastropoda includes herbivores, omnivores, a wide variety of specialized carnivores, scavengers, and even some parasites.
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Class Pelecypoda (Bivalvia)
This class contains the mollusks known as bivalves, including the mussels, oysters, scallops, and clams. All have shells composed of two pieces known as valves. In most, the valves are of similar size, but in some sedentary species, such as the oysters, the upper valve, which covers the left side of the body, is larger than the lower valve, which covers the right side and is attached to the substratum. Two large muscles, called adductors, hold the valves together at the top of the body. Pelecypod shells vary greatly in size, color, and ornamentation.
This class contains the mollusks known as bivalves, including the mussels, oysters, scallops, and clams. All have shells composed of two pieces known as valves. In most, the valves are of similar size, but in some sedentary species, such as the oysters, the upper valve, which covers the left side of the body, is larger than the lower valve, which covers the right side and is attached to the substratum. Two large muscles, called adductors, hold the valves together at the top of the body. Pelecypod shells vary greatly in size, color, and ornamentation.
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Scaphopods
This small class of marine mollusks includes 200 species of burrowing animals commonly known as the tusk, or tooth, shells. The shell is long, cylindrical and tooth- or tusk-shaped, and open at both ends. The foot and the small head project from the larger end. Threadlike tentacles hang from the head and are used for gathering the microscopic organisms on which tusk shells feed. Most scaphopods are tiny, usually only several inches (about 6 cm) long. They are found in both shallow and deep waters; they burrow into the bottom, with only the upper opening protruding.
This small class of marine mollusks includes 200 species of burrowing animals commonly known as the tusk, or tooth, shells. The shell is long, cylindrical and tooth- or tusk-shaped, and open at both ends. The foot and the small head project from the larger end. Threadlike tentacles hang from the head and are used for gathering the microscopic organisms on which tusk shells feed. Most scaphopods are tiny, usually only several inches (about 6 cm) long. They are found in both shallow and deep waters; they burrow into the bottom, with only the upper opening protruding.