Surface scutes are epidermal structures, like our fingernails, made of the tough protein keratin. Underneath these scutes are the dermal tissue and calcified shell, or carapace, which is actually formed by fusion of vertebrae and ribs during development. By weight, such bone consists of about 33 percent protein and 66 percent hydroxyapatite, a mineral composed largely of calcium phosphate with only some calcium carbonate.
Why exoskeletons of snails and clams are calcium carbonate while the endoskeleton of vertebrates like turtles are primarily calcium phosphate is not known. Both shells are strong, allow for protection, attachment of muscles and resist dissolution in water. Evolution works in mysterious ways. Unlike seashells, turtle shells have living cells, blood vessels and nerves, including a large number of cells on the calcareous shell surface and scattered throughout its interior.
Bone cells that cover the surface and are dispersed throughout the shell secrete protein and mineral and more or less entomb themselves. The bone can grow and reshape continuously. And when a bone breaks, cells are activated to repair the damage. In fact, turtle shell grows from within just like leg bones in humans.
Nutrients such as protein and calcium are supplied by blood vessels within the bone, not from outside of the bone tissue. Damaged seashells, on the other hand, use secretions of proteins and calcium from the mantle cells underneath the shell for repair. They secrete calcium carbonate, which hardens on the outside of their bodies, creating a hard shell.
The shell stays attached to the mollusk but it is not actually part of its living body because it is made of minerals, not mollusk cells unlike most animal structures. The mollusk continues to take in salt and chemicals from the sea and secrete calcium carbonate, which makes its shell grow even bigger. When a mollusk dies it discards its shell, which eventually washes up on the shore.
This is how seashells end up on the beach. A seashell is made mostly of calcium, with no more than 2 percent of protein. It forms from the bottom up, creating three clear layers an uncalcified outer proteinaceous periosteum similar to human fingernails , a calcified prismatic layer and an inner pearly calcified layer of nacre. Seashells are self-repairing; they use the calcium carbonate secretions from their mantle tissue to fix any damage.
Crabs and other crustacea have an exo-skeleton, not a "shell" in the sense in which clams and snails do. Most species do not change considerably over millions of years because evolution is a branching process. In other words bacteria didn't become extinct once multi-cellular organisms arose.
Think of it this way, trees don't lose their roots as they grow up and out of the soil and start growing limbs and branches. Primates branched off into a number of larger brained species, including the extinct species, homo habilus, homo erectus, and homo neanderthalus. But even though larger brained primates like homo sapiens evolved, there are still monkeys and apes.
By the way, Answers in Genesis warns modern day creationists in their article on " Arguments Creationists Should Not Use ," not to bother using the argument, "If humans evolved from apes, why are there still apes?
Starfish reproduce sexually and a-sexually, and have an exo-skeleton too. Echinoderms are armored animals that have a hard exoskeleton made of interlocking calcium carbonate plates and spines. These slow-moving creatures have a water-vascular system a rudimentary circulatory system , water-filled channels that go through the body. The oldest-known fossil echinoderm is Arkarua, which lived in pre-Cambrian times, during the Vendian period, roughly million years ago, in what is now Australia.
Some echinoderms include sea stars also called starfish , brittle stars which includes basket stars , sea cucumbers, sea urchins, sand dollars, and crinoids also called feather stars, which have a soft body surrounded by upwards-facing arms.
Classification: Kingdom Animalia animals , Phylum Echinodermata. Echinoderm Starfish: First submission Starfish may reproduce either sexually or asexually. When asexual reproduction takes place, the animal breaks itself into two pieces. The way the shell forms helps explain where the color comes from. For example, seashells from warm waters tend to be more colorful than those from cold areas. This might have to do with their diet. Warm Caribbean waters have more colorful foods than the cold ocean near Maine.
Search for:. Videos Activities Podcasts. And why are they different colors? Can seashells live or die?
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