The exoskeletons of snails and clams, or their shells in common parlance, differ from the endoskeletons of turtles in several ways. Seashells are the exoskeletons of mollusks such as snails, clams, oysters and many others. Such shells have three distinct layers and are composed mostly of calcium carbonate with only a small quantity of protein--no more than 2 percent. These shells, unlike typical animal structures, are not made up of cells. Mantle tissue that is located under and in contact with the shell secretes proteins and mineral extracellularly to form the shell.
Think of laying down steel protein and pouring concrete mineral over it. Thus, seashells grow from the bottom up, or by adding material at the margins.
Since their exoskeleton is not shed, molluscan shells must enlarge to accommodate body growth. This pattern of growth results in three distinct shell layers: an outer proteinaceous periosteum uncalcified , a prismatic layer calcified and an inner pearly layer of nacre calcified. In comparison, turtle shells are part of the vertebrate animal's so-called endoskeleton, or skeleton from within the body. 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. Unlike bivalves, whose soft inner body is completely surrounded by a protective shell, gastropods can extend and withdraw their soft bodies through the aperture of the shell. There is also a third less well-known group of mollusks called the cephalopods.
Cephalopods include species such as squid, octopuses, and cuttlefish. Cephalopods are essentially mollusks that have evolved over time resulting in the internalization or complete loss of their outer shells. Tune in next week for more information on some of the most common mollusks found along the shores of North Carolina. As a result, the anus of the adult animal is located over the head. Torsion is an independent process from coiling of the shell.
Figure 6. Members of the genus Conus produce neurotoxins that may one day have medical uses. Marine snails of the genus Conus Figure 6 attack prey with a venomous sting.
The toxin released, known as conotoxin, is a peptide with internal disulfide linkages. Conotoxins can bring about paralysis in humans, indicating that this toxin attacks neurological targets. Some conotoxins have been shown to block neuronal ion channels. These findings have led researchers to study conotoxins for possible medical applications.
Conotoxins are an exciting area of potential pharmacological development, since these peptides may be possibly modified and used in specific medical conditions to inhibit the activity of specific neurons. For example, these toxins may be used to induce paralysis in muscles in specific health applications, similar to the use of botulinum toxin. Since the entire spectrum of conotoxins, as well as their mechanisms of action, are not completely known, the study of their potential applications is still in its infancy.
Most research to date has focused on their use to treat neurological diseases. They have also shown some efficacy in relieving chronic pain, and the pain associated with conditions like sciatica and shingles. The study and use of biotoxins—toxins derived from living organisms—are an excellent example of the application of biological science to modern medicine. Cephalopods are a class of shell-bearing animals as well as mollusks with a reduced shell.
They display vivid coloration, typically seen in squids and octopi, which is used for camouflage. All animals in this class are carnivorous predators and have beak-like jaws at the anterior end. All cephalopods show the presence of a very well-developed nervous system along with eyes, as well as a closed circulatory system. The foot is lobed and developed into tentacles, and a funnel, which is used as their mode of locomotion.
Suckers are present on the tentacles in octopi and squid. Ctenidia are enclosed in a large mantle cavity and are serviced by large blood vessels, each with its own heart associated with it; the mantle has siphonophores that facilitate exchange of water. Locomotion in cephalopods is facilitated by ejecting a stream of water for propulsion.
A pair of nephridia is present within the mantle cavity. Sexual dimorphism is seen in this class of animals. Members of a species mate, and the female then lays the eggs in a secluded and protected niche. Females of some species care for the eggs for an extended period of time and may end up dying during that time period. Cephalopods such as squids and octopi also produce sepia or a dark ink, which is squirted upon a predator to assist in a quick getaway.
Reproduction in cephalopods is different from other mollusks in that the egg hatches to produce a juvenile adult without undergoing the trochophore and veliger larval stages. In the shell-bearing Nautilus spp. These chambers are filled with gas or water to regulate buoyancy. The shell structure in squids and cuttlefish is reduced and is present internally in the form of a squid pen and cuttlefish bone, respectively.
Examples are shown in Figure 7. Figure 7. The a nautilus, b giant cuttlefish, c reef squid, and d blue-ring octopus are all members of the class Cephalopoda. Baecker; credit b: modification of work by Adrian Mohedano; credit c: modification of work by Silke Baron; credit d: modification of work by Angell Williams.
Figure 8. Scaphopods are usually buried in sand with the anterior opening exposed to water. These animals bear a single conical shell, which has both ends open. The head is rudimentary and protrudes out of the posterior end of the shell. These animals do not possess eyes, but they have a radula, as well as a foot modified into tentacles with a bulbous end, known as captaculae. Captaculae serve to catch and manipulate prey. Ctenidia are absent in these animals.
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