Mollusks
The mollusks constitute one of the largest phyla of animals, both in numbers of living species (at least 47,000, and perhaps many more) and in numbers of individuals.
A significant characteristic of mollusks is their possession of a coelom, a fluid-filled cavity that develops within the mesoderm. The coelom not only functions as a hydrostatic skeleton but also provides space within which the internal organs can be suspended by the mesenteries. All mollusks have a soft body, which is generally protected by a hard, calcium containing shell. In some forms however, the shell has been lost in the course of evolution, as in slugs and octopuses, or has been greatly reduced in size and internalized, as in squids.
A significant characteristic of mollusks is their possession of a coelom, a fluid-filled cavity that develops within the mesoderm. The coelom not only functions as a hydrostatic skeleton but also provides space within which the internal organs can be suspended by the mesenteries. All mollusks have a soft body, which is generally protected by a hard, calcium containing shell. In some forms however, the shell has been lost in the course of evolution, as in slugs and octopuses, or has been greatly reduced in size and internalized, as in squids.
Structurally, mollusks are quite distinct from all other animals. However, all modern mollusks have the same fundamental body plan. There are three distinct body zones. A head-foot, which contains both the sensory and motor organs; a visceral mass, which contains the well-developed organs of digestion, excretion, and reproduction; and a mantle, a specialized tissue formed from folds of the dorsal body wall, that hangs over and enfolds the visceral mass and that secretes the shell. The mantle cavity, a space between the mantle and the visceral mass, houses the gills, the digestive, excretory, and reproductive systems, which discharge into it. Mollusks are also characterized by a toothed tongue, the radula, composed primarily of chitin. The radula scrapes off algae and other food materials and also to move them backward to the digestive tract. In some species, it is also used in combat. The digestive
tract of a mollusk is complete and ciliated, with a mouth, anus and complex stomach.
The stomach varies according to the mollusk's diet.
Food is taken up by cells lining the digestive glands arising from the
stomach, and then is passed into the blood. Undigested materials
are compressed and packaged, then discharged through the anus into the
mantle cavity and are carried away from the animals in the water currents.
This packaging of wastes in solid form prevents fouling of the water passing
over the gills.
Excretory functions are carried out by a pair of nephridia, tubular structures
that collect fluids from the coelom and exchange salts and other substances
with body tissues as the fluid passes along the tubules for excretion.
Octopus vulgaris
One notable species is the Octopus vulgaris in the Cephalopoda class. This is the common octopus and it is found on the coasts of the Mediterranean Sea, The Azores, Canaries and also as far south as Senegal. Octopuses are noted for their eight arms, which bear suction cups. The arms of octopuses are often distinguished from the pair of feeding tentacles found in squid and cuttlefish. Their limbs are muscular hydrostats. Unlike most other cephalopods, the majority of octopuses, those in the suborder Incirrina, have almost entirely soft bodies with no internal skeleton whatsoever. They have neither a protective outer shell, nor any vestige of an internal shell or bones, like a squid. A beak, similar to a parrot's is the only hard part of their body. This enables them to squeeze through very narrow slits between underwater rocks, which is very helpful when they are fleeing from predatory fish. Octopuses are the most intelligent invertebrates as they are the only ones ever to use tools. Octopuses also have three hearts. Two secondary hearts pump blood through each of the two gills, while the third pumps blood through the body. Octopus blood contains the copper-rich protein hemocyanin for transporting oxygen which is more effective in the colder temperatures of the sea. They also tend to not live very long in nature as octopuses die after reproduction, though this is a behavioral issue and they can live longer even after reproduction in controlled environments.
Loligo vulgaris
Loligo vulgaris better known as the common squid is another branch of the Mollusca phylum in the Cephalopoda as well but in the Teuthida class. Their systems work largely the same as the octopods. They have three hearts which are surrounded by renal sacs, the main excretory system. The kidneys stretch from the hearts to the liver. Their mouth is equipped with a sharp horny beak and is used to kill and tear prey into manageable pieces, but does not contain minerals, unlike the teeth and jaws of many other organisms and also contains a radula. Squid have two eyes on either side of their head, and also appear to have limited hearing.
Mytilus edulis
Mytilus edulis is a Mollusc of the class bivalvia. The bivalves are a highly successful class of invertebrates and are found throughout the world. The majority live buried in sediment on the seabed or in fresh waters. A sandy beach may superficially appear void of life but there is often a large number of shells and other invertebrates living beneath the surface of the sand. Most bivalves are filter feeders, using their gills to capture particulate food such as phytoplankton from the water. Some, however, feed by scraping detritus from the seabed, and this may be the original mode of feeding for the whole group before the gills became adapted for filter feeding. These primitive bivalves hold onto the substratum with a pair of tentacles at the edge of the mouth, each of which has a single palp, or flap. The tentacles are covered in mucus, which traps the food, and cilia then transports the particles back to the palps. Like most other molluscs, the excretory organs of bivalves are nephridia. There are a pair of these, each consisting of a long, glandular tube, which opens into the body cavity just beneath the heart, and a bladder. Metabolic waste is voided from the bladders through a pair of openings near the front of the upper part of the mantle cavity, from where it is washed away in the stream of exhalant water.
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