CATDOLL : CATDOLL: What are the shellfish?

The lifestyle of shellfish varies from species to species. Land-dwelling species belong to the class called gastropods, and all use their muscular feet to crawl on land.

Aquatic species have different lifestyles, such as floating, swimming, crawling, sessile, piercing and parasitic. Species that live a planktonic life all float in the water with the current. Generally, they are small, with thin shells or no shells. Some species have specialized feet into fins, such as many species in Pteropoda and Heteropoda; some species can secrete a buoyancy bladder to carry animals floating on the surface of the ocean, such as sea snails (Janthina). Species that live a swimming life can migrate long distances in the ocean, such as squid, calamari, and squid (Ommastrephes) in the cephalopoda. Their feet are specialized into arms and funnels, and there are fins on both sides of their bodies. They can swim quickly and steadily by spraying water from the funnel and swinging their fins. Although some bivalves such as scallops, Chlamys, Amussium, and Lima are not swimming species, they can swim in the sea in a butterfly-like manner by relying on the rapid opening and closing of their shells and the action of the mantle tentacles when necessary. Most aquatic shellfish live in the bottom of the water, crawling or crawling on the bottom of the water, or living in seclusion by digging holes in the bottom, or living attached to other foreign objects. For example, jade snails and mud snails (Bullacta) crawl on the muddy bottom, abalone, horseshoe snails (Trochus), and turbins (Turbo) crawl on rocks, and some nudibranchs such as Doris, Radix, and Planorbis living in freshwater crawl on aquatic plants. Their foot muscles are particularly developed, and their surface is wide and flat, which is suitable for crawling. Many benthic shellfish live buried, and most bivalves belong to this type. Their feet are well-developed and axe-shaped, suitable for digging mud and sand in the muddy beach and burying their entire body underground. For example, Venus, Tellina, Solen, Mya, etc. They rely on well-developed water inlets and outlets to communicate with the bottom surface for food and breathing. Some benthic shellfish live attached lives, such as mussels, scallops, Anomia, etc. Their feet can secrete foot threads to attach to rocks, coral reefs, other shells or objects. Oysters, Chama, Spondylus, etc. use a scallop shell to attach to external objects. These species generally do not move after attaching. Some benthic shellfish make holes in rocks, coral reefs, shells, bamboo and other external objects to live in, also known as perforated organisms, such as Lithophaga, some species in the Pholadidae, Saxicava, Teredo, Martesia, Xylophaga, etc., all rely on well-developed water pipes to communicate with the outside of the cave, absorb seawater for breathing and ingest microorganisms and organic debris in the water as food. There are also shellfish that live parasitic lives. External parasites such as the cylindrical snail (Stilifer) parasitize in the ambulatory groove of the arm of echinoderms; internal parasites such as the endovalva parasitize in the esophagus of anchor sea cucumbers.

Shellfish is a type of mollusk. It is an animal with three germ layers, bilateral symmetry, and true body cavity. The true body cavity of mollusk is formed by the schizocoelom, that is, the body cavity formed by the mesoderm. However, the true body cavity of mollusk is not well developed and only exists in the pericardial cavity and the gonadal cavity. Mollusks vary greatly in morphology, but in structure they can be divided into four parts: head, foot, visceral sac and mantle. The head is located at the front of the body, the foot is located behind the head and on the ventral side of the body, and is a muscular locomotion organ extending from the body wall. The visceral sac is located on the back of the body and is an internal organ surrounded by a soft body wall. The mantle is a membrane or a pair of membranes formed by the extension and drooping of the body wall on the back of the body. The cavity between the mantle and the visceral sac is the mantle cavity. Calcium carbonate is secreted from the mantle to the body surface to form one or two shells that surround the entire body. In a few species, the shell is surrounded by the body wall or the shell disappears completely. These basic structures vary and differ greatly in different classes. Molluscs have a complete digestive tract, respiratory and circulatory systems, and metanephridium, which is more advanced than protonephridium. Molluscs are of various types and widely distributed.

Chinese name

shellfish

boundary

Animalia

Door

Mollusca

Distribution area

Mainly distributed in the ocean

Common types

Oysters, mussels, clams, squid, snails

Appearance characteristics

The body of shellfish is soft, bilaterally symmetrical, and unsegmented. It consists of five parts: head, axe foot, visceral sac, mantle, and shell. The head has sensory organs such as mouth, eyes, and antennae. The axe foot is located on the ventral side of the body and is composed of strong muscles. It is an organ for crawling, digging mud or swimming. The visceral sac is located on the back of the body and includes internal organs such as the heart, kidneys, stomach, intestines, digestive glands, and gonads. The mantle covers the outside of the body and is composed of two layers of epidermis, connective tissue in between, and a small amount of muscle. The epidermal cells of the mantle secrete shells. The mantle and shells are both protective organs of shellfish.

The nervous system of shellfish is composed of four pairs of ganglia and the nerves that communicate with them: brain, foot, lateral, and visceral. The brain ganglion is located on the dorsal side of the esophagus and sends nerves to the head and front of the body; the foot ganglion is located in the front of the foot and sends nerves to the foot; the lateral ganglion is located in the front of the body and sends nerves to the mantle and gills; the visceral ganglion is located in the back of the body and sends nerves to the internal organs. The primitive species of shellfish have a simple nervous system without obvious ganglia. The more advanced species form ganglia, and the more advanced species have ganglia concentrated in the head to form a "brain". The sensory organs mainly include antennae, eyes, balance sacs, olfactory detectors, etc.

The digestive system includes the mouth, radula, esophagus, gastrointestinal tract, anus and accessory digestive glands. Among them, the radula is a relatively special organ of shellfish for licking and grinding food, and is one of the important bases for classification.

Shellfish breathe through gills and lungs. Aquatic species have gills, which are usually formed by the extension of the skin on the inner surface of the mantle, called the proper gills. Each gill has gill filaments on both sides or one side of the gill axis, and cilia on the gills. The movement of the cilia allows the breathing water to flow through the gills along a certain route for gas exchange. Some species have lost their proper gills and breathe through the skin surface or secondary gills formed on the skin surface (opisthobranchs). A part of the mantle of terrestrial species forms a lung chamber with dense venation network, which is used to breathe in the air.

The circulatory system is generally open-tube, but in higher cephalopods, the arteries and veins are connected by capillaries to become closed-tube. The center of the shellfish circulatory system is the heart, which has one ventricle and one, two or four auricles. The blood contains hemocyanin and is generally colorless. Only a few species, such as bivalve clams and gastropod flat snails, have hemoglobin and their blood is red.

The main organ of the excretory system is the kidney. The kidney is formed by ciliated renal tubules, one end of which is connected to the pericardial cavity and the other end opens into the mantle cavity. The number of kidneys varies depending on the species, and there are 6 pairs, 2 pairs, 1 pair, or only 1. In addition to the kidneys, some species also have glands on the wall of the pericardium or part of the liver that also have excretory functions.

The reproductive system includes gonads, reproductive ducts, copulatory organs and some accessory glands.