If you copy materials from this page!
To avoid misunderstandings, please read the rules for using and copying materials from the website www.ecosystema.ru

MOLLUSCS: INTERNAL STRUCTURE, REPRODUCTION AND CLASSIFICATION

Features of the internal structure.

The body of any mollusk has digestive, circulatory, excretory and other organ systems. Digestive system begins with the oral cavity, which passes into the pharynx (with a grater), esophagus, stomach with the digestive gland, liver, middle and hind intestine, which opens outward through the anus into the mantle cavity. Many species of mollusks have salivary glands.

Circulatory system molluscs are not closed. It most often consists of a two-chambered heart and blood vessels extending from it. The nervous system is formed by several pairs of ganglia with nerves. Metabolic products that are unnecessary for the body come from the blood of mollusks to the kidneys, and then into the mantle cavity and are removed outside. There may be one, two or four kidneys.

Reproduction.

Among mollusks there are both hermaphrodites and dioecious animals. Ponds and reels are hermaphrodites. Small snails emerge from the eggs they lay, glued together with a gelatinous substance. Most species of pearl barley are dioecious. Fertilization of their eggs occurs in the mantle cavity of the female. From fertilized eggs, larvae develop, which are pushed out through the siphon by the pearl barley when any fish swims past it. The larvae attach to the skin and gills of the fish and develop on its body for 1 - 2 months. This adaptability of pearl barleys and barnacles contributes to their distribution in nature in the larval phase. This is due to the sedentary lifestyle of adults.

Main classes of mollusks.

The phylum of mollusks is divided into three main classes. Pond snails, cattails and similar mollusks belong to the class of gastropods. These animals have a complete shell (less often it is reduced, like in slugs). Most of the ventral side of the body is occupied by a muscular leg (hence the name of the class). The eyes of gastropods are located either at the base of the tentacles (in pond snails), or at the ends of the tentacles (in coils, slugs). If a mollusk has two pairs of tentacles, then the eyes are located on the first pair. The gastropod class is the most numerous class in the phylum molluscs. It unites about 90 thousand species.

Pearl barleys, toothless mussels, oysters, and mussels are representatives of the bivalve class. The body of these animals is enclosed in a bivalve shell. They do not have a head, eyes, a tongue with a grater, or salivary glands. In the class of bivalves there are about 30 thousand species of freshwater and marine mollusks.

Squids, octopuses and cuttlefish belong to the class of cephalopods. There are about 600 species of these mollusks. Cephalopods inhabit mainly warm and fully saline seas. One part of their leg is turned into tentacles with suction cups that are located on the head and surround mouth opening. The other part of the leg forms a funnel. Some cephalopods have remains of a shell under their skin, which disappeared due to the transition of their ancestors to an active lifestyle.

The nervous system in cephalopods, compared to other mollusks, achieves the greatest development: the nerve ganglia merged and formed a large brain. Their sense organs are highly developed. The eyes of cephalopods resemble the eyes of fish in the complexity of their structure, and are not inferior to human eyes in visual acuity.

Scientists believe that mollusks originated from ancient annelids. One of the proofs of this is the similarity in the structure of sea larvae gastropods and marine larvae polychaete worms. In addition, some primitive mollusks have common features buildings with annelids.

Shellfish- invertebrate animals with a soft, unsegmented body, often protected by a shell. They live in fresh (pond snails, toothless mussels, pearl barley), salty (mussels, oysters, octopuses, squids) bodies of water or on wet areas of land (grape snail, naked slugs) . Most mollusks crawl slowly or lead an attached lifestyle, but some species (for example, squid) can swim quickly in the water column, reaching speeds of up to 70 km/h.

Among mollusks there are small species whose body does not exceed a few centimeters in length, but there are also real giants. For example, a marine bivalve tridacna can reach a length of up to 1.5 m, and its body weight - up to 280 kg! This mollusk lives up to 200 years. deep sea giant squid together with the tentacles it can reach a length of up to 18 m!

About 130 thousand species of mollusks are known. Among them there are herbivorous species, predators, filter-feeding species, etc.

External structure. The body of mollusks consists of three sections: head, torso and legs. (Fig. 141). The head contains sensory organs (a pair of eyes and tentacles), as well as a mouth opening. The leg is a muscular organ of movement located on the ventral side of the body.

The trunk is a sac-like outgrowth of the dorsal side of the body - surrounded by a fold of integument - mantle. Between the mantle and the body of the mollusk there is mantle cavity, communicating with environment. It contains the respiratory organs, the intestines, and the openings of the excretory and reproductive systems open into it.

The glandular cells of the mantle in most mollusks secrete protective sink. The outside of the shell is covered with a layer of horn-like organic matter. But the bulk of the shell consists of calcium carbonate, which forms its middle and inner layers. Middle layer - porcelain-shaped- formed by plates of calcium carbonate located perpendicular to the surface of the shell. Interior pearl The layer is formed by calcareous plates arranged in several layers, between which lie layers of organic matter. Refraction of rays occurs in these structures, as a result of which the mother-of-pearl shines and shimmers in different colors.

The symmetry of the body of mollusks is bilateral, however, in most gastropods the shell is spirally twisted, so the body also loses symmetry.

Veils The bodies of mollusks are represented by a single-layer epithelium, under which connective tissue is located. Epithelial tissue contains many glandular cells that secrete mucus, substances from which the shell is formed, etc. The epithelium of the mantle and gills is predominantly ciliated. Thanks to the work of cilia, a flow of water is created in the mantle cavity. Material from the site

The external structure of mollusks also has the following characteristics:

• the body is usually covered with a shell, which is secreted by the glands of the skin fold covering the body - the mantle;
• the skin-muscular sac is absent, the muscles are differentiated into separate bundles;
• the secondary body cavity is mainly filled with connective tissue.

Features of the internal structure.

The body of any mollusk has digestive, circulatory, excretory and other organ systems. The digestive system begins with the oral cavity, which passes into the pharynx (with a grater), esophagus, stomach with the digestive gland, liver, middle and hind intestine, which opens outward through the anus into the mantle cavity. Many species of mollusks have salivary glands.

The circulatory system of mollusks is not closed. It most often consists of a two-chambered heart and blood vessels extending from it. Nervous system formed by several pairs of ganglia with nerves. Metabolic products that are unnecessary for the body come from the blood of mollusks to the kidneys, and then into the mantle cavity and are removed outside. There may be one, two or four kidneys.

Reproduction.

Among mollusks there are both hermaphrodites and dioecious animals. Ponds and reels are hermaphrodites. Small snails emerge from the eggs they lay, glued together with a gelatinous substance. Most species of pearl barley are dioecious. Fertilization of their eggs occurs in the mantle cavity of the female. From fertilized eggs, larvae develop, which are pushed out through the siphon by the pearl barley when any fish swims past it. The larvae attach to the skin and gills of the fish and develop on its body for 1–2 months. This adaptability of pearl barleys and barnacles contributes to their distribution in nature in the larval phase. This is due to the sedentary lifestyle of adults.

Main classes of mollusks

The phylum of mollusks is divided into three main classes. Pond snails, cattails and similar mollusks belong to the class of gastropods. These animals have a complete shell (less often it is reduced, like in slugs). Most of the ventral side of the body is occupied by a muscular leg (hence the name of the class). The eyes of gastropods are located either at the base of the tentacles (in pond snails), or at the ends of the tentacles (in coils, slugs). If a mollusk has two pairs of tentacles, then the eyes are located on the first pair. The gastropod class is the most numerous class in the phylum molluscs. It unites about 90 thousand species.

Pearl barleys, clams, oysters, and mussels are representatives of the bivalve class. The body of these animals is enclosed in a bivalve shell. They have no head, eyes, tongue with a grater, salivary glands. In the class of bivalves there are about 30 thousand species of freshwater and marine mollusks.

Squids, octopuses and cuttlefish belong to the class of cephalopods. There are about 600 species of these mollusks. Cephalopods inhabit mainly warm and fully saline seas. One part of their leg is turned into tentacles with suction cups, which are located on the head and surround the mouth opening. The other part of the leg forms a funnel. Some cephalopods have remains of a shell under their skin, which disappeared due to the transition of their ancestors to an active lifestyle.

The nervous system in cephalopods, compared to other mollusks, achieves the greatest development: the nerve ganglia merged and formed a large brain. Their sense organs are highly developed. The eyes of cephalopods resemble the eyes of fish in the complexity of their structure, and are not inferior to human eyes in visual acuity.

Scientists believe that mollusks evolved from ancient annelids. One of the proofs of this is the similarity in the structure of the larvae of marine gastropods and the larvae of marine polychaete worms. In addition, some primitive mollusks have common structural features with annelids.

3. External structure of mollusks

External structure of chitons

They live mainly in the surf. Bilaterally symmetrical oval, flattened in the dorso-ventral direction, the body is covered with a shell of eight movably connected calcareous plates. The plates overlap each other in a tiled manner. Due to this arrangement of the plates, chitons can be rolled up on the ventral side, which is important when living in the surf zone. The shell within the class undergoes partial reduction: it decreases in size or becomes overgrown with integument. The body consists of three sections. The head is weakly separated from the body, covered with a shell, and lacks eyes and tentacles. Most of the ventral surface of the body is occupied by the leg. The mantle covers the entire body. Between the mantle and the leg there is a mantle groove, in which there are numerous (from 6 to 88 pairs) gills. The last, longest pair of gills is considered real. On the dorsal side of the body there are aesthetes (microesthetes and megaloesthetes) - the organs of touch. For some representatives, aesthetes are modified into eyes.

External structure of gastropods

Inhabitants of seas, fresh water bodies, land. The body shape is varied, in many cases the animals are asymmetrical. The body is divided into three sections. The head is separate, with 1–2 pairs of tentacles and eyes located on it. The foot often has a wide sole. Depending on your lifestyle, the shape of your leg changes. The body is in the form of a large internal sac, in lower gastropods it is vaguely delimited from the leg. Many gastropods are characterized by an anopedic curvature of the splanchnic sac. The shell is developed; in all subclasses of gastropods, a reduction of the shell is often observed, reaching complete atrophy in some animals. The sink has two or three layers. The shell is divided into: apex, whorl and mouth. The turns of the spiral are located in different planes (turbospiral) or in one plane - placospiral. There are right-handed (dexiotropic) and left-handed (leiotropic) shells. The axis of the spiral is represented either by a dense column - a column (columella) or a hollow column with a hole - a navel. If the last whorl of the shell covers the previous ones, the shell is involute; if the whorls of the spiral are open, it’s evolute. The mantle cavity is developed.

LECTURE 13. MOLLUSCS. GENERAL SIGNS OF THE TYPE. CLASSIFICATION. EXTERIOR BUILDING

3. External structure of mollusks

External structure of bivalves

Exclusively aquatic inhabitants. The body consists of two sections, the head is reduced, and in stationary forms the leg is also reduced. The body is covered with a mantle.

The mantle usually covers the entire body; the folds below are fused or free. WITH external environment the body is connected by holes for the legs and two siphons: inlet and outlet. The animal uses retractor muscles (there are two of them) to retract the leg, and a protractor to push it out. The shell consists of two symmetrical or asymmetrical valves, in some cases it is reduced.

The shell is often three-layered. The thickness of the valves depends on the habitat of the animals. The valves are connected with the help of a ligament, teeth (taxodont and heterodont teeth) and adductors - 1-2 closing muscles. For many bivalves There is a byssal gland, it is located on the leg. The secretion of the gland allows the animal to attach to the substrate.

External structure of cephalopods

Sea predators. The body of cephalopods is divided into three sections. The eyes are located on the head, and bibranchs have olfactory dimples and tentacles. The leg is modified into a funnel and tentacles. Nautiluses have many tentacles, they are smooth; higher cephalopods have 8 or 10 tentacles with suckers on inner surface. Decapod mollusks have two hunting tentacles. In male octopuses and argonauts, one tentacle is modified into a sexual tentacle (hectocotyl). The funnel (non-fused in four-branched mollusks and fused in bi-branched mollusks) provides jet propulsion animal, water is pushed out of the mantle cavity through it. The funnel is movable. There are cufflinks on the inner surface of the mantle, and cufflink holes at the base of the funnel. Using cufflinks, the robe is “fastened” to the body.

The evolution of cephalopods is associated with the reduction of the shell. The shell is developed in nautilids; it is spirally twisted, multi-chambered, and filled with gas. The animal is placed in the last chamber. The shell of cuttlefish is in the form of a calcareous plate, squids have a horny plate (gladius), and is absent in octopuses. There is a spirally twisted shell in female Argonauts, but it is a derivative not of the mantle, but of the epithelium of the tentacle blades. The integument contains pigment cells with which dilator muscles are associated; they surround the chromatophore.

LECTURE 13. MOLLUSCS. GENERAL SIGNS OF THE TYPE. CLASSIFICATION. EXTERIOR BUILDING

Control questions

1. habitats, species diversity and classification of mollusks.

2. General characteristics of the type of mollusks.

3. Features of the external structure of endocervical mollusks.

4. External structure of gastropods (body parts, shell structure and types of shells, modifications in the structure of the legs due to lifestyle).

6. External structure of cephalopods (body parts, leg transformation, shell of nautiluses and higher cephalopods).

7. The structure of the integument of endocervical mollusks.

8. The structure of the integument of shell mollusks.

General characteristics of the type

Mollusks are a large type of animal in terms of the number of species (130 thousand). They live mainly in the seas (mussels, oysters, squids, octopuses), fresh water bodies (toothless snails, pond snails, livebearers), and less often in a humid terrestrial environment (grape snail, slugs). Body sizes of adult mollusks different types vary significantly - from a few millimeters to 20 m. Most of them are sedentary animals, some lead an attached lifestyle (mussels, oysters), and only cephalopods able to move quickly in a reactive manner.

The type of mollusk is divided into three classes. Animals have differences in the structure of the shell and the way they feed.

Basic character traits structures of gastropods:

The body of the mollusk is enclosed in the sink protecting the animal and providing support for muscle attachment. Outer layer The shells are horny, middle (porcelain) and inner (mother-of-pearl) - calcareous. Gastropods have a solid shell in the form of a cap or a spirally curled turret.

External structure of gastropods.

The body of the mollusk is covered with a fold of skin - mantle, the epithelium of which secretes the substance of the shell. The body of representatives of this type is divided into the head, torso, and muscular legs. Most species have one or two pairs of tentacles on their heads. As for the body, it is asymmetrical and is completely located in the shell. The shell of gastropods is solid and forms a spiral with different quantities curls. The leg is used for movement and fixation - there is a special sole on the underside.

Internal structure of gastropods.

The digestive system consists of three sections: the foregut, midgut and hindgut. Most mollusks have a muscular tongue in the pharynx, covered with a horny plate with numerous denticles - grater With it they actively capture and crush plant and animal food. Ducts open into the pharynx salivary glands, and into the stomach - the duct of a special digestive gland - liver. Circulatory system open and consists of hearts And vessels. The heart has a ventricle and 1-2 (less often 4) atria. In addition to the vessels, blood passes part of the way in the slit-like cavities between the organs. Discharge - two buds. Respiratory organs in aquatic mollusks - gills, for terrestrials - lung, representing a section of the mantle cavity. The wall of the lung contains a dense network of blood vessels through which gas exchange occurs. The lung opens to the outside with a breathing opening called the spiracle.

Nervous system scattered-node type: five pairs of large ganglia are located in vital organs (head, leg, mantle, respiratory organs and sac) and are connected by nerve trunks. Of the sense organs, the most developed are the organs of chemical sense, touch, balance, and in mobile predators - vision.

Reproduction occurs sexually.

Exercise.

1. Study the lesson material (read the text of the textbook if necessary);

2. Examine the drawing and write down the names of the body parts of the mollusk on a separate sheet.

Sources:

1. Text - http://znanija.com/task/8417672
2. Sink http://upload.wikimedia.org/wikipedia/commons/thumb/d/d4/Charonia_tritonis_22-4-2005_15-02-29.JPG/800px-Charonia_tritonis_22-4-2005_15-02-29.JPG
3. Movement of gastropods -