How are mollusks similar and different? Type Soft-bodied or Molluscs. Class Cephalopods

1. General information

The sizes of mollusks range from 0.5 mm (some gastropods) to 15 meters (giant squid Architeuthis dux (Steenstrup, 1875) - adult individuals of this species were repeatedly caught by fishing ships and found remains washed ashore). Mollusks have a secondary body cavity (coelom), which is represented by the pericardial cavity (pericardium) and the cavity of the gonads (gonads). Mollusks are bilaterally symmetrical animals, however, some of them (class Gastropoda) lost their symmetry for the second time as a result of the displacement of a number of organs (torsion process). The body of mollusks is not segmented (only in some groups signs of metamerism appear) and consists, as a rule, of three sections - the head, trunk and legs. The chair can be partially or completely reduced. The head contains a mouth, tentacles and eyes. The leg is a muscular, thickened outgrowth of the abdominal wall of the body, which in most cases performs a locomotor function (movement). The tulub is kept above the leg.

2. Veil

Clam shells

It is believed that in the hypothetical ancestor of mollusks the cover was represented by a certain so-called protoperinotum: a cuticle with aragonite spicules. A similar structure of the cover is characteristic of representatives of the classes Caudofoveata and Solenogastres. However, in all classes of mollusks, except Caudofoveata, a ciliated animal surface appears - a leg (on this basis they are united in the Adenopoda group). In Solenogastres the foot is represented by an inhibitory groove. Polyplacophora also possess cuticular integument, but only on the lateral surfaces, called perinatal folds. The dorsal surface is covered with eight shell plates.

5. Circulatory system

6. Respiratory system

Most aquatic species (rapana, murex) breathe with gills (ctenidia), while terrestrial species (grape snail) and some aquatic species (large and small pond snails) breathe with lungs. Air penetrates through the breathing hole into the lungs, where the blood is saturated with oxygen and freed from carbon dioxide. Shellfish have blood of blue color. This determines high level copper in the blood.

7. Nervous system

Nervous system gastropods consists of nerve ganglia connected to each other and nerves. This type of nervous system is called scattered-nodal.

photo:Art

In living nature, mollusks are found almost everywhere - they are found in the thickness of ocean waters, high in the mountains, in salty and fresh water, on the ground and underground. They don't live only in sandy deserts and in snow cover.
The diversity of the habitat explains that mollusks differ from each other in body structure, its color and shape, methods and speed of movement and other features.

photo:jacinta lluch valero

But despite this, they have some common features: The mollusk consists of a head, body and legs. Most species are “equipped” with an external shell, which consists of protein substances and calcium carbonate. The shell plays a protective function - the mollusk hides in it at the slightest danger. It can be single or double-leafed. The body of the mollusk, like the shell, in most individuals is a spiral.

photo: divemecressi

The internal organs of mollusks are located in the body; in some species they are shifted to the leg. There are mollusks that do not have a head - it disappeared as unnecessary. The absence or presence of eyes, tentacles, the shape and size of the shell, the location of the internal organs - all this depends on the method of evolution, habitat, and nutrition of certain species of these invertebrate animals. It should be noted that mollusks are the first animals to have a liver.

photo: Martin LaBar

The first fossil mollusks date back to the Cambrian period. Their ancestors are considered to be annelids, which have external segmentation. At the beginning of their evolution, cephalopods predominated, later bivalves and gastropods appeared. Modern scientists pay special attention to the study of the evolution of mollusks, which can be traced from preserved shells. This makes it possible to understand the developmental features of not only invertebrates, but also the rest of the animal world, because mollusks are an important component of the diet of fish, birds and mammals.

photo:Art

Today, there are seven classes of mollusks. The most popular of them are gastropods, bivalves and cephalopods.

Gastropods- one of the many species that live literally everywhere. These are aquatic and land snails. Moreover, in aquatic gastropods, the respiratory organ is the gills, while in land animals they are transformed into lungs, which allows the snails to be independent of the reservoir and live a full life on land. Bivalve mollusks are difficult to confuse with other species - they have a characteristic shell consisting of two closed valves. The body of the mollusk is enclosed between the valves. At the slightest danger, the doors slam shut, and normal condition they are slightly open. Bivalves live in aquatic environment, both in salt and fresh water at different depths.

Features of the internal structure. The muscles of mollusks are represented by separate bundles that perform specific functions: some of them pull the leg and head inside the shell, the leg muscles provide movement. Special closing muscles contract to close the shell valves of bivalves.

Body cavity shellfish, as well as annelids, secondary. But the spaces between internal organs, mainly filled with loose connective tissue.

Digestive system molluscs begins with the oral opening and ends with the anal opening, opening into the mantle cavity. On the border between mouth opening and the pharynx may contain horny jaws. The pharynx has a muscular outgrowth - a tongue covered with rows of horny denticles. This creates a grater, with which the mollusks scrape food from various surfaces. Ducts open into the pharynx salivary glands. In cephalopods and some gastropods, the saliva, in addition to digestive juices, contains toxic substances that kill prey - various marine invertebrate animals and fish. The middle section of the intestine forms a pouch-like expansion - stomach. The ducts of the digestive gland open into it - liver (Fig. 142).

Excretory organs shellfish - kidneys Each kidney opens with one duct into the pericardial cavity, and with another into the mantle cavity. The end products of metabolism are removed there.

Circulatory system Mollusks, like arthropods, are not closed. There is a muscular organ - the heart, consisting of chambers: one or more atria and a ventricle (Fig. 142). Blood moves from the heart to arteries, and returns to the heart by veins Like arthropods, the blood of mollusks mixes with cavity fluid. Blood is most often colorless, sometimes red.

Since most mollusks are inhabitants of water bodies, respiratory system they have - gills. Therefore, they breathe oxygen dissolved in water. In terrestrial and some freshwater mollusks, the respiratory organ is lung. This is a section of the mantle in which blood vessels branch. The lung allows you to breathe atmospheric oxygen. Therefore, inhabitants of reservoirs that have lungs are forced from time to time to float to the surface of the water to capture air.

In most mollusks nervous system consists of nerve ganglia located in different parts bodies connected by nerve trunks (Fig. 143). This type of nervous system is called scattered-nodular. Mollusks that lead an active lifestyle (for example, cephalopods) develop a complex brain.

Sense organs mollusks are diverse. The skin contains receptors that perceive mechanical, chemical and physical stimuli (various chemical substances, touch, temperature, etc.). There are especially many receptors on the tentacles located on the head. The organs of vision - the eyes - are the most complexly arranged in cephalopods. In bivalves, due to their sedentary lifestyle, the sense organs are poorly developed.

Reproduction and development. Among mollusks there are both dioecious species and hermaphrodites. Fertilization can be either internal or external. For most marine species characterized by indirect development. They have larvae that live in the water column and ensure the spread of the species. In most freshwater and all terrestrial species, development is direct. Since mollusks grow throughout their lives, the size of their shells gradually increases. Material from the site

Complications of the structure and life processes of mollusks compared to annelids:

• the body consists of sections: head, torso and legs;
• the circulatory system is not closed, there is a heart that provides blood circulation;
• There are specialized respiratory organs - gills or lungs.

The internal structure and life processes of mollusks also have the following characteristics:

• there is a through intestine and digestive glands - salivary and liver;
• excretory organs - kidneys;
• nervous system of scattered-nodular type;
• There are both dioecious and hermaphrodite species; fertilization - both external and internal, development - direct or indirect;
• They live in fresh and salt water bodies, as well as on wet land areas.

On this page there is material on the following topics:

• Table of the evolution of the digestive system from unicellular organisms to mollusks

• What are the nervous system and sensory organs of mollusks

• Organs of secretion and reproduction of mollusks

• Compare the structure of arthropods and mollusks

• Report on the structural features of the organs of the digestive system of mollusks

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