There are many types of animals. These are flatworms, and coelenterates, and annelids, and arthropods, and echinoderms, and chordates. The science that studies them is called biology. Mollusks are also one of the types of animals. They will be discussed in this article. There is also a special branch of biology that studies this group of animals. It's called malacology. And the science that studies mollusk shells is conchology.
General characteristics of molluscs
Representatives of this type are also called soft-bodied. They are quite varied. The number of species is approximately 200 thousand.
This group of multicellular animals is divided into eight classes:
- Bivalves.
- Papace.
- Striated Belly.
- Pittails.
- Monoplacophores.
- Gastropods.
- Shoveled.
- Cephalopods.
The body of all these animals is arranged according to the same principle. Next, the characteristics of mollusks will be considered in more detail.
Organ systems and organs
Mollusks, like many multicellular animals, are built from various types of tissues that are part of the organs. The latter, in turn,form organ systems.
The structure of molluscs includes the following systems:
- circulatory;
- nervous system and sense organs;
- digestive;
- excretory;
- respiratory;
- sexual;
- body covers.
Let's look at them one by one.
Circulatory system
In molluscs, it is of an open type. It consists of the following bodies:
- heart;
- vessels.
The heart of mollusks consists of two or three chambers. This is one ventricle and one or two atria.
In many soft-bodied, the blood has an unusual bluish color. This color is given to it by the respiratory pigment hemocyanin, the chemical composition of which includes copper. This substance performs the same function as hemoglobin.
Blood in mollusks circulates in this way: from the blood vessels it flows into the gaps between the organs - lacunae and sinuses. Then she again gathers in the vessels and goes to the gills or the lung.
Nervous system
In mollusks, it comes in two varieties: ladder and scattered knot type.
The first one is built in such a way: there is a peripharyngeal ring, from which four trunks extend. Two of them innervate the leg, and the other two innervates.
The nervous system of the scattered-nodular type is more complicated. It consists of two pairs of nerve circuits. Two abdominal are responsible for the innervation of the internal organs, and two pedal -legs. On both pairs of nerve circuits there are nodes - ganglia. Usually there are six pairs of them: buccal, cerebral, pleural, pedal, parietal and visceral. The first innervate the throat, the second - the tentacles and eyes, the third - the mantle, the fourth - the leg, the fifth - the respiratory organs, the sixth - other internal organs.
Sense Organs
There are such organs of mollusks that allow them to receive information about the environment:
- tentacles;
- eyes;
- statocysts;
- osphradia;
- sensory cells.
Eyes and tentacles are located on the animal's head. Osphradia are found near the base of the gills. These are the organs of chemical sense. Statocysts are organs of balance. They are on the leg. Sensory cells are responsible for touch. They are located on the edge of the mantle, on the head and leg.
Digestive system
The structure of molluscs provides for the following organs of this tract:
- throat;
- esophagus;
- stomach;
- midgut;
- hindgut.
Liver is also present. Cephalopods also have a pancreas.
In the throat of soft-bodied there is a special organ for grinding food - radula. It is covered with teeth made of chitin, which are renewed as the old ones are worn down.
Organs of excretion in mollusks
This system is represented by the kidneys. They are also called metanephridia. The excretory organs of mollusks are similar tothose of worms. But they are more complex.
The excretory organs of mollusks look like a collection of tortuous glandular tubes. One end of the metanephridium opens into the coelomic sac, while the other end opens outward.
Excretory organs in mollusks may be present in different quantities. So, some cephalopods have only one metanephridium, located on the left side. Monoplacophorans have as many as 10-12 excretory organs.
Excretion products accumulate in the metanephridia of molluscs. They are represented by lumps of uric acid. They are excreted from the body of the animal every two to three weeks.
Also, part of the excretory system in molluscs can be called the atria, which are responsible for filtering blood.
Respiratory system
In different molluscs, it is represented by different organs. So, most soft-bodied have gills. They are also called ctenidia. These are paired bilaterally pinnate organs. They are located in the cavity of the mantle. Mollusks that live on land have a lung instead of gills. It is a modified mantle cavity. Its walls are permeated with blood vessels.
Skin respiration also occupies an important place in the gas exchange of molluscs.
Reproductive system
It can be arranged in different ways, since among mollusks there are both hermaphrodites and dioecious species. In the case of hermaphroditism, during fertilization, each individual acts simultaneously as both a male and a female.
So we looked at all organ systemsshellfish.
Integuments of the body of mollusks
The structure of this element varies among representatives of different classes.
Let's look at the different body coverings that mollusks can have, examples of animals that belong to one class or another.
Thus, in furrow-bellied and pit-tailed integuments are represented by a mantle that covers the entire body, with a cuticle consisting of glycoproteins. There are also spicules - a kind of needles that are made of lime.
Bivalves, gastropods, cephalopods, monoplacphors and spadefoots lack a cuticle. But there is a shell, which consists of one plate or two in the case of bivalves. Some orders of the gastropod class lack this part of the integument.
Sink structure features
It can be divided into three layers: outer, middle and inner.
The outside of a shell is always built from an organic chemical. Most often it is conchiolin. The only exception to this rule is the mollusc Crysomallon squamiferum from the class of gastropods. Its outer shell is composed of ferrum sulfides.
The middle part of a mollusc shell is made of columnar calcite.
Internal - from lamellar calcite.
So we examined in detail the structure of mollusks.
Conclusion
As a result, we will briefly consider the main organs and systems of soft-bodied organs in the table. We will also give examplesshellfish belonging to different classes.
| System | Organs | Features |
| circulatory | vessels, heart | Open type circulatory system, two- or three-chambered heart. |
| nervous | nerve circuits and ganglia | Two nerve circuits are responsible for the innervation of the leg, two - for the internal organs. There are five pairs of ganglions, each attached to specific organs. |
| digestive | pharynx, esophagus, stomach, intestines, liver, pancreas | A radula is present in the pharynx, which helps to grind food. The intestine is represented by the middle and hindgut. |
| excretory | Metanephridia | Glandular tubes that open outward at one end and into the coelomic sac at the other. |
| respiratory | gills or lung | Located in the cavity of the mantle. |
| sexual | ovaries, testes | Among the molluscs there are hermaphrodites, which have both male and female gonads. There are also dioecious species. |
Now let's look at representatives of various classes of the Mollusk type and their structural features.
| Class | Examples | Features |
| Bivalves | Mussels, oysters, Japanese scallop, Icelandic scallop | Have a two-plate shell made of calcium carbonate,have well-developed gills, they are filter feeders by type of food. |
| Gastropods | Prudoviki, slugs, coils, snails, bittiny | They have an asymmetric internal structure due to the twisted shell. On the right side, the organs are reduced. So, many species lack the right ctenidium |
| Cephalopods | Nautilus, squid, octopus, cuttlefish | They are characterized by bilateral symmetry. These mollusks do not have an external shell. The circulatory and nervous systems are the most well developed of all invertebrates. The sense organs are similar to those of vertebrates. The eyes are especially well developed. The excretory organs of molluscs of this class are represented by two or four kidneys (metanefridia). |
So we examined the structural features of the main representatives of the Mollusk type.
