Type Arthropods are the largest group of animals that inhabit our planet (more than 1 million species). They are found everywhere (in the ground-air environment, in all seas and oceans, in fresh water bodies).
External structure, integuments
Arthropods are bilaterally symmetrical animals.
They have a segmented body and articulated limbs (hence the name “arthropods”). Such limbs are also called lever-type limbs.
The outer cover of arthropods is saturated with a special organic substance – chitin. It protects the body and acts as an external skeleton (shell): muscles are attached to it from the inside.
The chitinous cover is weakly stretchable and interferes with the growth of the animal. Therefore, arthropods molt from time to time.
In the body of most arthropods, three sections can be distinguished:
the head, on which the main sense organs are located, necessary for orientation in space (eyes, antennae), as well as horn organs.
The chest, on which the legs are located, and in insects, the wings, performs the function of movement.
The third (posterior) part of the body – the abdomen – took over the function of digestion and reproduction.
In Arachnids and Decapod crayfish, the head and chest have grown together to form the cephalothorax.
Ticks have no boundaries between body parts.
The body cavity of arthropods is mixed: it is formed in the early stages of development when the primary and secondary cavities merge.
Arthropods have digestive, circulatory, respiratory, excretory, nervous and reproductive systems of organs.
The digestive system in arthropods consists of the anterior section (mouth, pharynx, esophagus, stomach), the middle and posterior intestines, ending with the anus.
The ducts of the liver or special tubular appendages – malpighian vessels – flow into the middle part of the intestine in Arachnids and Insects.
The organs of the excretory system of arthropods are different:
in crayfish (Class Crustaceans), it is a pair of buds (green glands), the ducts of which open at the base of the antennae.
The spider spider (Class Arachnids) has kidneys and excretory tubes.
The May beetle (Class Insects) has only excretory tubes (malpighian vessels).
In aquatic Arthropods (Crustaceans), the respiratory organs are gills.
Spiders have lung sacs and trachea that communicate with the external environment through the respiratory openings.
In insects, the respiratory organs are the thinnest respiratory tubes – the trachea.
Oxygen from the respiratory system is delivered to all organs by the circulatory system. The circulatory system is open and consists of the heart lying on the dorsal side of the body and the blood vessels extending from it. The heart is tubular, consisting of several chambers.
Nervous system and senses
The nervous system of arthropods is similar to the nervous system of annelids – the nodal type (periopharyngeal nerve ring and abdominal nerve chain).
Arthropods orient themselves in space with the help of well-developed sense organs.
The organs of smell and touch in arthropods are antennae and numerous hairs on the surface of the body.
Eyes, as a rule, are complex (faceted), formed by a large number of simple ocelli.
The vision of arthropods is called mosaic: the image of an object is composed of separate images perceived by simple eyes.
Some arthropods have good hearing. They perceive sound signals with fine auditory hairs located on the surface of the body and limbs.
Arthropods are characterized by complex behavior based on instinctive actions.
An instinct is a complex chain of hereditarily fixed (innate) reflexes – responses to the effects of the external environment.
Most arthropods are dioecious animals.
From the fertilized eggs laid by females, larvae develop, which grow, develop and turn into adults. Some arthropods, such as spiders, develop without the larval stage.
Many signs of arthropods, for example, body arthritis, abdominal nerve cord, the presence of a circulatory system, indicate their relationship with annelids, origin from common ancestors – primitive ancient small-segmented worms. At the same time, arthropods are much higher in organization than annelids. In the course of a long historical development, due to the fusion of the segments, body parts were formed, articulated limbs, chitinous cover, various respiratory organs developed, the nervous system and sensory organs became more complex.
These and other signs contributed to their prosperity on Earth. Originally, Arthropods were aquatic animals that gave rise to all the diversity of modern groups of arthropods: crustaceans, arachnids and insects.