The arthropods They are the broadest and most diverse phylum of animals in the animalia kingdom. This phylum was described as such, for the first time, by the French entomologist Pierre Latreille. So far there are approximately 1,250,000 described species, although specialists agree that there are still thousands of them to be discovered. It is such a diverse edge that the animals that comprise it are found in all existing habitats.
In general, arthropods are characterized by presenting a rigid protective covering (exoskeleton), the body divided into segments (tagmas) and articulated appendages that specialize in various functions such as locomotion..
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The taxonomic classification of arthropods is as follows:
- Domain: Eukarya.
- Animalia Kingdom.
- Subkingdom: Eumetazoa.
- Superphylum: Ecdysozoa.
- Panarthropoda.
- Phylum: Arthropoda.
The group of arthropods is made up of organisms with eukaryotic cells, whose DNA is delimited in a cellular structure known as the nucleus. They are also multicellular beings, since during the stages of their development, their cells diversify and specialize in different functions such as digestion, reproduction or transmission of nerve impulses, among others..
Arthropods are considered triblastic animals because they present the three embryonic germ layers: ectoderm, mesoderm and endoderm. From these layers the different organs of the adult animal are formed.
Likewise, arthropods belong to the group of protostomes, since mainly the blastopore gives rise to the mouth and in some species simultaneously to the anus..
When it comes to habitat, arthropods are such a large and diverse group that they have colonized practically every habitat on the planet. They are distributed throughout the world geography.
They are preferably herbivores, feeding on algae and terrestrial plants. Despite this, a smaller number of species are carnivorous, including some arachnids..
Likewise, this group of animals has bilateral symmetry. This means that, by drawing an imaginary line along its longitudinal plane, two exactly equal halves are obtained. Like mollusks and annelids, arthropods are coelomate, presenting a reduced coelom in adult individuals.
Despite the fact that arthropods constitute the largest group of animals in the animal kingdom and therefore the most diverse, they present common morphological characteristics that distinguish them from any other phylum..
First, the body of arthropods is divided into segments known as metamers. These are repetitive, some equal to each other. However, beyond this division of the body, what characterizes them even more is that there is a specialization of certain regions.
In the body of arthropods you can see several well differentiated areas. Some species have a head and trunk, others have a cephalothorax and abdomen, and many others have a head, thorax, and abdomen. This process of differentiation is known by the name of tagmatization and each segment is called tagma.
Likewise, arthropods have a rigid and hard cover, a cuticle that is identified by the name of exoskeleton. Structurally, the cuticle is made up of two layers:
- Epicuticle, which contains proteins and waxes. Is thin.
- Procuticle, which is made up of chitin and some proteins. It is also divided into two layers, the exocuticle and the endocuticle..
This exoskeleton is made up of plates, which in the most primitive animals are only limited to each metamer and are connected to the others through an internal system of membranes. In the case of more complex animals, the plates of each metamer fuse, forming large segments that cover a complete tagma..
Every so often, arthropods undergo a shedding process. This is because the exoskeleton does not grow as the animal does. In this sense, it is necessary to create a new exoskeleton that adapts to the new size of the individual as it develops and enlarges..
Other characteristic morphological elements of arthropods, which also contribute to give this taxonomic group its name, are the articulated appendages. Usually two pairs of appendages per metamer are located, although the most primitive arthropods follow the pattern of one pair of appendages per metamer.
The appendices are made up of pieces called artejos. These are articulated with each other by some auxiliary anatomical structures such as membranes, among others..
In general and according to their structure, there are two types of appendices:
- Appendices unirrámeos: as their name implies, they are those that have a single axis. They occur mainly in arthropods that inhabit terrestrial environments such as arachnids.
- Birrámeos appendages: they are those that present two axes. They have two branches, endopod and exopod. These articulate with the protopod (proximal area of the appendix). They are typical of arthropods in aquatic habitats such as crustaceans.
Likewise, over time and as the group of arthropods has evolved and diversified into species, the appendages have been modified or transformed to fulfill specific functions, beyond simple locomotion..
Thus, for example, crustaceans and myriapods have modified appendages in jaws, chelicerates have pedipalps, and scorpions have combs, and crustaceans and myriapods have maxillae, just to name a few..
Arthropods have a complete digestive system, with sections specialized in the various functions that make up the digestion process. The digestive tract is segmented into three areas or zones: stomodeum, mesentery and proctodeum..
It is the first portion of the digestive tract of arthropods. It is made up of the oral appendages (of different morphology, depending on the species), the esophagus, the pharynx and, in some species, the stomach, called the crop..
Similarly, there are species that have salivary glands that synthesize and release enzymes that contribute to the digestion process, since they begin to disintegrate and convert nutrients into simple substances that can be assimilated by the animal..
Depending on the type of diet, the pharynx may be highly developed or have special musculature. Likewise, the stomach is not considered as such, but rather a widening of the esophagus.
The boundary between the stomodeus and the mesentery is marked by the presence of the so-called esophageal or ventricular valve.
It is the site where the absorption of nutrients already processed by digestive enzymes takes place..
Depending on the species, the mesentery will have different configurations. For example, in the simplest arthropods the mesentery is simply a straight tube.
On the other hand, in the more complex animals of this phylum the mesentery presents structures called cecum in which digestion and absorption take place. These increase the absorption surface of the animal's mesentery..
At the end of this structure, between it and the proctodeum is the pyloric valve, which allows or restricts the passage of substances.
It is covered with cuticle. Its length is very short, compared to the mesentery. This is where the stool forms in the digestive tract. Culminates in the anus.
Again, depending on the type of arthropod, the proctodean may be specialized in other functions such as the absorption of water and salts..
The respiratory system of arthropods is simple and varied. This means that, depending on the habitat that the animal occupies (aquatic or terrestrial), its respiratory system will present a specific anatomy.
In the case of aquatic arthropods such as crustaceans, the gas exchange with the external environment takes place through gills. Through these highly vascularized structures they extract oxygen from the water.
In another vein, terrestrial arthropods can present two types of respiration: tracheal or book lungs..
In organisms that present this type of respiration, the respiratory system is made up of a system of branched and interconnected tubes called tracheas. These open outwards through holes, the spiracles.
The tracheae, as they branch out inside the animal, progressively reduce their diameter, becoming tracheae. Likewise, they are covered with cuticle.
In tracheal respiration, the tracheae carry oxygen directly to the cells and are responsible for gas exchange.
Among the arthropods that have this type of respiration, insects and myriapods can be mentioned, among others..
In this type of respiration, gas exchange occurs in structures that are made up of a series of invaginations of the integument that are organized in a similar way to the pages of a book. These communicate with the outside through spiracles.
The most representative arthropods of book lung respiration are scorpions and spiders, among others.
In arthropods, the fluid that circulates through the blood vessels is hemolymph. The only specialized cell that these animals present are the so-called amoebocytes. These have functions related to clotting and immunity.
Likewise, the circulatory system of arthropods is of the open type, also known as lagunar. In this, the hemolymph reaches a kind of lagoon (hemocele) that is a secondary cavity within the body of the animal..
They also have a kind of heart that is responsible for pumping hemolymph throughout the body through various blood vessels. The heart is not a complex organ like that found in other types of animals, but consists of a tube with contractile capacity that is located in the dorsal position..
The nervous system of arthropods is similar to that of annelids. It is made up of a kind of brain made up of the union of three nerve ganglia: protocerebro, deutobrain and tritobrain..
The proto-brain is related to the secretion of endocrine substances, the ocelli and the eyes. Likewise, the deutobrain emits nerve fibers that innervate the antennae of the arthropods that have them and the tritobrain has fibers that innervate the chelicerae and the second pair of antennae of the arthropods that present them..
It also has a periosophageal nerve ring that connects through nerve fibers with the primitive brain already mentioned..
At the ventral level, two nerve chains are observed that run longitudinally through the entire animal. These chains have a pair of nerve ganglia in each metamer. However, these nerve cords are not dissociated, but communicate through transverse nerve fibers.
With regard to the sense organs, arthropods have them very well developed. They present several types of eyes, among which the compounds stand out. They also have receptors distributed throughout the body that allow them to perceive tactile and chemical stimuli (smell and taste).
Most of the species that make up the phylum of arthropods are dioecious, that is, they have female and male individuals.
Although due to the wide variety of species that make up this phylum, the anatomy of the reproductive system is very diverse, it has certain aspects in common.
First, they generally have a single pair of gonads. Likewise, they have ducts on both sides of the body, which merge in the midline of the body and lead to a single hole called the gonopore..
Females have a structure known as the spermatheca, which functions as a storage site for the male's sperm. Likewise, depending on the species, certain glands that produce structural substances for eggs, as well as pheromones, may be present..
In the case of males, they present a seminal vesicle, as well as certain glands that are responsible for secreting some chemical compounds such as those that make up the spermatophore.
Likewise, depending on the species, males may present anatomical structures that fulfill the function of holding the female for the copulation process..
The excretory system is widely varied, according to each of the species of this phylum.
Arthropods may have some glands such as the coxal and antennal, which have an excretory function. Likewise, some have a kind of ducts called Malpigio tubes. These are blind and are immersed in the hemolymph. They empty at the level of the proctodeum, pouring waste products such as urine..
Among the substances that arthropods discard through excretion are mentioned ammonia, urea and uric acid, among others.
Arthropods present a type of sexual reproduction, which consists of the fusion of the two gametes, female and male. In most species, fertilization is internal, although there are species that have external fertilization..
Likewise, arthropods can be oviparous or ovoviviparous. The oviparous are those that reproduce by laying eggs and the ovoviviparous develop in an egg, but which is placed inside the female.
In the case of arthropods that have internal fertilization, the male introduces the sperm into the female, with the help of modified appendages (gonopods). Later the female lays the eggs, within which the new individuals develop.
After some time, which varies by species, the eggs hatch. In species that have indirect development, larvae emerge from the eggs that must undergo a metamorphosis process until they reach the adult stage. For example, in the case of insects, the stages that comprise their development are larva, nymph and adult.
On the other hand, in species whose development is direct, individuals that already present the characteristics of adults come out of the eggs. This type of development is typical of some arachnids.
The phylum Arthropoda is divided into five subphiles:
- Trilobite: they are a group of arthropods that existed in abundance during the Paleozoic. They gradually died out. They were small, and had a flattened body, divided into three tagmas and ovoid in shape. This group is totally extinct.
- Chelicerata: they are a large group that is characterized by not having antennae or jaws. They have six pairs of appendages distributed as follows: a pair of chelicerae, four pairs of legs, and a pair of pedipalps. Mites, arachnids and scorpions belong to this subphylum.
- Crustacea: they are characterized by presenting a pair of mandibles and two pairs of antennae. They can also be divided into between 15 and 20 segments. This group includes lobsters, crabs and shrimp, among others..
- Myriapoda: they have a characteristic elongated and segmented body, which is why they are often confused with other types of animals. They have a pair of antennae and jaws. Covers animals such as centipedes and millipedes, among others.
- Hexapoda: they have a body divided into three tagmas (head, thorax and abdomen). They also have antennae, jaws and maxillae. This subphylum includes insects such as beetles and ants, among thousands of others..
Here are some examples of the species that make up the phylum Arthopoda.
- Chelicerata: within this subphylum there are spider species such as Sphodros rufipes, Aname, atra Y Atypus karshi. Likewise, it includes species of scorpions such as Androctonus crassicauda Y Hottentotta tamulus.
- Crustacea: encompasses crab species such as Procambarus clarkii, Callinectes sapidus and lobsters like Palinurus elephas, among other.
- Myriapoda: includes centipede species such as Scolopendra cingulata and millipedes like Illacme pienipes, among many others.
- Hexapoda: includes insects such as domestic musca, butterflies like Morpho menelaus and beetles like Lamprima aurata.
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