There are some unique characteristics of arthropods, that you are probably not aware of. Arthropods are the most diverse and abundant group of animals.
The 750,000 documented species account for more than three times the total number of animal species, owing to their high adaptive diversity, which has allowed them to colonize a wide range of environments since their debut in the Precambrian.
Arthropods are the most diverse and abundant group of animals, and it is estimated that approximately 90% of animal species are arthropods. Due to this enormous diversity, it is impossible to deal with them all in just one of our articles, so in this one we will describe the unique characteristics of arthropods and then we will talk about the Crustaceans, known to all perhaps due to their enormous economic importance throughout the world.
The Unique Characteristics of Arthropods
Let us come straight to the answer and show you the 5 unique characteristics of arthropods, before we continue to talk about arthropods in detail.
- They are segmented animals. This segmentation is visible throughout embryonic development and in the most rudimentary species; in more advanced forms, the segments may vanish, merge, or become structurally and functionally specialized.
- Each segment has a dorsal anterior brain and a ventral nerve cord that forms a ganglion.
- Embryonic development follows a spiral pattern.
- At least in the most primitive species, each segment bears a pair of appendages; more advanced species may have lost or differentiated one or both sets.
- Each appendage that functions as a lever is linked to transverse striated muscles.
The Exoskeleton of Arthropods
One of the unique characteristics of arthropods is the presence of a chitinous exoskeleton or cuticle that externally covers the body of the animal.
This skeleton is divided into separate plates with articulation that allows movement; in the most primitive forms, each plate was limited to one segment, connected to the adjacent ones by internal membranes, whereas in the most evolved forms, plate fusion of several segments is common to form body regions or tagmas such as the head, thorax, and abdomen.
Each segment’s cuticle is made up of four primary plates: a dorsal tergite, a ventral sternite, and two lateral pleurites. The appendage skeleton has been transformed into articulated tubular structures called artifacts, which are also connected by membranes and in which condyles and alveoli develop for muscular insertions; inside the body, these insertion zones are called apodemes, which are folds of the exoskeleton to the interior.
The exoskeleton of arthropods is composed of two epidermal layers, the epicuticle and the procuticle. Proteins and waxes create the first layer, which prevents water loss. The second, considerably thicker, can be separated into an exocuticle and an endocuticle, both generated by proteins and chitin, and accumulations of carbonates or phosphates can develop in some marine organisms, such as crabs.
The exocuticle, the procuticle’s outermost layer, accumulates phenols that strengthen and give it resistance, so it is usually lacking in areas where flexibility is required, such as the membranes that connect the limb artifacts and the suture lines where the skeleton will later break during molting.
The fundamental issue with the exoskeleton of arthropods is that it does not expand with the animal, so it must discard the old skeleton in order to produce a larger one.
This is known as molting or ecdysis, and it is regulated by the hormone ecdysone. It starts with the epithelium secreting enzymes that cause the old exoskeleton to be shed; this is followed by the production of the outermost layer of the new skeleton between the old and the epithelium, through which degraded components from the old cuticle are absorbed by many enzymes.
The new procuticle is formed after the absorption of the majority of the old exoskeleton of arthropods; at this point, the animal is covered by two skeletons, the new and the old, which is solved by the breaking of the old exoskeleton by numerous zones called suture lines, through which the animal leaves the molt behind.
Because the new exoskeleton of arthropods is fragile and wrinkled, it must be adjusted to the animal’s increased size by raising blood pressure or inhaling water or air. An arthropod will often molt multiple times during its life, with each phase in between referred to as a stage.
The Internal System of Arthropods
In contrast to annelids, the body cavity or coelom is greatly reduced in arthropods and is confined to the cavity containing the gonads and, in certain families, the cavity containing the excretory organs. This is because the hemocoel, or blood cavity, occupies the majority of the body and, along with the heart and blood arteries, constitutes the circulatory system.
The heart is a muscular tube with a series of lateral openings known as ostioles; diastole causes blood to enter the heart through these openings from the large sinus in which it is located, known as the pericardium; systole pumps blood from the heart to the tissues via blood vessels and from there to the hemocele, from which it returns to the pericardium.
Hemocyanin, a somewhat bluish pigment, is the major respiratory pigment.
The internal system of arthropods consists of two types of structures:
- Malpigy tubes: are blind tubules of the digestive system in the hemocele into which wastes from the blood are emptied; they are evacuated to the outside via the digestive tract and the anus.
- Saccules: are blind sacs that are grouped in pairs and finish near the body’s appendages. The saccular portion is present in the hemocele like the preceding ones, but they vary in that selective filtration is feasible.
The digestive system varies greatly amongst arthropod taxa, although it is distinguished by a relatively large anterior and posterior zone.
The neurological system is distinguished by cephalization, which occurs in arthropods and results in the existence of a relatively large brain associated with sensory systems. This brain is split into three sections:
- The nerves of the eyes arrive in the protocerebrum. It has three pairs of optic centers and is engaged in photoreception as well as vision and movement integration.
- The nerves of the main antennae arrive in the deutocerebrum.
- The tritocerebrum contains nerves from the secondary antennae, the lip (labium), and the digestive system. As the esophagus crosses it, its commissure is posterior to the digestive system. This indicates that it is an advanced segment ganglion connected to the brain.
The great majority of arthropods are dioecious, and fertilization is typically internal, especially in terrestrial species with modified appendages for copulation; in aquatic species, fertilization is normally external, although this does not result in a high number of gametes being produced.
Eggs are centrolytic, rich in yolk, and their development is produced by superficial segmentation; in this type of segmentation, no membranes are formed during cell divisions, so a syncytium of nuclei forms, which migrates to the superficial zone, where the membranes are formed, and the embryo is formed later.
Sensory System of Arthropods
The exoskeleton of arthropods poses a problem for the interaction of the animal with the environment, so the formation of sensory organs is linked to modifications of the skeleton.
The sensory system of arthropods is made of sensillae that are non-light and normally in the shape of hairs but can sometimes be pits or slits in the cuticle. They might have mechano- or chemoreceptors that are linked to neurons.
The ocelli, which contain few receptors and are the simplest receptors found in arthropods, are among the ocular receptors.
Crustaceans and insects have complex eyes that are made up of many elongated cylindrical components called osmatidia.
The cornea, which serves as a lens, is the outermost component; when viewed from the outside, its surface is generally hexagonal in form and is referred to as a facet. Behind the cornea is an extended and conical crystalline cone that also serves as a lens; the two lenses remain stationary since they are created by the changes to the exoskeleton of arthropods.
The photosensitive structure known as the retinula is located at the bottom of the cone, in the center of which is the rhabdome, a translucent cylinder around which there are about 8 photosensitive cells (retinula) whose inner surfaces have microvilli (rhabdomeres) arranged perpendicular to the axis of the osmatidium; axons emerge from each of these cells to the brain or ocular nerve centers.
These eyes can produce a mosaic image, formed by the numerous images captured by each osmatidium if the light is intense (image by apposition), since the photoreceptor pigment expands along the walls of the cone, reflecting the light to the interior of the cone and preventing it from passing to other adjacent osmatidia.
If the light is weak, the pigment retracts and the light reaching a rhabdomere may have entered through a facet other than the osmatidia it stimulates; thus, images are produced by superimposition.