Invertebrate animals seem to be the great misunderstood of the animal kingdom. Many people can dislike or even fear them without knowing that these organisms represent 95% of currently known animal species. If we continue talking about figures, there are about 35 phyla in the animal kingdom, of which all are from invertebrate animals except for one, the phylum of chordates.
General characteristics of invertebrate or boneless animals
What are the characteristics of invertebrate animals? The truth is that there are many, but these are some of the main ones. To begin with, their name refers to the fact that they lack a vertebral column (or notochord) and an internal skeleton, as is the case with vertebrates, so we can say that they are animals that do not have bones . In addition, they are usually small animals, although variable, and many of them have protective structures such as shells, carapaces or covers .
These invertebrates are found in almost all habitats found on Earth, from hot water in hydrothermal vents to the depths of the seas or the frozen surface of the Antarctic.
For a long time his research was scarce due to its complex description and the underestimation of its potential in various fields such as economic, scientific and, although it may not be shared in all cultures, the food field. In addition, to all this it was added that this group are often small animals that live discreetly, in the seas, sediments, soils, etc.
Despite all this, they constitute most of the known biodiversity on the planet. In 2005, between 1.7 and 1.8 million species were registered, of which about 990,000 were invertebrate animals or animals without bones . However, these figures are constantly changing.
This term is frequently used to refer to invertebrates that inhabit fresh water , such as river beds, ponds, lakes and lagoons … It includes the following groups of aquatic macroinvertebrates :
The abundance and diversity of these macroinvertebrates are factors used as bioindicators, that is, they are indicators of the state of the ecosystem and local biodiversity. They are essential as they serve as food for other organisms and are transformers of organic matter and, in addition, they function as predators in small ponds in which the absence of fish makes them the main responsible for the predation of zooplankton.
Invertebrates or animals that have no bones: the groups
There are a large number of invertebrate groups , but the largest of them is the arthropods. Thus, of the immense variety of invertebrate phyla (about 34), the vast majority are found in the arthropod group (80%) of animals. Some of the most common phyla of invertebrates are (8 phyla):
- Poriferous (9,000 species or sp).
- Cnidarios (10.000 sp)
- Platelmintos (20.000 sp)
- Molluscs (100,000 sp)
- Annelids (16,500 sp)
- Nemátodos (25,000 sp)
- Arthropods (more than 1,000,000 sp).
- Echinoderms (7,000 sp).
Poriferous or sponges are the most primitive and simple animals . They are aquatic organisms with sessile life, that is, they grow and live rooted in the soil or rocks. Its body is made up of numerous small holes as pores (which is why they are called poriferous). They have a larger hole called an osculum.
Sea sponges feed on the particles found in the water, which pass by currents through the internal channels of their body. These currents are created by cells that have a flagellum surrounded by a fold called choanocytes. Learn more about them in this other GreenEcology article about the Starfish: what it is and its characteristics .
All of them are aquatic , the cnidarians include corals, jellyfish and hydras . They are animals whose body is shaped like a sac with a single opening that constitutes the mouth and that connects with the gastrovascular cavity (similar to a primitive stomach). Around the mouth they have tentacles with specialized cells called cnidoblasts with stinging substances that serve both as defense and attack towards their prey.
It is a type of invertebrate that does not move because, for example, jellyfish are carried away by the current and corals remain anchored to the ground or rocks.
Flatworms or flatworms, other boneless animals
Also called “flat worms”, they are the group of tapeworms, planarians and trematodes . They present forms of free life and forms of parasitic life (tapeworm or solitary). The novelty that they present with respect to the previous groups is the presence of a central nervous system , which is capable of processing the information collected by the eyes and other sensitive structures. Its body can be divided into two identical halves lengthwise.
Mollusks, another well-known group of invertebrate animals
Mollusks, the group of snails, clams, squid or octopuses , have a soft body, and there are species in which it is protected by a hard shell.
Its internal organs form a visceral mass and its body consists of a muscular foot, the mantle, the paleal cavity that functions as a respiratory system and, finally, a shell of limestone materials that can be formed by a single piece or valve or, for two pieces. This can be found internally in some groups or it can be non-existent.
Mollusks consist of 8 classes, of which the ones with the greatest number of species are:
- Gastropods (75,000 species): terrestrial or aquatic, they have a wide foot and eye tentacles. They have a shell formed by a single piece. Example: snails and limpets.
- Bivalves (13,000 species): all of them aquatic organisms that are buried in the seabed. They have an ax-shaped foot and lack a distinct head. They feed on the organic particles that are floating in the water. Example: clams, mussels, oysters …
- Cephalopods (800 species): all of them marine. They are the group of octopuses, squid and cuttlefish. They have a highly developed foot from which the tentacles with so characteristic suction cups start. The shell is internal or non-existent, depending on the species. To move they consist of a propellant siphon.
This group of animals that have no bones is the group of the famous earthworm . They are animals with a cylindrical body and divided into rings. They present metameria, that is, certain organs are repeated in each of the rings. Its body is covered by mucus, a viscous substance secreted by certain glands, which facilitates its movement. Most are free-living, that is, they are not parasites.
Nematodes or round worms, animals without bones and parasites
Nematodes or ” round worms ” are very abundant and diverse organisms found in soils and aquatic habitats. Its body is unsegmented and has characteristic muscles for its movement. They can be free-living or parasites . From the appearance of nematodes, a new body cavity called coelom appears in invertebrates, which makes the existence of hollow internal organs possible.
Other large groups of boneless animals: arthropods or insects, arachnids and crustaceans
It is the most diverse and heterogeneous group, found in almost all existing habitats. It is of great importance because they encompass the vast majority of known animal species (80% of animals are arthropods), such as insects, crustaceans or arachnids .
They have a very elaborate body design, with a chitin exoskeleton divided into segments and appendages (legs, antennae and mouthparts).
When the individual grows, it is detached from its exoskeleton by molting, after which it produces a new one. This is repeated several times throughout your life. In addition, it is common for the juvenile to be very different from the adult animal, so it undergoes a process of metamorphosis until it reaches its final adult appearance.
Arthropods are divided into several groups. According to the type of appendages and the number, some of the most common arthropods are:
- Myriapods: its body consists of a head with a pair of antennae and an elongated trunk that is segmented. One or two pairs of legs are born from each of these segments. Example: centipedes and scolopendras.
- Arachnids: the body is divided into cephalothorax (head and abdomen are fused) and abdomen. They have four pairs of legs, lack antennae or jaws and in the mouth they have pincer-shaped structures, called chelicerae, and a pair of appendages (not legs) called pedipalps. Example: spiders, scorpions and mites.
- Crustaceans: the body consists of cephalothorax and abdomen. They have two pairs of antennas and they are all aquatic. Within the crustaceans we find lower crustaceans, such as barnacles or other species that make up zooplankton, and decapods, such as prawns, crabs or prawns. They have 10 legs on the cephalothorax and one pair of appendages per segment of the abdomen.
- Insects: divided into head, thorax and abdomen, they have a pair of antennae, a pair of compound eyes and several simple eyes. Its distinctive feature is the presence of 6 legs and a pair of wings (except in some groups). There are a large number of orders of insects that are classified based on the type of wings, the mouthparts, according to the type of feeding or the metamorphosis they undergo. Example: butterflies, grasshoppers, beetles, ants …
Echinoderms: stars and sea urchins
They include stars and sea urchins . They are the marine animals closest to vertebrates. Their bodies are made up of 5 identical parts and they do not have bones, but they do have an external skeleton formed by limestone plates, being able to present spikes in some species and, in others, with a great capacity for regeneration of those parts that have been lost.
They have an exclusive mechanism for their movement and feeding called the ambulatory apparatus. It is a hydraulic system made up of a network of internal channels that contain seawater that is pumped to different parts of your body. Small dilations called tube feet arise from these ducts which, when filled with water, exert an effect similar to that of a suction cup, allowing the animal to move.