Invertebrates | What Are Invertebrates? 8 Groups of Invertebrate Animals
What are invertebrates? Invertebrates, the oldest and most abundant animals, derive their name from the absence of an internal skeleton, specifically the vertebral column.
Invertebrates vs. Vertebrates?
The natural world boasts a vast array of animal species, estimated to number around 4 million. To facilitate their study and description, scientists have organized them into distinct classes based on their shared characteristics. Each animal falls exclusively into one such class.
The primary classification separates animals into vertebrates and invertebrates. Vertebrates possess an internal bony skeleton centered around the vertebral column, while invertebrates lack this internal bone structure. Invertebrates, owing to their considerable differences, have been further categorized into various types.
Considering the significant distinctions among the animals within each type, these categories have been subdivided into smaller groups, which are subsequently broken down into additional subgroups. Ultimately, these classifications yield clusters of animals that share striking similarities. For instance, classifying a dog involves identifying it as a vertebrate, a mammal, a carnivore, a canid, a member of the dog genus, and a domestic species.
Groups of Invertebrates
Invertebrates are widespread in every environment and are divided into eight main groups:
- Porifera
- Coelenterates
- Platelminti
- Nematodes
- Annelids
- Arthropods
- Molluscs
- Echinoderms
Porifera
Porifera or sponges inhabit seabeds, drawing nourishment and oxygen through their pores. These aquatic, sessile organisms anchor themselves to the bottoms of both saltwater and freshwater bodies. Most reproduce asexually through budding, forming colonies. Their bodies feature calcareous or siliceous spicules in the form of needles and stars, arranged around a central cavity that maintains their body’s inflated shape.
Coelenterates
Coelenterates possess sac-shaped bodies with stinging tentacles, comprising:
- Jellyfish
- Anemones
- Corals
Polyps, a type of coelenterate, form coral reefs, such as the massive Great Barrier Reef in Australia, stretching 2000km in length, 100km in width, and rising 100m high. These reefs consist of the external skeletons of colonies formed by millions of interconnected polyps.
Platelminti, Nematodes, and Annelids
These invertebrates, commonly referred to as “worms,” share specific characteristics:
- Elongated, soft bodies
- Bilateral symmetry
- Creeping motion
Platelminti
Platelminti, known as flatworms, reside in both fresh and saltwater environments, sometimes acting as parasites. They possess a branched digestive and excretory system, with a single opening for both the intake of food and expulsion of waste. Breathing occurs through the skin, while primitive sensory organs, called eyespots, aid in foraging. Species include planaria and tapeworms, the latter capable of parasitizing the human intestine (reaching up to 2 meters in length).
Nematodes
Nematodes, or roundworms, feature slender, threadlike bodies. They inhabit diverse environments, including as parasites in numerous plant species and as pinworms, capable of infecting the intestines of children.
Annelids
Annelids, characterized by cylindrical bodies divided into identical segments (metamers), include various terrestrial, marine, and freshwater species. Notable examples are the earthworm, a hermaphrodite feeding on decaying matter, and the leech, a blood-sucking parasite.
Arthropods
Arthropods, comprising over a million classified species, represent the largest animal group, displaying the following features:
- Jointed legs
- Bilateral symmetry
- Exoskeleton composed of chitin
- Body segmented
Arachnids
Arachnids, possessing eight legs and typically lacking wings, feature a body divided into a cephalothorax and abdomen. Predatory by nature, they include spiders, scorpions, mites, and ticks.
Crustaceans
Crustaceans encompass a diverse group ranging from shrimps and lobsters to woodlice and barnacles. This group accommodates both small and substantial species, such as the Japanese crab, boasting a diameter of 3 meters.
Myriapods
Myriapods display elongated bodies divided into segments, each bearing numerous legs. Some feature a single pair of legs per segment (centipedes), while others possess two pairs (millipedes), like the scolopendra.
Insects
Insects represent a diverse array of animals, including fleas, dragonflies, beetles, grasshoppers, butterflies, ants, flies, and mosquitoes. Their body comprises a head, chest, and abdomen, featuring eyes, legs, and, in some cases, wings.
Molluscs
Molluscs, with soft bodies, reside primarily in aquatic environments, though some inhabit terrestrial habitats. They can be shielded by single or double shells or possess no shell at all.
Bivalves and Gastropods
Bivalves, such as mussels and clams, sport a shell composed of two valves, employing an ax-shaped foot for burrowing. They utilize two muscles to keep their valves closed. Gastropods, including limpets, sea snails, and land snails, feature a developed foot and a spiral-shaped shell. Terrestrial gastropods respire using lungs, while aquatic ones rely on gills.
Cephalopods
Among molluscs, cephalopods represent the most evolved and complex group. They feature a foot divided into numerous tentacles equipped with suckers for agile movement. Their propulsion mechanism involves expelling water from their bodies to generate movement in the opposite direction.
Echinoderms
Echinoderms, characterized by mobile spines, employ them for locomotion and defense. Common examples include starfish (asters) and sea urchins.
Starfish, with over 200 species, typically exhibit a round body extending into five identical arms, although some can possess more than 40 arms or appear pad-like due to their small size.
Sea urchins, marine echinoderms, display a globular body covered in spines, with approximately 850 species, varying in shape, size, and shell texture. Some sea urchin spines can exceed 30cm in length.
