Information on the Animal Class Invertebrates
What Are Invertebrate Animals?
Characteristics and Special Features of Invertebrates:
- No Backbone: The primary characteristic is the absence of an internal skeletal structure and a spine.
- Enormous diversity: Invertebrates make up the largest group in the animal kingdom – over 96 % of all known animal species fall into this category. They include:
- Mollusks: (snails, clams, octopuses)
- Echinoderms: (starfish, sea urchins)
- Arthropods: (insects, arachnids, crustaceans)
- Simple organisms: (sponges, worms, cnidarians)
- Diverse support systems: Instead of bones, they often use exoskeletons (chitin), calcium shells, or water-filled body cavities (hydrostatic skeleton).
- Habitats: They inhabit nearly every corner of the Earth – from the deepest oceans to the highest mountains.
Did You Know?
Only about 4 % of all living beings on Earth are vertebrates (such as mammals, birds, reptiles, amphibians, and fish). The world is essentially owned by invertebrates!
What Are Mollusks?
Characteristics and Special Features of Mollusks:
- Invertebrates: Mollusks do not have an internal skeleton or bones.
- Soft body: Their bodies are generally quite flexible.
- Protective shell: Many species have a protective shell or casing made of calcium (like snails and clams).
- Radula: Almost all mollusks use a structure called a radula to break down their food.
- Muscular foot: Most of them move by crawling on a strong muscular foot.
Habitat
Where Can Mollusks Be Found?
Mollusks primarily inhabit water, with a few species also found on land (like snails).
- Freshwater (great ramshorns)
- Saltwater (squids)
- On land (snails)
Movement
How Do Mollusks Move?
Mollusks have developed various methods of movement. They:
- glide (snails)
- propel themselves (squids)
- pull themselves (like clams)
- swim (scallops)
Diet
What Do Mollusks Eat?
Mollusks feed on plankton, algae, or lichens. Depending on the species, they may also consume other mollusks, crustaceans, or fish. There are:
- Herbivores (sea snails)
- Omnivores (nautiluses)
- Carnivores (octopuses)
- Filter feeders (clams)
Reproduction
How Do Mollusks Reproduce?
Most mollusks lay eggs, from which larvae hatch.
How Can You Distinguish Mollusks From Other Animal Species?
What invertebrates generally share with other animal species is the radula. They use it to scrape, shred, and ingest food. Mollusks lack a skeleton. Their bodies are primarily composed of soft flesh held together by muscles and skin (known as the "mantle"). Not all have a hard calcium shell on the outside, like octopuses. Additionally, snails do not always carry a shell, as seen in slugs.
Mollusc Species
There are more than 130,000 species of molluscs:
| Snails: | 100,000 |
| Mussels: | 10,000 |
| Cephalopods/squids: | 1,000 |
| Chitons: | 900 |
What Are Echinoderms?
Characteristics and Special Features of Echinoderms:
- Calcium skeleton: They have a tough skin made of calcium plates and often striking spines.
- Marine inhabitants: Echinoderms exclusively live in saltwater (such as starfish and sea urchins).
- Five-rayed symmetry: Their body structure is typically star-shaped or rounded, divided into five sections.
- Water vascular system: They possess a unique canal system (ambulacral system) for movement.
- Reproduction: Most species lay eggs, from which larvae hatch in open water.
The Evolution of Echinoderms
Echinoderms evolved over 500 million years ago.
Habitat
Where Can Echinoderms Be Found?
They live exclusively in the sea. There are no echinoderms in freshwater or on land. Most species are found on the ocean floor, ranging from shallow coastal waters to great depths.
- Shallow waters (starfish)
- Deep waters (sea cucumbers)
Diet
What Do Echinoderms Eat?
Depending on the species, they consume either plants or meat. Sea urchins are omnivores or herbivores. Sea cucumbers ingest sand in hopes of finding something edible within. Starfish hunt for clams. Sea lilies and feather stars filter the water for food.
- Carnivores (starfish)
- Herbivores (sea urchins)
- Omnivores (sea Cucumbers)
- Filter feeders (sea Lilies)
Movement
How Do Echinoderms Move?
Echinoderms have a unique method of locomotion: they move using their own water vascular system. They can:
- swim (brittle stars)
- walk (sea urchins)
- use their tube feet (starfish)
Reproduction
How Do Echinoderms Reproduce?
Echinoderms lay eggs.
Echinoderm Species
There are about 6,300 species of echinoderms:
| Brittle stars: | 2,000 |
| Starfish: | 1,500 |
| Sea cucumbers: | 1,200 |
| Sea urchins: | 950 |
| Sea lilies: | 620 |
| Feather stars:: | 550 |
What Are Crustaceans?
Characteristics and Special Features of Crustaceans:
- Invertebrate arthropods: Crabs belong to the arthropods and lack a backbone.
- Chitin exoskeleton: Their bodies are protected by a hard outer skeleton that is shed as they grow.
- Gill breathing: Most crustaceans extract oxygen from water using gills.
- Limbs: They have two pairs of antennae (feelers) and often possess claws for defense.
- Development: Crabs hatch from eggs and typically go through various larval stages.
The Evolution of Crucstaceans
Crabs have existed for over 500 million years on Earth and are related to insects. They breathe using gills, even those crabs that live on land.
Habitat
Where Can Crustaceans Be Found?
Crabs inhabit all bodies of water on Earth, with a few species also found on land.
- Saltwater (krill)
- Freshwater (crayfish)
- On land (woodlouse)
Diet
What Do Crustaceans Eat?
Crustaceans are primarily omnivores. They feed on algae, aquatic plants, insect larvae, worms, tiny crustaceans, fish, crabs, and snails.
- Omnivores (hermit crabs)
- Carnivores (mantis shrimps)
- Herbivores (coconut crabs)
- Filter feeders (barnacles)
Movement
How Do Crustaceans Move?
Crabs either walk or swim. An exception is the barnacle, which as a larva swims but remains stationary for its entire adult life. Therefore, crustaceans can:
- walk (lobsters)
- swim (shrimps)
- stay put (barnacles)
Reproduction
How do crustaceans reproduce?
Crabs develop from eggs and initially exist as larvae, which then transform into adult creatures.
Crustacean Species
There are about 67,000 species of crustaceans:
| Malacostracae: | 28,000 |
| Mussel shrimps: | 15,000 |
| Copepods | 12,000 |
| Tongue worms: | 130 |
| Fish lice: | 130 |
| Rempipeda: | 24 |
| Horse shoe shrimps: | 11 |
