Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in which their body consists of a hollow sphere of cells, the blastula, during embryonic development.
Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 micrometers (the miniature crustacean Stygotantulus stocki) to 33.6 meters (the blue whale).
Animals are categorized into 8 major groups, based on common characteristics like body structure, digestion, locomotion, reproduction etc. The major animal groups are:
Sponges
Sponges are simple animals that live in aquatic environments and have porous bodies. There are over 10,000 known species of sponges. They live attached to surfaces like rocks and corals. Sponges filter and consume food particles suspended in water through their porous system. They do not have differentiated tissues or organs. While most reproduce sexually, some can reproduce asexually as well through budding. Sponges are considered to be the sister group of all other animals.
Key Features
- Lack true tissues and organs
- Poriferans – have holes or pores all over the body
- Filter feeders – consume food particles from water
- Asexual and sexual reproduction
- Simplest multicellular animals
Cnidarians
Cnidarians include animals like jellyfish, sea anemones and corals. There are over 11,000 species of cnidarians. They are aquatic animals found in both freshwater and marine environments. Cnidarians possess stinging cells called cnidocytes that help in capturing prey. They have a sac-like unbranched body with a single opening that serves as both mouth and anus. While some cnidarians live alone, others form colonies like coral reefs. Cnidarians have specialized tissues but lack organs. They exhibit radial symmetry.
Key Features
- Radial symmetry
- Diploblastic – two embryonic layers
- Stinging cells – cnidocytes
- Sessile or free-swimming
- Predatory – kill prey using stinging cells
Flatworms
Flatworms, or platyhelminthes, are soft-bodied invertebrates that live in marine, freshwater and damp terrestrial environments. There are over 25,000 species of flatworms. Most are predators or scavengers feeding on live or dead animals. Free-living flatworms have bilateral symmetry while parasitic ones are asymmetrical. They have a rudimentary digestive cavity with a single opening that serves as both mouth and anus. Some flatworms exhibit primitive cephalization – concentration of nervous tissues at the anterior end. Flatworms lack circulatory and respiratory systems.
Key Features
- Bilateral symmetry
- Triploblastic – three embryonic layers
- Acoelomates – lack body cavity
- Have simple eyespots
- Exhibit primitive cephalization
Roundworms
Roundworms or nematodes constitute one of the most diverse groups of animals with over 25,000 species identified so far. They are typically elongated and cylindrical in shape resembling a worm, hence the name. Roundworms are pseudocoelomates as they have a body cavity that is not completely lined by mesoderm. They have a complete digestive system with specialized excretory and nervous systems. Roundworms are extremely widespread and can be free-living in soil or parasitic in plants and animals.
Key Features
- Elongated cylindrical bodies
- Pseudocoelomates – partial body cavity
- Digestive system with specialized excretory system
- Live in diverse habitats including as parasites
- Hermaphrodites – can self fertilize
Molluscs
Molluscs, or mollusks, include shelled invertebrates like snails, slugs, clams, octopuses and squids. There are over 100,000 recognized living species of molluscs. They have soft unsegmented bodies and secrete a calcareous shell for protection. Molluscs are triploblastic protostomes with organ systems like a circulatory, digestive and nervous system. They inhabit marine and freshwater habitats. Some terrestrial molluscs have adapted to live on land as well.
Key Features
- Soft unsegmented bodies
- Secrete calcareous external shells
- Triploblastic protostomes
- Have organ systems – digestive, circulatory etc.
- Inhabit diverse aquatic and terrestrial habitats
Segmented worms
Segmented worms or annelids are a large group of over 22,000 species that include earthworms, leeches and polychaetes. They have elongated bodies divided into segments. Each segment has its own set of excretory, circulatory, nervous and reproductive organs. This internal segmentation allows them great flexibility and adaptability to diverse environments including terrestrial, marine and freshwater habitats. Segmented worms play vital ecological roles including biomass production, nutrient cycling and serving as food source for other animals.
Key Features
- Metamerism – body divided into segments
- Setae – bristles extending from segments
- Moist skin for gas exchange
- Closed circulatory system
- Play vital ecological roles
Arthropods
Arthropods are the most diverse and numerous animal phylum with over 1 million described species including insects, crustaceans, arachnids and myriapods. They have segmented bodies with paired, jointed appendages. The hard exoskeleton provides protection as well as points for muscle attachment enabling efficient locomotion. Adaptability of body segments and appendages contribute to vast diversity of arthropods. They inhabit terrestrial, aquatic and aerial habitats. Ecologically vital, they function as herbivores, predators, parasites and more in food chains.
Key Features
- Segmented bodies and appendages
- Hard exoskeleton
- Open circulatory system
- Most diverse and abundant animal phylum
- Inhabit diverse habitats
Echinoderms
Echinoderms are exclusively marine organisms comprising about 7,000 living species. This includes animals like sea stars, sea urchins and sea cucumbers. Adult echinoderms exhibit pentaradial symmetry and have an endoskeleton made of calcareous plates. They have a unique water vascular system for locomotion, feeding and gas exchange. Echinoderms are important members of aquatic ecosystems including coral reefs. Most echinoderms are mobile and play vital ecological roles as predators and filter feeders.
Key Features
- Endoskeleton of calcareous plates
- Unique water vascular system
- Pentaradial symmetry as adults
- Important members of aquatic ecosystems
- Mobile predators, grazers and filter feeders
Chordates
Chordates comprise over 95,000 species of animals and include several familiar vertebrates like fish, amphibians, reptiles, birds and mammals along with lancelets and tunicates. They are defined by the presence of a notochord, dorsal nerve cord and pharyngeal slits during some stage of their development. Chordates exhibit bilateral symmetry and are mobile with muscular bodies. They have specialized organ systems and higher intelligence compared to other invertebrates. Chordates occupy both aquatic and terrestrial habitats. The vertebrates form the most advanced group of chordates.
Key Features
- Notochord, nerve cord and pharyngeal slits
- Bilateral symmetry
- Muscular bodies
- Well developed organ systems
- Higher intelligence and adaptability
Table Summarizing the 8 Major Animal Groups
Group | Example Species | Key Features |
---|---|---|
Sponges | Sea sponges | Lack tissues and organs, filter feeders |
Cnidarians | Jellyfish, corals | Stinging cells, radial symmetry |
Flatworms | Planaria, tapeworms | Bilateral symmetry, simple eyespots |
Roundworms | Nematodes, pinworms | Pseudocoelomates, parasitic |
Molluscs | Snails, clams, octopuses | Secrete shells, organ systems |
Segmented worms | Earthworms, leeches | Metamerism, setae |
Arthropods | Insects, spiders, crabs | Exoskeleton, jointed appendages |
Echinoderms | Sea stars, sea urchins | Endoskeleton, water vascular system |
Chordates | Fish, birds, mammals | Notochord, higher intelligence |
Conclusion
Based on body plan, symmetry, presence of tissues and organ systems, mode of reproduction and other characteristics, animals can be classified into 8 major groups – sponges, cnidarians, flatworms, roundworms, molluscs, segmented worms, arthropods, echinoderms and chordates. These animal groups showcase the incredible diversity of the kingdom Animalia, occupying an amazing variety of ecological niches on Earth. From the simplest sea sponges to highly advanced mammals like humans, animals continue to fascinate and amaze us!