9 Major Groups of Invertebrate Animals

In contrast to bony vertebrates, invertebrates are animals with no vertebral column or backbone. Invertebrates comprise approximately 97% of the animal kingdom. Most invertebrate animals are soft-bodied and do not develop rigid internal skeletons. Many invertebrates do, however, possess hard exoskeletons to protect their bodies from their environments. Some common examples of invertebrates are snails, sponges, earthworms, squids, sea-stars, centipedes, butterflies, spiders, and jellyfish.

1. Phylum Porifera (Sponges)

Sponges, or phylum Porifera, are one of the most common types of invertebrate animals. Currently, there are about 3,000 documented sponge species. The phylum name comes from the Latin words porus, which means "pore," and ferre which means "to bear." It is so named because most sponges bear holes.

Porifera are multicellular animals that consist primarily of tissue and lack complex organs. Most reside in the ocean anchored to coral reefs, rocks, or shells. Sponges occasionally grow on oysters, cover their shells, and prevent them from feeding. This can result in the deaths of the affected oysters and may have an impact on the oyster industry.

2. Phylum Cnidaria (Coelenterates)

Cnidarians or Coelenterates are jelly-like aquatic invertebrate animals that are radially symmetrical and have tentacles that encircle a mouth at one end of the body. The life-cycle of a cnidarian includes multiple distinct stages. During the sessile stage, cnidarians resemble cylindrical polyps, while during the medusa stage, they are free-swimming and look like jellyfish.

The name Cnidaria is derived from the Greek word knide, which means "nettle" or "stinging." The word refers to specialized stinging cells called nematocysts that can be projected by certain cnidocytes as a defense. Nematocysts contain poison that can paralyze and sometimes kill other animals. Other examples of cnidarians include corals, hydra, and sea anemones.

Examples of Cnidarians (Coelenterates)

• Jellyfish differ in shape, size, and color. Some have shorter tentacles, some have longer tentacles, and some have limbs with nematocysts.
• Hydra are small, freshwater organisms that resemble jellyfish. Interestingly, hydra have regenerative properties and don't seem to "age" in a traditional sense.
• Corals live in large colonies that serve as sanctuaries and nurseries for fish and other marine organisms.

3. Phylum Platyhelminthes (Flatworms)

Platyhelminthes are structurally simple, wormlike invertebrate animals that lack anuses and circulatory systems. The name Platyhelminthes derives from the Greek terms platys, which means "flat," and helmin, which means "worm." There are about 15,000 known species of flatworm. Flatworms' bodies are flattened from the back, or dorsal side, to the belly, or ventral side.

Examples of Platyhelminthes (Flatworms)

• Planaria maculata is a free-living invertebrate that occupies freshwater habitats. Adult Planaria are usually about one centimeter in length but can be longer.
• Flukes are parasitic worms that live as ectoparasites or endoparasites in various parts of their hosts' bodies, including the intestines, blood, and liver. Flukes have a very simple digestive cavity.
• Tapeworms are endoparasites that usually live in the organs of their hosts. They feed on their hosts' digested food but do not have digestive cavities of their own.

4. Phylum Echinodermata (Echinoderms)

Phylum Echinodermata is a group of invertebrate marine animals that have spiny skins. The word Echinodermata comes from the Greek terms echinos, which means "hedgehog," and derma, which means "skin." There are about 5000 species of echinoderms in the world. Members of this group display radial symmetry; most individuals' bodies are divided into five equal parts that encircle a central axis.

Examples of Echinodermata (Echinoderms)

• Starfish inhabit the shallow waters near ocean shores. They come in a variety of colors, shapes, and sizes. There are blue, bright red, reddish-brown, and flesh-colored starfish. They usually have five arms, but some have more than ten. Their arms encircle a small central disk. Some have no visible arms at all and are shaped like balls.
• Sea-urchins are the spiniest of all echinoderms. There are several kinds of sea-urchins. The most common sea-urchins are black and have short spines. Some black and purplish sea-urchins with relatively small bodies and very long spines are dangerous. Their spines can irritate or injure human skin.
• Brittle stars sport non-dangerous spines. However, their arms are fragile, so it is not advisable to hold or touch them. That is how the brittle star got its name. Through a process known as regeneration, brittle stars are able to regrow lost arms.
• Sea cucumbers are not spiny-skinned. They have an ovoid body, similar to the shape of a real cucumber. Sea cucumbers have a habit of shooting out milky and sticky fluid as a defense tactic when disturbed.

5. Phylum Mollusca (Mollusks)

Phylum Mollusca is a group of invertebrate animals whose members can be found both in water and on land. The word Mollusca comes from the Latin term mollis, which means "soft." Because mollusks are soft-bodied animals, most have hard shells made of calcium carbonate to protect their bodies from predators and the environment. Shelled mollusks are divided into two categories: univalves and bivalves. Most also have a muscular ventral foot. There are about 70,000 species of mollusks in the world.

Examples of Mollusca (Mollusks)

• Snails are abundant on land and in water and vary in shape, size, color, and design. Snails have muscular ventral feet that they use to move themselves over surfaces.
• Squids and octopi are examples of mollusks that don't have outer shells. The skin of these species is more durable than that of shelled mollusks. Squids have ten muscular arms that they use to capture prey.

6. Phylum Nematoda (Nematodes)

Nematoda is a group of invertebrate animals commonly known as nematodes. Nematodes are unsegmented roundworms that are elongated and slender. The word Nematoda comes from the Greek term nematos which means "thread." Nematodes live in a wide variety of environments, including soil, freshwater, saltwater, and in the bodies of plants and animals as parasites.

Examples of Nematoda (Nematodes)

• Ascaris lumbricoides is one of the easiest nematodes to find in pigs. Ascaris usually live in large numbers in pigs' intestines. They are also known as common roundworms. In humans, they affect children more often than adults. This is thought to be because children are generally less careful than adults in their hygiene habits.
• Vinegar eels are not actually eels. They are nematodes whose scientific name is Turbatrix aceti. Generally, vinegar eels are smaller than Ascaris.
• Filaria worms, hookworms, and pinworms are all common nematodes that act as parasites in human and animal bodies.

7. Phylum Arthropoda (Arthropods)

Phylum Arthropoda is a group of invertebrate animals with jointed limbs and exoskeletons made of chitin. It is the largest phylum in the animal kingdom.

There are an estimated 750,000 species in this phylum. Of this number, 700,000 species belong to Class Insecta, 25,000 belong to Class Crustacea, 15,000 belong to Class Arachnida, 800 belong to Class Chilopoda, and 200 belong to Class Diplopoda.

The word Arthropoda comes from the Greek terms arthron, which means "joint," and pous, which means "foot." Butterflies, spiders, millipedes, centipedes, crabs, barnacles, scorpions, and ticks all belong to Phylum Arthropoda.

8. Phylum Annelida (Annelids)

Phylum Annelida is a group of invertebrate animals with segmented, muscular bodies. The name Annelida comes from the Latin term annulus, which means "ring." This name refers to the ringlike segments of their bodies. The digestive systems of annelids stretch from the mouth to the anus, and different sections of the system have different roles.

Examples of Annelida (Annelids)

• Earthworms belong to the phylum Annelida and are abundant in soil. They have cylindrical, segmented bodies. These annelids are beneficial to humans—they help enrich the soil by allowing the air to enter as they burrow through the earth.
• Leeches, or Hirudo medicinalis, are another common annelid species. In the past, they have been used to remove blood from individuals for medical purposes.
• Marine sandworms, Nereis virens, and Aelosoma are also annelids.

9. Phylum Chordata (Chordates)

While many species in Phylum Chordata have backbones, there are some that are invertebrates. The name Chordata comes from the Greek word chorde, which means string. While invertebrate chordates don't have backbones, they do have notochord structures that support their bodies.

Examples of Invertebrate Chordates

• Sea squirts are animals that squirt water from openings in their body covers when they are touched suddenly. Adult sea squirts live underwater attached to harder objects near beaches. Sea squirts have muscular coats over their bodies that are referred to as "tunics". Members of their subphylum are called tunicates.
• Lancelets, or amphioxus, is another type of invertebrate chordate. These slender, fish-like chordates live at the edge of the sea and burrow in the sand.