9 Major Groups of Invertebrate Animals

In contrast to bony vertebrates, invertebrates are animals with no vertebral column or backbone. Invertebrate animals are very diverse animal forms composing about 98% of the animal kingdom. Ordinarily, invertebrate animals are soft-bodied animals having no rigid and developed internal skeleton but usually have a hard exoskeleton. Their hard exoskeletons serve as body protection from the environment. Some examples of invertebrates are snails, sponges, earthworms, squids, sea stars, insects, butterflies, spiders, and jellyfish.

What Is Phylum Porifera?

Phylum Porifera commonly knows as sponges are one of the most common types of invertebrate animals. There are about 3000 species of sponges present in marine and aquatic life. The name phylum comes from the Latin word "Porus" which means pore and "Ferre" which means to bear. It is named as such because sponges are full of holes, or pores.

Porifera are multicellular animals consisting of tissues but have no organs and is present in the ocean distinctly attached to coral reefs, rocks, and shells. However, sponges have an indirect negating effect on humans. Sponges sometimes grow on oysters, covering their shells and thus preventing them from getting food. When this happens, it results in the death of oysters. Therefore, such cases may affect the oyster industry.

What Is Phylum Cnidarian?

Cnidarians or Coelenterates are jelly-like aquatic invertebrate animals radially symmetrical having tentacles encircling the mouth at one end of the body. In a life cycle of a Phylum Cnidarian, they appear in two fundamental forms - the sessile and free-swimming medusa. Sessile resembles a cylindrical polyp while a free-swimming medusa looks like a jellyfish.

Cnidarians come from the Greek word "Knide" which means nettle or stinging. The word pertains to cnidocytes that contain these stinging nematocysts. Nematocysts contain a poison which paralyzes and sometimes kills the animals hit by them. However, jellyfishes which belong to the group are not harmful to people, but some are. Other examples of Cnidarians are corals, hydra, and sea anemones.

Examples of Phylum Cnidarians (Coelenterates)

• There are various kinds of jellyfishes in the world. Jellyfishes differ in shape, sizes, and color. There are jellyfishes with short tentacles, and some have long. Some jellyfishes have limbs with nematocysts.
• There are about 9200 species of Phylum Cnidarians. Hydra which also belongs to Phylum Cnidaria (formerly Coelenterata) is the tiny counterpart of jellyfish living in freshwater. Hydra mirrors a jellyfish vertically indicating its tentacles are above the body.
• Besides jellyfish and hydras, Cnidarians include corals and sea anemones. The corals you ordinarily see along beaches are the exoskeletons of tiny Cnidarians which used to live there. Corals are major sanctuary and nursery place of fish and other marine organisms.

What Are Phylum Platyhelminthes?

Phylum Platyhelminthes are structurally simple wormlike invertebrate animals having no anus and circulatory system. Phylum Platyhelminthes comes from the Greek terms "Platys" which means flat and "Helmin" which means worm. There are about 15000 species of flatworms. These animals are diverse, having bodies flattened from the back, dorsal side, to the belly-side, or ventral side. Some examples of Phylum Platyhelminthes are Planaria or flatworms, Flukes, and Tapeworms.

Examples of Phylum Platyhelminthes (Flatworms)

• Planaria or flatworms are free-living invertebrate flatworms which live in the freshwater. The scientific name of this flatworm is "planaria maculata." The adult Planaria is about one centimeter in length or longer.
• A fluke is a group of parasitic worms. Some flukes live as ectoparasites, and some as endoparasites in various parts of the host's body including the intestines, blood, and liver. Flukes have a very simple digestion cavity.
• Tapeworms are endoparasites wherein adults of most of them live in the organs of the host. Tapeworms, which feed on the digested food of the host, have no digestive cavity at all. Unlike the free-living flatworms, the bodies of flukes and tapeworms blend into the parasitic life.

What Is Phylum Echinodermata?

Phylum Echinodermata is an invertebrate group of marine animals which has spiny skins. Phylum Echinodermata comes from the Greek term "Echinos" which means hedgehog and "Derma" which means skin. A hedgehog is an animal which has many spines. There are about 5000 species of echinoderms in the world. This group has a radial symmetry in which their bodies have five equal parts encircling a central axis.

Examples of Phylum Echinodermata (Echinoderms)

• Starfishes are the examples of Echinoderms that are present in shallow water near the shore. They come in all colors, shapes, and sizes. There are blue, bright red, reddish brown, and flesh-colored starfishes. They usually have five arms, but there are those having more than ten long and slender arms encircling a small central disk. There are also those having no visible arms at all, shaped like a ball.
• Sea-urchins are the spiniest of all Echinoderms. There are several kinds of sea-urchins. The most common sea-urchins are the black sea-urchins with short spines. The black and purplish sea-urchin with a relatively small body and very long spines are dangerous. Their spines are poisonous.
• Brittle stars are also Echinoderms having 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 regeneration, brittle stars regenerate their chopped arms.
• Sea cucumbers are Echinoderms which 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 when disturbed. It serves as the sea cucumber's attack tactics and defense from its attackers.

What Is Phylum Mollusca?

Phylum Mollusca is a group of invertebrate animals found both in water and land. Phylum Mollusca comes from the Latin term "Mollis" which means soft. Meaning to say, Mollusca means soft-bodied animals. Most mollusks have a hard shell protecting their soft bodies from attackers and ventral muscular foot. There are about 70000 species of mollusks in the world.

Examples of Phylum Mollusca (Mollusks)

• Mollusks Shells are examples of Phylum Mollusca. Mollusks shells are mollusks having soft body protected by an exoskeleton made of calcium carbonate. Mollusks shells have two categories - Univalves and Bivalves. Clams or mollusks shells have ventral muscular foot used for burying themselves into the sand or mud.
• Snails are abundant on rocks at the shore varying in shapes, sizes, colors, and design. Snails have ventral muscular foot used for creeping over surfaces.
• Squids and octopuses are examples of mollusk having no outer shell. The body wall of these species is hard compared to that of shelled mollusks. The muscular foot of the squid has ten arms used in catching preys.

What Is Phylum Nematoda?

Phylum Nematoda is a group of invertebrate animals commonly known as Nematodes. Nematodes are unsegmented roundworms that are elongated and slender. Phylum Nematoda comes from the Greek term "Nematos" which means thread. Nematodes live in a wide variety of environments - soil, fresh water, salt water, and in bodies of plants and animals as parasites.

Examples of Phylum Nematoda (Nematodes)

• Ascaris is one of the easiest Nematodes to find in pigs. Ascaris lives in the intestines of pigs and usually in large numbers. Its scientific name is Ascaris lumbricoides. Ascaris, known as the common roundworm, more often affect children. That is because children are generally less careful in their health habits.
• Vinegar eel is not an "eel." It is a nematode with a scientific name of Turbatrix aceti. Generally, vinegar eels are smaller in length and size than Ascaris.
• Other examples of Nematodes are Filaria worms, hookworms, and pinworms and are acting as parasites in the human and animal bodies.

What Is Phylum Arthropoda?

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

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

Phylum Arthropoda comes from the Greek terms "Arthron" which means joint and "Pous" which means foot. Insects, spiders, millipedes, centipedes, crabs, barnacles, scorpions, and ticks belong to Phylum Arthropoda.

What Is Phylum Annelida?

Phylum Annelida is an invertebrate group of animals with segmented muscular bodies. Phylum Annelida comes from the Latin term "Annulus" which means ring. The rings refer to the ringlike segments of the body. The digestive system of Annelids stretches from the mouth to the anus, differentiated with different regions, each of different role.

Examples of Phylum Annelida

• Earthworms belong to the group Phylum Annelida. These are abundant in the soil. They have a cylindrical body that is segmented. These annelids are the most essential to people. They help enrich the soil by allowing the air to enter as it burrows through the earth.
• Leeches are another common annelid. Scientists call leeches as Hirudo medicinalis. It serves as a natural medicine for sucking blood from some patients.
• Other examples of Phylum Annelida are the marine sandworm, Nereis virens, and Aelosoma.

What Is Phylum Chordata?

Phylum Chordata is a group of invertebrate animals which have backbones, or the vertebrates. That is because most of the members of this phylum do not have spines. However, some chordates do not have spinal columns. Phylum Chordata comes from the Greek word "Chorde" which means string. Chordates have notochord structures which support their bodies.

Examples of Phylum Chordata

• Sea squirts are animals that squirt water from openings in the body cover when touched suddenly. Adult sea squirts live attached to objects underwater near the beach. Sea squirts refer to the "tunic" which is a muscular coat over the body. The term for the members of this subphylum is tunicates.
• Lancelets or amphioxus are another chordates. It is one of the several species of fishlike chordates that are slender. It lives at the edge of the sea. They burrow in the sand reached by water, with their anterior ends above the surface.