Types of Invertebrates

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Annelida -> Segmented worms with a muscular body wall used for burrowing. External hairs called setae aid in traction during burrowing. An internal coelom is divided into compartments by walls known as septum. The digestive system stretches from the mouth to the anus, differentiated into regions, each with a different function. Reproduction is sexual. – Examples: Lugworms, earthworms, leeches

Pogonophora -> Deep-sea worms that live in chitinous tubes attached to the ocean floor. Their long, slender body has a beard of tentacles at the head end. There is no mouth or digestive system and these animals absorb all nourishment through the body surface. They reproduce sexually. – Examples: Bead worms

Vestimentifera -> Giant deep-sea worms that live in chitinous tubes attached to the ocean floor. They derive nutrition using a specialized organ called a trophosome to digest sulfide-oxidizing bacteria. – Examples: Llamellibrachs

Sipuncula -> Marine worms with a saclike body and a long proboscis. These worms withdraw their narrow head into the fatter posterior portion. The head end bears tentacles used in burrowing and gathering food particles. Lacking a cardiovascular system, they use internal fluid to transfer oxygen and food to body tissues. They reproduce sexually and some have a larval form. -> Peanut worms

Echiura -> Plump marine worms that take shelter in sand burrows or rock crevices. They use a mucous net or a scoop-shaped proboscis to capture food particles. Reproduction is sexual. – Examples: Spoon worms

Priapulida -> Cucumber-shaped, marine worms with spiny heads. During movement the barrel-shaped proboscis withdraws into the trunk of the worm. Reproduction is sexual. – Examples: Priapulus

Tardigrada -> Microscopic animals with four pairs of stubby legs that live in marine and freshwater sediments and on the surface of mosses and lichens. They use sharp stylets protruding from their mouths to suck food from plant cells. These animals have a remarkable ability to withstand extreme dryness and low temperatures. Reproduction is sexual. – Examples: Water bears

Pentastomida or Linguatulida -> Parasitic worms that live in the lungs of snakes, crocodiles, and some mammals and birds, feeding on blood and tissue. The head bears four leglike claws and a snoutlike mouth. The body is covered by a cuticle that is molted during larval development. They reproduce sexually. – Examples: Tongue worms

Phoronida -> Cylindrical, marine worms that live in a chitinous tube embedded in sand or attached to rocks, shells, or other objects in shallow water. Protruding out of the tube is the animal's lophophore, a structure of ciliated tentacles arranged in a horseshoe shape, that is used for feeding. They have a U-shaped digestive tract. Adults are sedentary and larvae are free-swimming. Reproduction is asexual in at least one species, but most species reproduce sexually. – Examples: Horseshoe worms

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Tardigrada

Tardigrade

Tardigrade, any of a group of tiny arthropod-like animals characterized by four pairs of stubby legs ending in large claws, a stout body, rounded back, and lumbering gait. They are also known as “water bears.” Tardigrades live worldwide in moist land habitats, along rocky shorelines, and on the bottoms of streams, lakes, and oceans. Their most striking feature is their ability to temporarily enter a nearly lifeless state known as anabiosis, which helps them survive environmental extremes. About 400 species of tardigrades are known and they range in length from 0.1 to 0.5 mm (0.004 to 0.02 in).

Tardigrades are most commonly found in association with the water film on mosses, liverworts, and lichens. Their densities may reach 2 million individuals per sq m (185,874 per sq ft) of moss. Some live in hot springs and a few live in a symbiotic relationship with or on the bodies of other animals. Tardigrades use a pair of oral stylets, or needle-like mouthparts, to pierce the walls of plant cells and feed on the liquid inside. Most species are plant eaters, but some are predators, feeding on tiny invertebrates and bacteria, and a few are detritivores, feeding on dead tissue and debris.

Scientific classification: Tardigrades make up the phylum Tardigrada. It is not clear to what other animals tardigrades are most closely related, but they are most similar to arthropods (insects, crustaceans, etc.), annelids (segmented worms), and gastrotrichs.