Onychophora

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Velvetworm, any of a group of soft-bodied, many-legged, worm-like animals known for their velvety cuticle, or outer covering, and their resemblance to both arthropods (insects, spiders, and crustaceans) and annelids (segmented or true worms). They live in moist tropical and temperate habitats south of the tropic of Cancer. Velvetworms are also known as onychophorans or peripatuses, after the most familiar genus. About 80 species of velvetworms are known.

Mature velvetworms range from 1.5 to 15 cm (0.6 to 5.9 in) in length and may be blue, orange, green, or black in color. Their thin, flexible cuticle, or outer skin, bears many small, scaly, wartlike tubercles and sensory hairs, which produce a velvety appearance. Velvetworms have 14 to 43 pairs of stubby, claw-tipped legs called lobopods. The lobopods contain no muscles. Velvetworms control their body and limb movements with hydrostatic pressure changes within their body cavity. Velvetworms use these pressure changes to lift and lower their legs in waves, as their body creeps along with slug-like motion.

When conditions are too dry or too wet, velvetworms hide in burrows or protected niches until conditions improve. They are nocturnal, appearing only at night, and prey on other small invertebrates. The velvetworm's jaws hold the prey while it is being eaten. Salivary glands produce enzymes that partly digest tissues so the meal can be sucked up. When disturbed or threatened, velvetworms shoot a sticky, quick-hardening slime from two oral papillae, wartlike bumps on either side of the mouth. They can shoot the slime up to 50 cm (19.7 in). The slime is produced by slime glands originating at the base of the oral papillae and may also be used to immobilize prey.

The velvetworm has a pair of fleshy, ringed antennae on its head and a pair of eyes near the bases of the antennae, each with a lens and retina. Blood is circulated by a tube-shaped dorsal heart (located near the top of the body cavity) and flows freely within the body cavity around the internal organs; it carries no oxygen. Gases are exchanged through a system of minute tracheae, or breathing tubes, which open into pits located throughout the cuticle. The pits cannot be closed and are a major source of water loss for the moisture-dependent velvetworm. The velvetworm takes in water by drinking and by absorption through special ballooning sacs on the lobopods. Wastes are excreted from paired sacs called coelomoducts or nephridia, which are present on most body segments and which lead to pores on the legs. The slime glands and female sexual ducts are modified coelomoducts. The brain gives rise to a pair of ventral nerve cords, which run along the bottom of the body cavity and are connected by ladderlike cross nerves.

The sexes are separate in velvetworms; males are generally smaller and may have fewer legs than females. In the species where sexual behavior is known, fertilization occurs through copulation.

Scientific classification: Velvetworms make up the phylum Onychophora. The most familiar species are in the genus Peripatus.

Pictures of Invertebrates

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Scorpion
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Sheep Tick
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Starfish
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Tapeworm
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Velvetworm
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