The velvet worm is a furry, caterpillar-like worm, covered in a thin chitinous skin, with a pair of slender antennae on the head. Just behind the antennae are simple eyes. Roughly 43 pairs of stubby, clawed legs, allow the velvet worm to walk over almost any surface.
The anatomy of velvet worms is different from arthropods as it does do not have a chitinous outer skeleton. Their body is filled with fluid and functions as a skeleton. The outer skin is a thin, water-resistant cuticula. Beneath the outer skin is a single layer of ectodermal cells that secretes a new cuticle. Moulting of the skin takes place every 14 days.
Velvet worms are haemocoels, which means they do not have a vascular system. All organs are situated in a blood-filled body cavity. A muscular tubular heart pumps blood to all parts of the body. Breathing is through holes called spiracles that lead to fine tubes or tracheae, which bring oxygen to the different organs. Spiracles are always open, which can easily result in dehydration. For this reason, Onycophorans are dependent on humid environments.
In the mouth cavity is a pair of jaws, surrounded by ridged lips. Each jaw is a papilla with chitinous teeth. The roof of the mouth cavity forms the tongue which has a row of small teeth on its surface. Slime-glands open at the ends of oral papillae.
When prey is caught, it is dismembered and dissolved. The muscular throat is the start of the intestine and contains two salivary glands. Liquefied food is transported via the throat to the oesophagus and into the central intestine. Further digestive enzymes are secreted and the released nutrients are absorbed. Indigestible remnants discarded through the anus, which is underneath the body and at the rear. Accumulated waste products are excreted by the nephridiopore found at the beginning of each leg.
The entire body, including the stub feet, is covered by numerous sensitive bristles that are responsive to mechanical stimuli. Each bristle is connected to sensory nerve cells. Sensory cells are found on the antennae and at the openings of the two slime glands in the mouth papillae.
Locomotion is achieved by conical stubby legs that have a retractable foot, which end in a pair of retractable claws and three tubercles.
Behind each antenna is a simple eye that is composed of a lens and a cornea. The retina consists of differentiated cells such as photoreceptors and is connected to an area of the brain that deals with visual processing.
Both sexes have a genital opening called the gonopore, located underneath and towards the back. Females have two ovaries. In the anatomy of live-bearing worms, each ovary branches into a separate oviduct and a separate uterus where embryos develop. Both uteri lead into a vagina that opens to the gonopore. Egg laying species have an egg laying organ called the ovipositor. After the ovipositor is the gonopore through which eggs are laid.
Males have two testes, along with sperm vesicles that widen into an ejaculatory channel, leading to gonopore. The genus Paraperipatus is the only species recorded thus far to have a penis-like organ.
Scientists have described the anatomy of velvet worms as having many characteristics common with segmented worms such as earthworms and arthropods including crabs, spiders and scorpions.