The phylum Onychophora is a small phylum consisting of strange, caterpillar-like invertebrates that share traits with both arthropods and annelids (worms).
Velvet worms, also known as walking worms or peripatus, are neither made of velvet nor are they actual worms. They are long-bodied invertebrates, roughly 0.6-5.9 in (1.5-15 cm) in length, and while they may resemble worms they have several features not present in any annelid. For example, they have between 14-43 pairs of "legs" known as lobopods, much like the false legs of a caterpillar. However, unlike caterpillars, or any other arthropod for that matter, velvet worms lack a chitinous exoskeleton. Instead, their body is covered with a thin, flexible cuticle that is not water resistant. This skin is usually blue, orange, green, black, or white in colour, and is covered with scaly tubercles and sensory hairs. These sensory hairs give the velvety appearance and hence the name.
The short, claw-tipped, hollow lobopods are kept rigid by hydrostatic pressure, as they lack muscles. The lobopods are not joined together. Locomotion is acquired through changes in hydrostatic pressure within the body, which causes the lobopod pairs to rise in waves while the body is contracted. This movement resembles that of a caterpillar, and velvet worms are equally as slow.
Ringed antennae are located on the top of the head, with eyes positioned near their base. The mouth is surrounded by six finger-like projections, two of which are mandibles and two of which are oral papillae. Both of these projections are used for hunting, as velvet worms are carnivorous in nature. They feed upon other invertebrates, including isopods, termites, and slugs. When they have found suitable prey, the oral papillae release a quick-hardening, slimy adhesive at quick speeds. They can shoot the adhesive at distance of up to 19.7 in (50 cm). When the adhesive has dried, the prey is cemented to the ground allowing the velvet worm to eat at leisure. To feed, it jabs its prey with its mandibles, and then secretes digestive enzymes into the invertebrate's body. It then sucks up the partially-digested tissues.
Internally, velvet worms have a tube-shaped heart that pumps blood through an open circulatory system. The blood, however, does not carry oxygen. Instead, oxygen is brought in through minute trachea located all over the surface of the skin. The openings to these trachea are sites of major water loss, but similarly can easily cause the drowning of a velvet worm. Thus, the phylum Onychophora is the only phylum in which none of its living members are aquatic.
Velvet worms are sexually dimorphic, with the females being larger than the males.
Fertilization may occur internally or externally, depending on the species. In the external case, the male deposits a spermatophore (sperm packet) onto the female's skin, where it is then absorbed into the body. The sperm then travels through the bloodstream to get to the ovaries.
Most velvet worms give birth to live young. The young either develop internally, being nourished via a placenta-like organ, or develop in eggs that hatch inside the female. Other species lay eggs which hatch outside of the female.
Velvet worms are found in humid, shady areas such as rotting logs, caves, or underground areas, throughout South America, Africa, and Australia. They are nocturnal.
There are between 90 and 140 known species of velvet worms (figures vary) in two families:
Peripatidae (South American velvet worms)
Peripatopsidae (Australian and African velvet worms)
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