|Onychophorans - Velvet Worms
|The Onychophorans (Velvet Worms) are particularly fascinating because they are
terrestrial worms with anatomical features intermediate between those of the arthropods
(e.g. millipedes) and the annelids (e.g. polychaetes). They occur in tropical and south
temperate areas. Their adaptation to terrestrial life is only partial and they prefer damp
habitats. The body is more-or-less cylindrical and a little over 1 cm to 15 cm (6 inches0 in
length. The animal is covered in a thin flexible chitinous cuticle which is not divided into
plates and segments like those of arthropods. The ridge-like rings of cuticle around the
body do not mark-out segments, but are . The internal arrangement is segmented but with
The anterior end bears a pair of antennae, the cuticle covering of which is annulated
(marked into rings). A pair of slime papilla end in pores which connect via ducts which
sit above the gut, one on either side, to a pair of slime glands. These slime papillae can
squirt slime as far as 15 to 30 cm (6 to 12 inches) which hardens almost immediately,
entangling prey. Velvet worms feed on snails, insects, worms and the like. Salivary
secretions, from a pair of salivary glands, are passed into the prey and the partially
digested tissues sucked up by the mouth. The mouth is ventral and surrounded by the
labrum (upper lip) a pair of jaws, an outer and an inner lip and cuticular scales. These
structures form a preoral cavity which leads to the mouth.
There are 14 to 43 pair of limbs, each bearing two claws, which look superficially like
legs, but all the muscles operating the leg originate from inside the main body and these
appendages are not jointed like those of the arthropods, but are lobe-like extensions of
the body, more like the parapodia that occur in polychaetes. These leg-like limbs are
called lobopods. Velvet worms crawl slowly, the lobopods are positioned more ventrally
than the parapodia of polychaetes and hold the body off the ground. Locomotion is
assisted by extension and contraction of the flexible body: when part extends the legs
here are lifted off the ground and moved forwards. The soft flexible cuticle allows the
animals to squeeze through small spaces in earthworm-like fashion. The antennae, slime
papillae and jaws represent the first three pairs of modified appendages, the claws of the
third pair being modified into the cutting blades of the jaws.
Above: a longitudinal section through a lobopod of Peripatus, showing the muscles
entering the limb from the main body (so-called extrinsic muscles as they originate
outside the limb in the main body). The friction pads near the end of the limb can also be
seen (top right).
The ventral organs are paired areas of thickened cuticle (in the adult, they possibly
serve a different function during development) found in each segment and serve as
attachment sites for the posterior limb depressor muscles (which lower the lobopods). A
similar, but smaller, paired structure, the preventral organ occurs just in front of the
ventricular organ and serves as an attachment site for the anterior depressor muscles
which also help lower the lobopods.
The genital pad bears the genital opening (gonopore) and separates the ventricular and
proventricular organs from one-another in the penultimate lobopod-bearing segment. This
opening is cross-like in males (as above) and slit-like in females.
Homeostasis (maintaining the internal environment of the body)
A pair of small openings, the nephridiopores, also occur in each segment: on the ventral
surface, each pore near to the base of a lobopod. These are excretory pores which
connect to tubular excretory (and/or osmoregulatory) organs called nephridia. There is
one pair of nephridia per body segment. A coxal gland opens via a slit-like pore at the
base of each lobopod through which coxal sacs evaginate. These are thought to be used
to absorb moisture from the substrate.
Tubes called trachea carry oxygen to the tissues. These trachea open via numerous tiny
openings (spiracles) scattered all over the body surface in between bands of tubercles.
(The tubercles are small wart-like papillae which cover the body surface - see further
below). The spiracles can not be closed as they can in many insects. Also, each spiracle
supplies only local tissues, there is no arrangement of tracheae into bundles for long
distance transport of oxygen as there is in insects.
The cross-section below is through the hind-region of a female velvet worm. The sexes are
always separate (velvet worms are dioecious) but one species consists of parthenogenetic
females (females giving 'virgin birth' without a male to fertilise the eggs). The two ovaries are
generally fused and situated dorsally and this section is just in front of them. The ovaries each
give off a muscular oviduct (gonoduct) which fuse into a common uterus
The cuticle is covered by wart-like papillae called tubercles, each covered in cuticular
scales. These are arranged in rings around the body and appendages. Beneath the
cuticle, the remainder of the body wall is divided into the epidermis, a layer of cells which
secretes the cuticle, a dermis (stained blue above), a layer circular and diagonal
muscles (visible on the right in the above section) and then a thick layer of longitudinal
DV = dorsal vessel (heart)
I = intestine
LH = lateral haemocoel
LM = longitudinal muscle
NC = nerve cord (x 2)
NT = nephrocytic tissue 9part of nephridial system)
O = oviducts (gonoducts)
P = pericardium (pericardial sinus) enclosing the
PH = perivisceral haemocoel, a fluid-filled cavity
bathing the organs
TM = transverse muscle (dorsoventral muscle),
dividing the perivisceral haemoceol from the lateral
U, my best guess is that this is the uterus (or seminal
receptacle which some species have) containing
received spermatozoa: Peripatus has a small seminal
receptacle attached to each oviduct.
The 'heart' is, as in arthropods, a contractile dorsal vessel which is open at both ends and
bears a pair of lateral pores or ostia in each body segment. The 'blood' is colourless and
better referred to as haemolymph. Haemolymph enters the ostia to be pumped by waves of
peristaltic contractions in the muscular dorsal vessel. Blood leaves the dorsal vessel through
the anterior end, entering the haemocoel, which is a system of sinuses which conveys the
heamolymph backwards where it re-enters the dorsal vessel via the ostia. Channels connect
the various sinuses or haemolymph cavities together, for example, the lateral haemocoels are
separated off from the perivisceral haemocoel by transverse muscles, through which openings
connect the lateral haemocoels to the perivisceral haemocoel. The blood plays a minimal
function in oxygen transport which is carried out by the tracheae. Haemolymph also flows
through a system of channels beneath the cuticle: the hemal channels which are situated
beneath the ridges and rings of the cuticle (see below).
Above: haemal channels can be found beneath the cuticle, here seen as a white space
beneath the dermis with a few cell (haemocytes) within it.
A pair of ventral longitudinal nerve cords run the length of the animal, each confined to a
lateral haemocoel (see close-up view below). Note the cell bodies located on the outside and
the fibrous mass of axonal fibres (neuropil) more centrally located.
Pairs of fused ganglia make up the brain (cerebral ganglia) which is situated right at the front
of the head, largely anterior to the mouth and dorsal and is connected to the two ventral nerve
cords by transverse commissures. The dorsal cords are connected by transverse
connectives, like rungs in a ladder, and in each segment paired swellings (segmental
ganglia rather like local brains) give off nerves to supply the segment.
Reproduction and Development
Males have a pair of elongate testes, each of which opens into a sperm duct. The two sperm
ducts fuse into a common duct which where sperm are packaged into spermatophores (up to
1 mm long). During mating the males usually deposit spermatophores (via their gonopore)
randomly over the surface of the female. This triggers phagocytes to break down tissue in the
female body wall beneath the spermatophores and the sperm enter her haemocoel and then
find the ovaries. In other species, the male deposits the sperm inside the uterus of the female
which may be enlarged as a seminal receptacle to store sperm until needed.
The female has a pair of fused ovaries, each of which opens into an oviduct. The two oviducts
fuse into the uterus which opens to the outside via the gonopore. Alternatively, in some species
the uteri may remain separate and then fuse into a common vagina which opens via the
oviduct, as in Peripatus. In some species the female lays large yolky eggs through an
ovipositor. In others the eggs are retained and hatch inside or else live young are produced
which may be attached to the wall of the oviduct via a placenta and feed at the expense of the
mother. The hatchlings or juveniles have the full complement of adult body parts and organs
and develop directly into adults.
There is one pair of simple eyes or
ocelli: one ocellus near the base of each
antenna. Velvet worms are nocturnal
and avoid light. The larger tuberculae or
papillae end in a sensory bristle.
These microscope sections are of the
velvet worm Peripatus.
It is difficult identifying structures in section, especially for an organism for which little literature
is readily available. A more anterior section would reveal ducts of the slime glands where the
oviducts are at this level, but these have distinctly different morphology in the literature I have
seen, which makes me think we are seeing the oviducts here. Also, an more anterior section
might reveal the pair of salivary glands which are the pair of modified nephridia of the
segment bearing the slime papillae and run along about half or so of the length of the animal in
the lateral haemocoels. When the slime glands themselves are sectioned then they appear as a
mass of tubules lateral to the gut. Slime glands, salivary glands and oviducts are all elongated
tubular organs and so may be encountered over a considerable length of the animal, making
identification of organs more difficult.
The two halves of the brain appear quite separate in this section, but they do fuse at another
Above: a section through a lobopod showing some of the subterminal and ventral friction pads
(of which there are 3 to 6 per leg) upon which the lobopod rests in contact with the substrate.
Above and below: sections through the body wall. The layers, from outside to inside are:
cuticle, epidermis, dermis, circular and diagonal muscle fibres, longitudinal muscle fibres.
The preoral cavity or depression opens via the mouth into the pharynx. Jaws, each consisting
of a pair of blade-like structures, manipulate and tear up food, which consists of small
invertebrates and carrion. The pharynx connects to the oseophagus which opens directly into
the long straight intestine, which opens into the short rectum which opens to the outside via
the ventral anus. The single pair of salivary glands are thought to be modified nephridia.