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Invertebrate
Zoology Kingdom: Animalia, Phylum: Onychophora (Contact)
The velvet
worms (Onychophora —"claw bearers". Also known as Protracheata) are a minor Ecdysozoan phylum. The
segmented caterpillar-like organisms have minute eyes, antennae, many pairs of
legs and slime glands. They are common in tropical regions of the Southern
Hemisphere where they prey on small animals such as insects that they capture
by with an adhesive slime. In modern zoology, they are known for their
strange mating habits and for bearing
their young alive.
They are popular as pets due to their interesting appearance and eating
habits. They do not occur in North
America. Two families are Peripatidae and
Peripatopsidae. They show a peculiar distribution, with the peripatids being
mainly equatorial and tropical, while the peripatopsids are all found in what
was the former Gondwanaland Velvet worms have been considered
close relatives of the Arthropoda and Tardigrada, with which they form the
taxon Panarthropoda. Thus they are of
palaeontological interest in the reconstruction of the ancestral arthropod. Body Plan. -- The body is soft and many segmented, with antennae
and eye spots. They seem to be an
intermediate form between Annelida and Arthropoda. The head has one pair of anennae, one pair of jaws, one pair of
papillae and eyes that are similar to the Annelida. The legs are not segmented but stumpy like those of caterpillars. The legs end in tufts of claws. A haemocoel is present. Body Wall. -- There is a chitinous cuticle that forms the
exoskeleton, which is periodically shed and renewed. There are circular and longitudinal muscle
layers under the epidermis. Digestive System. -- This is a straight tube. Heart.
-- The heart is dorsal and with one pair of ostia. Respiration. -- Tracheae are present as in the Arthropoda. They have an open circulatory system. Excretion. -- By nephridia, one pair per segment. Nervous System. -- Similar to that of both the Annelida and
Arthropoda. Protection. -- When irritated they are able to eject a slime from
their oral papillae for up to several centimeters. This slime is also used to trap insects on which they feed. Reproduction. -- The sexes are separate and gonads
paired, but otherwise this is not related to either the Annelida nor
Arthropoda. The ducts unite to form a
median passage opening just before the anus.
In males filiform spermatozoa are bound up in spermatophores in the
upper part of the vas deferens. The
lower part is muscular and functions in ejaculations. Fertilization is normally internal but in
some species perrmatophores are deposited on the skin of the female giving hypodermic impregnation.
Ovaries are enclosed by a funnel.
They are fertilized at the proximal end of the oviduct. They are variable in size among the different
species. In large species development
occurs with yolk depletion and secretions of the uterine wall. However, embryos from smaller eggs are
attached to the uterine wall and a placenta is formed. ------------------------------------ Please see
following plates for Example Structures of the Onychophora: Plate 76 = Phylum: Onychophora -- Peripatus capensis structures Plate 77 = Phylum: Onychophora --
Fossil Terataspis from New York
Devonian ============== |