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Introduction Arthropoda


Invertebrate Zoology


Kingdom: Animalia, Phylum: Onychophora




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






Introduction Arthropoda