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Onychophora (ZOOA CC-2-3-TH) Introduction Onychophora (ZOOA CC-2-3-TH) Introduction - Onychophorans or velvet worms are carnivorous, terrestrial invertebrates, which inhabit tropical and temperate forests of the southern hemisphere and around the equator, where they are mainly found in rotted logs and leaf litter (Ruhberg 1985, Reid 1996, Mayer 2007, Oliveira et al. 2012a). So far, about 200 species of Onychophora have been described (Mayer and Oliveira 2011, 2013, Oliveira et al. 2012a, 2013a). They are classified in two major subgroups, the Peripatidae and Peripatopsidae, which diverged prior to the break-up of Gondwana over 175 million years ago (Hamer et al. 1997, Allwood et al. 2010, Braband et al. 2010, Jeffery et al. 2012, Mayer and Oliveira 2013, Murienne et al. 2014). Onychophorans have a typical worm- shaped body with 13–43 pairs of stumpy, unjointed legs (= lobopods), the number of which varies inter- or intra-specifically (Ruhberg 1985, Reid 1996, Oliveira et al. 2012a, 2012b). The anterior-most limbs have been modified to three pairs of specialized cephalic appendages: the sensory antennae, the jaws situated within the mouth cavity, and the slime papillae, which are used for defence and prey capture (Manton and Heatley 1937, Read and Hughes 1987, Storch and Ruhberg 1993, Mayer et al. 2010, Oliveira and Mayer 2013). Onychophorans are predators that use a sticky slime secretion produced in large glands and ejected via the slime papillae to entangle the prey, such as crickets and other small invertebrates (Manton and Heatley 1937, Lavallard and campiglia 1971, Ruhberg and Storch 1977, Read and Hughes 1987, Baer and Mayer 2012). After the prey has been immobilized using the glue-like slime, its cuticle is punctured with the jaws and digestive saliva is injected into its body (Manton and Heatley 1937, Baer and Mayer 2012). The liquefied contents are then ingested using a sucking pharynx (Mayer et al. 2013a, Nielsen 2013). Besides the pharynx and the mouth cavity containing the jaws and the tongue, the digestive tract of onychophorans exhibits atubular oesophagus, followed by a thick-walled, straight midgut, and a short posterior hindgut (Balfour 1883, Lavallard 1986, Storch et al. 1988, Mayer et al. 2013a). The hindgut opens to the exterior via a terminal anus. The renal organs (= nephridia) of onychophorans are segmental structures that typically open to the exterior at the basis of each leg (Gabe 1957, Storch et al. 1978, Lavallard 1981, Mayer & Koch 2005, Mayer 2006a). The salivary, anal, uterine, and accessory genital glands, as well as the gonoducts, are generally regarded as derivatives of nephridia of the corresponding body segments (Storch et al. 1978, 1979, Reid 1996, Mayer and Koch 2005, Mayer 2006a, 2007). In females, the gonoducts are associated with paired uteri that are connected to the ovarian tubes via a pair of oviducts, whereas the testes of males are associated with large seminal vesicles (Herzberg et al. 1980, Storch and Ruhberg 1990, Storch et al. 1995, Brockmann et al. 1997, 2001, Walker et al. 2006, Mayer and tait 2009). The male and female genital tracts open to the exterior via an unpaired genital opening, which is located either between the last leg pair in peripatopsids or the penultimate leg pair in peripatids (Storch and Ruhberg 1993, Oliveira et al. 2012b, Ruhberg and Mayer 2013). The body wall of onychophorans exhibits well-developed musculature consisting of an outer circular layer followed by diagonal and inner longitudinal muscle layers (Hoyle and Williams 1980). The longitudinal musculature is organized into two dorsal, two lateral, and three ventral bundles without any metameric arrangement, whereas the muscles associated with limbs are clearly segmental (Hoyle and Williams 1980, Oliveira et al. 2013b). The transverse musculature and the pericardial septum subdivide the body cavity of onychophorans (= haemocoel) into one large perivisceral sinus, two lateral sinuses, and one dorsal pericardial sinus, which contains a tube-shaped, longitudinal dorsal vessel (= heart) (campiglia and Lavallard 1975, Seifert and Rosenberg 1977, nylund et al. 1988, Storch and Ruhberg 1993, Ruhberg and Mayer 2013). during embryogenesis, the haemocoel of onychophorans arises by mixocoely, i.e. a fusion of the primary and secondary (= coelomic) body cavities (von Kennel 1888, Mayer et al. 2004, 2005, Mayer 2006a). Together with tardigrades (water bears), onychophorans are regarded as the closest relatives of arthropods (spiders, centipedes, crustaceans, insects and allies) – the largest and most diverse animal groups on Earth. However, in contrast to arthropods, the anatomy of onychophorans has changed little since the Early Cambrian , rendering them important for addressing various evolutionary and other scientific questions, for example: • How did the panarthropod ancestor look like? • What are the origins of vision and colour vision? • How are the major arthropod groups related to each other? • What is the developmental basis of animal segmentation? • What is the phylogenetic position of tardigrades? • How did the arthropod head evolve? • What are the origins of the arthropod nervous system? • How did the mitochondrial genomes evolve? • To what extent are velvet worms useful for conservation? • What is the actual species diversity of Onychophora? • Can the onychophoran slime be used for bioengineering? Time of origin: Onychophora evolved from the marine fossil onychophoran-like organism Aysheaia pedunculata from the Mid-Cambrian period about 520 million years ago. Zoological Importance of Onychophora: Onychophora show a great zoological importance because: 1. They furnish an example of discontinuous distribution and 2. They represent an example of living connecting link between the two phyla— Annelida and Arthropoda. Anatomical Peculiarities of Onychophora: A. Primitive features: 1. Onychophora are worm-like body covered with thin, flexible, chitinous cuticle. 2. Onychophora are sluggish in nature. 3. Head segments are comparatively small (3 head segments in onychophores but in true arthropods head segments are 5 or 6). 4. Presence of segmentally arranged nephridia. 5. Presence of cilia in the reproductive tracts. 2. Onychophora are sluggish in nature. 3. Head segments are comparatively small (3 head segments in onychophores but in true arthropods head segments are 5 or 6). 4. Presence of segmentally arranged nephridia. 5. Presence of cilia in the reproductive tracts. B. Sole peculiarities: 1. Segmentation indistinct on external surface. 2. Head appendages include a pair of antennae, a pair of jaws and a pair of oral papillae. 3. Texture of the skin is present. 4. Numerous, un-jointed, stumpy walking legs, terminated into a pair of claws, quite unlike the parapodia of polychaeta. 5. Tracheae and disposition of the tracheal apertures are not arthropod-like. 6. Presence of a pair of slime glands opening at the ends of the oral papillae that secrete proteinaceous adhesive substance and helps to capture the prey. 7. Lacking of blood pigments. 8. Subcutaneous haemal channels. C. Salient features: 1. Caterpillar-like body, ranging from 5 mm to 15 cm in length. 2. Body soft and covered by a thin, flexible, chitinous cuticle which is moulted periodically. 3. Indistinct segmentation externally and marked only by the presence of paired, un-jointed, hollow stumpy appendages (13 to 43 pairs according to species). These un-jointed walking legs are called lobopods. 4. Each walking leg terminates in a pair of curved claws. 5. Integument with fine transverse wrinkles and with numerous conical large and small tubercles. 6. Simple eyes, similar with that of an-nelidan polychaetes. 7. Head with 3 pairs of appendages including a pair of annulated antennae, a pair of claw-like mandibles (jaws) which are the modified 2nd pair of appendages and a pair of oral papillae (3rd pair of appendage). 8. A pair of slime glands are present inside the body which open to the tip of the oral papillae that discharge the adhesive material, used for to capture prey and defence. 9. Body wall dermomuscular. Muscles are un-striated. 10. Reduced coelom. 11. Haemocoelomic body cavity. 12. Open circulatory system with lateral valvular ostia on the heart. 13. Elongated tubular heart which is surrounded by pericardial sinus occurring the entire length of the body. 14. Delicate un-branched, rarely branched tracheal tubes open by means of small spiracles, scattered irregularly. Spiracles are without any closing device. 15. A single pair of nephridia in each segment except the genital opening bearing segment. 16. Ladder-like nervous system. Brain is large, bilobed and situated dorsal to the pharynx. 17. Reproductive and excretory ducts are ciliated. 18. Dorsal coelomic gonads. 19. Sexes separate (gonochoristic). 20. Fertilization internal. 21. Oviparous or ovoviviparous with yolky or non-yolky eggs. 22. Viviparous with placenta. 23. Cleavage holoblastic in the eggs of viviparous species and superficial in the oviparous forms which lay their eggs in moist condition. 24. Development direct. 25. Nocturnal and carnivorous in habit. Characters of the living families: I. Peripatopsidae: Number of legs varies from 14-19 pairs; legs with complete spinous pads are 3; the absence of a diastema on the inner side of the jaws; primary dermal papillae without a constriction nephridial opening on 4th and 5th pairs of legs in between third spinous pads; genital opening between or behind last pair of legs, oviparous or ovoviviparous. II. Family Peripatidae: Number of legs varies from 19-43 pairs; legs
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