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Affinities of Onychophora B.Sc

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Affinities of Onychophora B.Sc Affinities of Onychophora B.Sc. Part-I, Paper-I Dr. Vandana Kumari, Department of Zoology, R.C.S. College , Manjhaul Introduction Onychophora has many characters of Annelida, particularly its general body organization. But it also exhibits many important features that take it closer to Arthropods. Peripatus shows annelidan, arthropodan and special characters. Because it shows more Arthropoda characters it was kept in Arthropoda by some. Because it shows both Annelida and Arthropoda characters, it was considered as a connecting link between Annelida and Arthropoda. Onychophorans are not worms and also they are not arthropods. Hence they are treated as a separate phylum. Onychophora-Peripatus A small phylum of terrestrial invertebrates which comprises the velvet worms such as peripatus. They share characteristics with the arthropods and annelids, having a long, soft segmented body with stubby legs (lobopods). Affinities Features of Onychophora have made it difficult to place it within any one of the ten major phyla. The detailed studies of Onychophores have now confirmed that in addition to its own peculiar features it has characters common with two other large groups, Annelida and Arthropoda. Annelidan Character ▪ Presence of worm like body with bilateral symmetry. ▪ Presence of thin flexible cuticle. ▪ Absence of distinct head region. ▪ Presence of dermo muscular body wall with circular and longitudinal muscles. ▪ Presence of a pair of simple eyes on the dorsal side of head. ▪ Presence of hollow stumpy appendages in the trunk segments. ▪ Presence of muscular pharynx. ▪ Presence of short stomodaeum and proctodaeum regions of alimentary canal. ▪ Presence of salivary glands which are modified Nephridia. ▪ Presence of paired Nephridia in every segment. ▪ Presence of segmentally-arranged coxal glands which are homologous with settiparous glands of chaetopods. ▪ Presence of cilia in the reproductive ducts. Dissimilarity with Annelida ▪ Ventrally placed mouth in Onychophores. ▪ Heart and ostia present in Onychophores. ▪ Clawed appendages in Onychophores. ▪ Presence of antennae in Onychophores. ▪ Ladder-like nervous system. ▪ Tracheal tube for respiration in Onychophora. ▪ Absence of true metamerism in Onychophora. ▪ Texture on the skin. ▪ Disposition of the gonads and ▪ Haemocoelomic body cavity. Arthropodan Characters ▪ Presence of a chitinous cuticle. ▪ Presence of jointed appendages arranged one pair in each segment. ▪ Presence of jaws in the mouth which are modified appendages. ▪ Presence of haemocoel. ▪ Presence of colourless blood. ▪ Presence of long, tubular and dorsal heart enclosed by pericardium. ▪ Presence of tracheal system with stigmata. ▪ Presence of tubular gonads around the gut Dissimilarities with Arthropods ▪ Arrangement of tracheae is not arthropod-like. Here in each segment there are numerous permanently opened spiracles (no closing mechanism). ▪ Jaw is the modification of second appendages and the movements of jaws operate from anterior end and proceed towards posterior end. ▪ Formation of skin is not like that of arthropods. ▪ Segments behind head are simple and identical. ▪ Segmentation not distinct in Onychophora. ▪ Absence of Malpighian tubules in Onychophores. ▪ Simply, un-jointed, numerous stumpy legs in Onychophores. ▪ Structure of eye is less complicated. ▪ Two ventral nerve cords are widely separated and without true ganglia. ▪ Body regions or tagmata are not well developed in Onychophores, which are well developed in Arthropoda. Similarities with Mollusca ▪ Slug-like appearance. ▪ Ladder-like nervous system resembling that of chiton and lower prosobranchia. ▪ 3. Antennae tentacle-like. ▪ Guilding (1826) first discovered a peripatus and considered it to be an aberrant mollusc. But according to many scientists the resemblances with molluscs are only superficial. Primitive Characters of Peripatus ▪ Presence of superficial segmentation of the body. ▪ Presence of one pair of jaws in the mouth cavity. ▪ Presence of trachea which open out with diffused stigmata. ▪ Presence of a ladder like nervous system. ▪ Presence of velvety skin. ▪ Genital organs are arranged in a manner which is not seen in any other animals. ▪ Some of these peculiar features of Onychophores which neither support annelids nor arthropods, given it a separate phylum status..
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