Navigation Arthropoda: Characters and Classifications | Invertebrate Zoology Article Shared by ADVERTISEMENTS: In this article we will discuss about Arthropoda:- 1. General Characters of Arthropoda 2. Classification of Arthropoda. General Characters of Arthropoda: 1. Arthropods are triploblastic, bilaterally symmetrical, metamerically segmented animals. 2. Body of Arthropoda is covered with a thick chitinous cuticle forming an exoskeleton. ADVERTISEMENTS: 3. Body segments usually bear paired lateral and jointed appendages. 4. Musculature is not continuous but comprises separate striped muscles. 5. Body cavity is haemocoel. The true coelom is reduced to the spaces of the genital and excretory organs. 6. Digestive tract is complete, mouth and anus lie at opposite ends of the body. ADVERTISEMENTS: 7. Circulatory system is open with dorsal heart and arteries but without capillaries. 8. Respiration through general body surface, by gills in aquatic forms, tracheae or book lungs in terrestrial forms. 9. True nephridia are absent; excretion by coelomoducts or Malpighian tubules or green or coxal glands. 10. Cilia are entirely absent from all parts of the body. 11. Sexes are generally separate and sexual dimorphism is often exhibited by several forms. 12. Fertilisation is internal. Development is usually indirect through larval stages. 13. Parental care is also often well marked in many arthropods. 14. Most diversified group inhabiting the land, water and air. Classification of Arthropoda: Phylum Arthropoda and its various groups have been classified differently by different workers. But the classification of Arthropoda followed in the present text is generally based on Vandel (1949), Snodgrass (1960) and Storer (1979). ADVERTISEMENTS: Onychophora, however, was considered to be arthropod for a long time but the modern trend is to consider it as an independent group of segmented animals. Subphylum I. Trilobitomorpha: (Gr., tria = three + lobos = lobe + morphe – form): 1. Represented by fossil trilobites only. 2. Body separated by two longitudinal furrows into three lobes. ADVERTISEMENTS: 3. Except the last segment all segments bear biramous appendages. 4. All were marine forms. 5. All were bottom dwellers, i.e., benthozoic and existed from Cambrian to Permian. Examples: ADVERTISEMENTS: Triarthrus, Dalmanites. Subphylum II. Chelicerata: (Gr., chele = claw + keros = horn + ata = group): 1. Body divisible into cephalothorax or prosoma and abdomen or opisthosoma. 2. First pair of appendages are chelicerae which are preoral and feeding in function. ADVERTISEMENTS: 3. Second pair of appendages are pedipalpi which are postoral. 4. Antennae and true jaws not found. 5. Mostly terrestrial and predaceous. Class 1. Merostomata: 1. Exclusively aquatic, all marine. 2. Prosoma bears compound eyes placed laterally; it is broadly jointed to abdomen. 3. Five or six pairs of appendages in abdomen modified as gills or branchiae for respiration. 4. Abdomen ends in a spike-like long telson. Subclass I. Xiphosura: 1. Prosoma is convex covered by a broad horse-shoe-shaped carapace. 2. Prosoma bears six pairs of appendages. 3. Opisthosoma hexagonal consisting of six mesosomatic segments and a vestigial un-segmented metasoma with a long narrow telson. 4. Genital openings paired covered by genital operculum. 5. Respiration by lamelliform gills or book- gills attached to the abdominal appendages. Example: Limulus (King crab). Subclass II. Eurypterida: 1. Extinct (Palaeozoic) forms. 2. Large sized arthropods. 3. Cephalothorax small covered by dorsal carapace. 4. Cephalothorax followed by 12 free segments and a terminal elongated narrow telson. 5. Cephalothorax with six pairs of appendages. Examples: Eurypterus, Pterygotus. Class 2. Arachnida (Gr., arachne = spider + oid = like): Class 2. Arachnida (Gr., arachne = spider + oid = like): 1. Mostly terrestrial, few aquatic. 2. Prosoma bears six pairs of appendages; one pair chelicerae one pair pedipalpi and four pairs of walking legs. 3. Abdomen usually without appendages. 4. Respiratory organs either tracheae, book- lungs or book-gills. 5. Excretory organs are Malpighian tubules or coxal glands or both. 6. Sexes are separate, sexual dimorphism not conspicuous and development mostly direct. Order 1. Scorpionida: 1. Terrestrial forms found under stones in tropical and subtropical regions; true scorpions. 2. Prosoma covered dorsally by carapace and bears a pair of chelicerae, a pair of pedipalpi and four pairs of walking legs. 3. Opisthosoma divisible into mesosoma and metasoma; mesosoma is broad consisting of seven segments and metasoma is narrow consisting of five segments with a poison- sting. 4. A pair of comb-like pectines occur ventrally on the second segment of mesosoma. 5. Respiration by four pairs of book-lungs. Examples: Buthus, Palamnaeus. Order 2. Pseudoscorpionida: 1. Minute false scorpions found under the bark of trees. 2. Prosoma formed of six fused segments covered dorsally by carapace. 3. Opisthosoma often to twelve segments. 4. Sting and telson not found. 5. A pair of spinning glands present. 6. Respiration by tracheae. Example: Chelifer. Order 3. Palpigradi: 1. Small arachnids. 2. Opisthosoma often segments and is jointed to the prosoma by a pedicle. 3. Telson with a long jointed flagellum. 4. Chelicerae are chelate and pedipalpi leg like. 5. Respiration by three pairs of book-lungs. Example: Koenenia. Order 4. Solifugae: 1. Body consists of prosoma and opisthosoma. 2. Prosoma consists of three fused and last three segments. 3. Opisthosoma composed of ten segments and is not marked off from prosoma. 4. Chelicerae are large and chelate, pedipalpi are elongated and leg-like. 5. Poison glands absent. 6. Respiration by tracheae. 7. Commonly referred to as sun spiders or wind spiders. Example: Galeodes. Order 5. Amblypygi: 1. Somewhat flattened body. 2. Carapace covering the prosoma undivided. 3. Pedipalpi large and heavy, chelicerae of moderate size. 4. First pair walking legs sensory in nature. 5. Opisthosoma bears twelve segments, flage​llum absent. Example: Charinus. Order 6. Uropygi: 1. Commonly referred to as whip scorpions. 2. Chelicerae two-jointed and moderate in size. 3. Pedipalpi large, heavy and usually with pincers. 4. Opisthosoma bears twelve segmentsa long flagellum present in the last segment. 5. Eyes one pair. Examples: Thelyphonus, Mastigoproctus. Order 7. Araneae: 1. Body consists of prosoma and opisthosoma. 2. Prosoma is unsegmented covered by cara​pace and bears six pairs of appendages. 3. Opisthosoma is usually unsegmented attached to prosoma by a narrow pedicle. 4. Chelicerae are subchelate with poison glands. 5. Pedipalps simple and six jointed. 6. Eight eyes are arranged dorsally in two rows on the carapace of prosoma. 7. Respiration either by book-lungs or book- lungs and tracheae both. 8. True spiders. Examples: Argiope (Writing spider), Aranea (House spider) Lycosa (Wolf spider), Agelena (Funnel-web spider). Order 8. Ricinulei: 1. Body consists of prosoma and opisthosoma. 2. Prosoma with movable anterior cucullus. 3. Opisthosoma composed of nine segments and connected to prosoma by a pedicle. 4. Chelicerae and pedipalpi are chelate. 5. Respiration by tracheae. Examples: Cryptocellus, Ricinoides. Order 9. Phalangida or Opiliones: 1. Spider-like in appearance. 2. Prosoma unsegmented, opisthosoma bears ten segments. 3. Pedicel absent between prosoma and opisthosoma. 4. Chelicerae chelate; pedipalpi leg-like. 5. Spinning glands not found. 6. Respiration by tracheae. Examples: Phalangium, Leiobunum. Order 10. Acarina: 1. Body unsegmented in which prosoma is fused with opisthosoma. 2. Chelicerae and pedipalpi are small and associated with the mouth-parts which are adapted for biting, piercing and sucking. 3. Respiration by tracheae or by skin. 4. Free living and parasitic forms. 5. Commonly referred to as ticks and mites. Examples: Chorioptes (Mite), Sarcoptes (Itch-mite), Idodex (Tick). Class 3. Pycnogonida: 1. Commonly referred to as sea spiders. 2. Very small in size. 3. Body mainly consists of cephalothorax, abdomen reduced. 4. Pedipalpi short segmented and chelicerae very small. 5. Usually eight pairs of long walking legs. 6. Mouth placed on a long proboscis. 7. Eyes simple and 4 in number. 8. Respiratory and excretory organs wanting. 9. Sexes separate (dioecious); females with a pair of ovigers for carrying eggs. Examples: Pycnogonum, Nymphon. Subphylum III. Mandibulata: (L., mandibula = mandible + at a = group): 1. Terrestrial and aquatic both freshwater and marine. 2. Body divisible into either cephalothhorax and abdomen or head, thorax and abdomen. 3. Appendages of head consist of one or two pairs of antennae, one pair of mandibles and one or two pairs of maxillae. 4. Eyes usually compound. 5. Respiration either by gills or tracheae. 6. Excretion by Malpighian tubules or green glands. 7. Sexes usually separate with sexual dimorphism. 8. Development usually involves larval stages. Class 1. Crustacea (L., crusta = a hard shell): 1. Mainly aquatic, generally marine but few freshwater and few live in moist places. 2. Generally free living but few are parasitic forms. 3. Head often fused with thorax to form cep​halothorax covered dorsally by carapace. 4. Head bears a pair of compound eyes and five pairs of appendages. 5. Thorax and abdomen often with a pair of biramous appendages in each segment; appendages modified for various purpose. 6. Respiration either by gills or general body surface. 7. Coelom greatly reduced, it is in the form of haemocoel. 8. Blood vascular system