Arthropoda

• Arthropoda can either be considered as a monophyletic clade of genetically diverse but evolutionarily linked species or an artificial, polyphyletic grouping of similar taxa evolving multiple times from different pre arthropod ancestors
• About 84 percent of all known species of animals are members of the phylum Arthropoda
• Arthropods are represented in every habitat on Earth and show a great variety of adaptations
• Approximately 900,000 species of arthropods have been recorded, with probable as many more remain to be described
• Arthropods


  • Several types live in aquatic environments and others reside in terrestrial ones; some groups are even adapted for flight
  • Insects, spiders, scorpions, pseudoscorpions, centipede, crabs, lobsters, brine shrimps, etc
• Arthropod exoskeleton


  • It is a non-living skeletal covering composed of chitin (a complex sugar) bound to protein, secreted by the underlying epidermis
• Arthropods lack locomotory cilia, even in the larval stages, probably because of the presence of the exoskeleton
• Arthropod body


  • It is usually segmented, and the segments bear paired jointed appendages, from which the name arthropod ("jointed feet") is derived
  • In modern members there is a fusion and modification of different regions of the body for highly specialised functions
• Arthropod great diversity and abundance

  • A versatile exoskeleton / cuticle
  • Segmentation and appendages for efficient locomotion
  • Air is piped directly into cells
  • Highly developed sensory organs
  • Complex behavious patterns
  • Reduced competition through metamorphosis
• Minute copepods (typically less than 1 millimetre long) are among the most abundant animals on Earth, especially in marine surface waters
• Minute crustaceans inhabit underground waters in many parts of the world, and deserts support a large arthropod fauna, especially insects and arachnids
• Collembolans and the oribatid mites are among the permanent inhabitants of Antarctica
• Brine shrimp are found in some saltwater lakes, and beetles, mites, and various crustaceans have been taken from hot springs
• Arthropods are the only invertebrates capable of flight
• Classification of arthropods


  • Subphylum Chelicerata
  • Subphylum Trilobitomorpha (trilobites)
  • Subphylum Crustacea
  • Subphylum Myriapoda
  • Subphylum Hexapoda
• Subphylum Trilobitomorpha (trilobites)


  • Head (or cephalon) composed of 5 segments bearing a pair of antennae and compound eyes; Oval, flattened body composed of cephalon, thorax and pygidium, each segmented; Dorsal surface molded longitudinally into 3 lobes; each segment bears a pair of similar, branched appendages; marine; More than 4,000 fossil species known
• Commonly recognized trilobite orders

  • Agnostida
  • Redlichiida
  • Corynexochida
  • Lichida
  • Odontopleurida
  • Phacopida
  • Proetida
  • Asaphida
  • Harpetida
  • Ptychopariida
• Subphylum Chelicerata


  • Body divided into prosoma (cephalothorax) and opisthosoma (abdomen); No antennae; First pair of appendages consists of chelicerae flanking the mouth; In most chelicerates the other prosomal appendages are a pair of pedipalps and four pairs of legs
• Class Merostomata (Horse shoe crab)


  • Large marine chelicerates with book gills on the underside of the opisthosoma; Prosoma covered by a dorsal carapace; Opisthosoma bears a long terminal spine; Three body segments, the prosoma, the opisthosoma, and the telson

  
• The horseshoe crab has ten eyes on the top of the prosoma and six pairs of appendages on the underside of the prosoma
• The telson or tail is attached to the opisthosoma and the horseshoe crab uses it to steer itself and to flip itself over if it gets turned on its back
• Orders of Merostomata


  • Xiphosura (horseshoe crabs, 4 species)
  • Eurypterida (Gigantostraca), which is extinct and includes 200 fossil species from the Paleozoic Era
• Class Arachnida


  • Chiefly terrestrial; Book lungs and/or tracheae as gas exchange organs; Opisthosoma (abdomen) segmented or unsegmented externally and broadly or narrowly joined to the prosoma; About 70,750 species; Size: 0.25 mm–l8 cm; Prosomal appendages consist of 1 pair of chelicerae, 1 pair of pedipalps, and 4 pairs of legs; Gonopore always on the lower side of second abdominal segment
• Orders of Arachnida


  • Acari: (Acariformes and Parasitiformes) mites and ticks
  • Amblypygi: whip-spiders
  • Araneae: spiders
  • Opiliones: harvestmen
  • Palpigradi: micro-whip scorpions
  • Pseudoscorpiones: pseudoscorpions (False Scorpions)
  • Ricinulei: hooded tick-spiders
  • Schizomida: schizomids
  • Scorpiones: Scorpions
  • Solifugae: sun-spiders, camel-spiders, or wind scorpions
  • Uropygi or Thelyphonida: vinegaroons
• Class Pycnogonida (Sea spiders)


  • Mostly Marine; Narrow trunk of 4 to 6 segments; Greatly reduced abdomen; Cephalon (head) with proboscis bearing a pair of chelicerae, palpi, and egg-carrying legs; Commonly found crawling over sessile animals, such as hydroids and bryozoans; Usually 4 pairs of walking legs attached to lateral projections of the trunk; Tubercle with 4 eyes located dorsally between the first pair of legs; No gas respiratory organs; About 1,000 described species; 1 mm–10 cm

  
• The correct taxonomy within the class Pycnogonida is uncertain, and it appears that no agreed list of orders exists. All Families are considered part of the single order Pantopoda
• Subphylum Crustacea


  • Mainly aquatic, generally marine but few freshwater and few live in moist places; Generally free living but few are parasitic; Head bears a pair of compound eye and five pairs of appendages; Thorax and abdomen often with a pair of biramous appendages in each segments; Respiration either by gill or general respiratory surface; Sex usually separate & sexual dimorphism is common; Development includes metamorphosis with free larval stages; Coelom greatly reduced, it is in the form of haemocoel; Excretory organs are modified coelomoduct which may be either maxillary gland or antennary (green) glands; 44,000 described species distributed among 6 subclasses
• Subclasses of Crustacea


  • Class Branchipoda
  • Class cephalocarida
  • Class maxillopoda
  • Class malacostraca
  • Class Remipedia
• Class Branchipoda


  • A small body (0.25 mm - 10 cm long), Simple mouth parts, Paired compound eyes, single simple eye, Leaflike or phyllopodous appendages, for feeding, locomotion, and respiration; Minimal body tagmosis
• The nervous system and sensory system of branchiopods are simple, although some species vibrate their compound eyes to gather more visual information
• Since most branchiopods are small with a thin cuticle, gas exchange can occur across the body wall as well
• Orders of Branchipoda


  • Phyllopoda (fairy shrimp, tadpole shrimp and brine shrimp)
  • Cladocera (Daphnia species)
• Class Remipedia


  • The first described remipede was the fossil Tesnusocaris goldichi (Lower Pennsylvanian), but, since 1979, at least seventeen living species have been identified with global distribution throughout the Neotropics; Their size is variable from 10–40 millimetres long and include a head and an elongated trunk of up to forty-two similar body segments; Swimming appendages are lateral on each segment, and they swim on their backs; They have fangs connected to secretory glands; They have a primitive body plan for crustaceans, and have been regarded as an ancestral crustacean group
• The size and complexity of the brain suggests that Remipedia might be the sister taxon to Malacostraca, regarded as the most advanced of the crustaceans
• Class Cephalocarida


  • Horseshoe shrimp are small, 2-4 mm; Have an elongated body and a large head, the posterior edge of which covers the 1st thoracic segment; Eyes are absent and the 2nd pair of antennae is located posterior to the mouth, which is unique for the Crustacea; Maxillae are unspecialized; The 1st pair of maxillae is very small and the 2nd pair has the same makeup as the following thoracic legs; The mouth is behind the upper lip and mandibles occur on either side; There are 10 thoracic segments and the abdomen bears a telson but no other appendages

  
• Class Malacostraca


  • Largest subclass of crustacea including most crabs, lobster, shrimps, crayfish, krill, sow bugs and beach flies; Large marine and fresh water crustacean; Thorax comprises eight segments, abdomen six segment, rarely seven; Exoskeleton of head unite with few or more thoracic segment to form cephalothoracic carapace
• Orders of Malacostraca


  • Syncarida "mountain shrimps"
  • Mysidacea
  • Cumacea
  • Leptostraca
  • Isopoda
  • Amphipoda
  • Stomatopoda
• Maxillopods are generally small animals
• Mouth


  Behind the upper lip and mandibles occur on either side
• Thoracic segments


  • 10 segments
  • Abdomen bears a telson but no other appendages