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• Arthropoda


  Phylum of animals with segmented/jointed appendages covered with a chitinous exoskeleton
• Arthropods


  • They are the most successful of all animal groups because of their huge population and their diversity
  • They can readily adapt to their particular kind of environment or habitat - be it aerial, terrestrial, or aquatic
• Theories regarding the development and evolution of arthropods


  • Arthropods have risen from Polychaete stock
  • Arthropods and annelids came from a common stock
• Fossil records of almost all animal phyla including Arthropoda, Cnidaria, Annelida, Echinodermata and Mollusca first appeared


  550 million years ago
• Linnaeus first designated all arthropods as belonging to the term Insecta
• Differences and variation in morphological and anatomical features divided the big group of arthropods into different major groups of arthropods
• Major groups of arthropods

  • Myriapoda (centipede and millipede)
  • Crustacea
  • Arachnoidea
• Phylum Arthropoda included all of the mentioned groupings


  1845
• Metamerism


  • Evident in the embryonic development of all arthropods and is conspicuous feature of many adults especially the higher primitive species
• Tagmatization


  • Resulted from the fusion of metamere, with the last 5 segments fused into a head, the nest 3 into the thorax with most of the abdominal segments retained
• Variations in tagmatization size


  • Cephalothorax (fusion of head and thorax)
  • Opisthosoma (fusion of thorax and abdomen)
• Segments of jointed appendages of arthropods


  • Coxa (most proximal)
  • Trochanter
  • Femur (usually longest)
  • Patella
  • Tibia
  • Metatarsus
  • Tarsus
  • Pretarsus (most distal and usually modified for a specific function)
• Chitinous exoskeleton


  • Secreted by the hypodermis
  • Composition: thin outer epicuticle (proteins & wax), thicker inner procuticle (outer exocuticle made up of chitin & protein forming a complex glycoprotein, can be impregnated with salts & tannins, inner endocuticle same as above but not tanned)
  • Makes movement possible by the division of the cuticle into separate plates
  • Articular membrane primitively connects the plate of one segment to the plate of the adjoining segment; a region in which the cuticle is very thin and flexible
  • Four primary plates of the cuticle of each segment: dorsal tergum, two lateral pleura, ventral sternum
  • Joints - tube-like segments or sections of the cuticular skeleton of the appendages, connected by an articular membrane; enable the segments of the appendages and body to move
  • Pores generally penetrate the body, where secretions pass through
  • Structures originating from the ectoderm and lined by the exoskeleton: tracheal tube, book lungs, proctodeum and stomodeum, certain portions of the reproductive tract
  • Color depends on deposition of pigments within the cuticle, subcuticular chromatophores, blood and tissue pigments are other sources of pigments
  • Ecdysis (molting) allows the arthropod to grow in size
• Endoskeleton


  • Infolding of the procuticle that produces inner projections or apodemes on which the muscles are inserted
  • May involve the sclerotization of internal tissue forming free plates for muscle attachment within the body
• Muscular System


  • Striated muscle bundles attach to the inner surface of the procuticle by tonofibrils
  • Innervated by neurons which may be phasic (fast), tonic (slow), or inhibitory (prevents contraction)
• Coelom


  • Schizocoelom - hemocoel that are sinuses or spaces in the tissue filled with blood, lined with cilia
  • Reduced to a cavity of the gonads or by the excretory organs
• Circulatory System


  • Open blood vascular system
  • Heart consists of one or more chambers with vascular walls arranged in a linear fashion
  • Ostia - pairs of lateral openings perforating the wall; enable the blood to flow into the heart from the pericardium (large surrounding sinus, a part of the hemocoel)
  • Blood contains amoebocytes and hemocyanin (respiratory pigment)
  • Direction of blood flow: heart ===> arteries ==> sinuses (which bathe the tissues) ==> pericardium => heart
• Digestive System


  • Foregut - derived from the stomodeal region of the ectoderm, concerned with ingestion, trituration and storage of food
  • Midgut - derived from the endoderm, site of enzyme production, digestion and absorption
  • Hindgut - derived from the proctodeal region of the ectoderm, functions in the absorption of water and the formation of feces
• Nervous System


  • Brain has three major regions: Protocerebrum (with one to three pairs of optic centers), Deutocerebrum (receives the antennal nerves), Tritocerebrum (gives rise to nerves that innervate the labium, digestive tract, chelicerae)
  • Ventral nerve cord with segmental ganglionic swellings
  • Sense organs include hairs, bristles, setae that may carry chemoreceptors, canals, pits, slits that may house chemoreceptors or detect vibrations/forces, proprioreceptors, simple and compound eyes
• Reproduction and Development


  • Mostly dioecious
  • Fertilization: generally internal in terrestrial forms, external in aquatic forms
  • Eggs: rich in yolk, centrolecithal (nucleus surrounded by a small island of non-yolky cytoplasm)
  • Cleavage: intralecithal or superficial
  • Development: stereogastrula
• Trilobitomorpha (Class: Trilobita)


  • Fossil trilobites, belonged to the Paleozoic era
  • Body has three divisions: cephalon (fusion of acron & four post-oral segments), thorax/trunk (segments separated, each bearing a pair of biramous appendages), pygidium (fused segments with reduced appendages)
  • Development has three larval stages: protaspia (planktonic with carapace), meraspia (carapace, pygidium & some developing trunk appendages present), holaspia (adult features evident)
• Chelicerata


  • Body divisions: cephalothorax or prosoma, abdomen or opisthosoma
  • Antennae absent
  • Chelicerae (first pair of appendages, feeding structures), pedipalps (second pair of appendages, perform various functions), four pairs of walking appendages
• Merostomata


  • Aquatic chelicerates
  • 5 or 6 pairs of abdominal appendages modified as gills & by a spike-like telson at the end of the body
• Xiphosura (Order)


  • Marine soft bottoms, shallow waters
  • Prosoma covered by a large horseshoe-shaped carapace
  • 2 pairs of eyes: a large dorsolateral & a small dorsomedian pair
  • Mouth is posterior to the labrum/upper lip (with a pair of trisegmented chelicerae on each side)
  • 6 pairs of cephalothoracic appendages: first 4 pairs identical chelate walking legs, fifth pair non-chelate with 4 leaf-like processes for burrowing, chilaria (degenerate, same function as 5th pair)
  • Opisthosoma: unsegmented, with 6 short mobile spines at the posterior ends; bears 6 pairs of appendages with the first pair forming the genital operculum
  • Telson: a long triangular spine like tail or caudal spine, not a true telson
  • Digestion: extracellular, intracellular within the hepatic ceaca
  • Excretion & osmoregulation: 4 pairs of coxal glands
• Chelate walking legs


  • Identical first 4 pairs
  • Coxae - most proximal segment with spines on the median side; macerate & move food anteriorly
  • Flabellum - short spatulate process on the median side of the last pair's coxae, for cleaning the gills
• Fifth pair of legs


  • Non-chelate with 4 leaf-like processes; for pushing, clearing, & pushing away the mud & silt during burrowing
• Chilaria


  • Located between the fifth pair, degenerate & same function of the 5th pair
• Opisthosoma


  • Unsegmented, with 6 short mobile spines at the posterior ends; bears 6 pairs of appendages with the first pair forming the genital operculum
• Telson


  • A long triangular spine like tail or caudal spine; not a true telson in horseshoe crabs since it does not bear the anal opening; may be used for pushing & for righting the body when it is accidentally turned over
• Digestion


  Extracellular, intracellular within the hepatic ceaca
• Excretion & osmoregulation


  4 pairs of coxal glands – common saclike chambers - coiled tubules ===> bladder ===>excretory pore at the base of the last pair of walking legs
• Reproduction


  Dioecious with external fertilization - male climbs on the dorsum of the female while the latter scoops out sand to lay from 200 - 300 large eggs which are simultaneously fertilized by the male, after which are covered with sand
• Development


  Centrolecithal egg ==> cleavage ==> trilobite larva==> from 13-14 instars ==> adult
• ORDER EURYPTERIDA - extinct giant arthropods that existed from the Cambrian to the Permian period
• Eurypterids


  • Aquatic, probably attained the largest size of any of the arthropods
  • Smaller cephalothorax
  • Abdomen divided into: pre abdomen (mesosoma) and post abdomen (metastoma)
  • Six pairs of gills, first pair forms the operculum
• CLASS ARACHNIDA - largest and most important of the chelicerate classes
• Migration from aquatic to a terrestrial environment


  1. Entailed morphological and physiological changes:

  2. Epicuticle became waxy

  3. Arachnid book lungs and trachea developed from book gills

  4. Appendages became better adapted for terrestrial locomotion

  5. Development of silk in spiders, pseudoscorpions; poison glands in scorpions, spiders, and pseudoscorpions
• Arachnid body


  • Divided into:
  • Prosoma- unsegmented, covered dorsally by a solid carapace; ventral surface covered by coxae of the appendages
  • Abdomen- segmented, divided into a preabdomen and a postabdomen
• Arachnid appendages


  • Arise from the prosoma and consist of a pair of chelicerae (for feeding), a pair of pedipalps, and 4 pairs of legs
• Arachnid respiration


  By book lungs and/or trachea