Summary Phylum Arthropoda

Phylum Arthropoda:
Arthropods include spiders, scorpions, ticks, mites, crustaceans and insects. Live in aquatic and terrestrial
environments. They are herbivorous, carnivorous or omnivorous.

Why have Arthropods achieved such great diversity and abundance?
- They have a versatile exoskeleton
- Segmentation and appendages provide for more efficient locomotion
- Air piped directly to cells
- Highly developed sensory organs
- Complex behaviour patterns
- Use of diverse resources through metamorphosis

Characteristics of Phylum Arthropoda:
 Jointed appendages
 Live in aquatic and terrestrial habitats
 Some can fly
 Free-living and parasitic
 Bilateral symmetry
 Segmented body divided into tagmata
 Triploblastic
 Reduced coelom, mostly filled with blood (haemocoel)
 Cuticular exoskeleton
 Complete digestive system
 Complex muscular system
 Nervous system consists of a dorsal brain and ganglia
 Well-developed sensory organs
 Sexes usually separate, with paired reproductive organs and ducts
 Usually internal fertilisation
 Respiration by body surface, gills, trachea or book lungs
 Open circulatory system with dorsal contractile heart, arteries and hemocoel

Subphylum Trilobita

 Dorsoventrally flattened bottom dwellers and scavengers
 Exoskeleton made of chitin, strengthened in some areas with calcium carbonate
 3 tagmata: head (cephalon), trunk and pygidium
 Cephalon had a pair of antennae, compound eyes, a mouth and 4 pairs of leglike appendages
 No true mouthparts
 Each body segment except the last bear a pair of biramous appendages

Subphylum Chelicerata:

Includes horseshoe crabs, spiders, ticks, mites, scorpions and sea spiders. Their bodies are composed of 2 tagmata: a
cephalothorax and an abdomen. They have 6 pairs of cephalothoracic appendages that include a pair of chelicerae, a
pair of pedipalps and 4 pairs of walking legs. They have no antennae. Most chelicerates suck liquid food from their
prey.

Class Merostomata:
Subclass Eurypterida:
- Giant water scorpions - 4 pairs of walking legs on their abdomen
- Head has 6 fused segments - 12 segments and a spike like telson
- Compound eyes - Some had anterior appendages modified into
- Chelicerae crushing jaws
- Pedipalps

, Subclass Xiphosurida: Horseshoe crabs
- Live in shallow water
- Unsegmented horseshoe-shaped carapace and a broad abdomen which has a long telson or tailpiece
- Their cephalothorax bears a pair of chelicerae, one pair of pedipalps and 4 pairs of walking legs
- Their abdomen bears 6 pairs of broad, thin appendages that are fused in the median line
- On 5 abdominal appendages, book lungs occur under the gill opercula
- 2 lateral rudimentary eyes and 2 simple eyes on the carapace
- Swims by means of abdominal plates and can walk with its walking legs
- Feeds at night on worms and small molluscs, which it seizes with its chelicerae and walking legs
- During mating season, they come to shore at high tide to mate. A female, burrows into sand and lays eggs. Males
add sperm to the eggs and the female covers it with sand. Eggs are warmed by the sun.
- Larva are segmented

Class Pycnogonida: sea spiders
- Small thin bodies
- 4 pairs of long thin walking legs
- Segments are duplicated in some, so they possess 5 or 6 pairs of legs
- Some males have a pair of legs (ovigers) on which they carry developing eggs
- Equipped with chelicerae and pedipalps
- The small head (cephalon) has a raised projection with 2 pairs of simple eyes
- Mouth at the tip of a long proboscis, which sucks juices from soft-bodied animals
- Circulatory system is limited to a simple dorsal heart
- No excretory or respiratory system
- Long thin body and legs provide large surface area in proportion to body volume for diffusion of gas and wastes
- Due to the small size of the body, the digestive system and gonads have branches that extend into the legs

Class Arachnida:
- Common in warm dry areas
- 2 tagmata: a cephalothorax and an abdomen, which may or may not be segmented
- The abdomen houses reproductive and respiratory organs
- Cephalothorax bears a pair of chelicerae, pair of pedipalps and 4 pairs of walking legs
- Mostly predaceous and have chelicerae modified into fangs and pedipalps modified into claws
- Strong sucking pharynx
- Spinning glands
Order Araneae: Spiders
 The spider body is compact
 Consists of a cephalothorax and abdomen, both unsegmented and joined by a slender pedicel
 Pair of chelicerae which have terminal fangs through which ducts run from venom glands
 Pair of leg-like pedipalps which have sensory function and are used by males to transfer sperm
 4 pairs of walking legs which terminate in claws
 All spiders are predaceous. They seize prey with their chelicerae, inject venom, liquify the prey’s tissues
with digestive fluid and sucks the resulting broth into its stomach
 Respire using book lungs or a system of internal tubes called tracheae.
 Book lungs open to the outside through a slit in the exoskeleton. Internal to the slit are many thin-walled
chambers filled with blood. Between a pair of chambers is a cavity where air enters
 Excretory system of Malpighian tubes and specialised resorptive cells in the intestinal epithelium
 8 simple eyes
 Web-spinning habits:
 2-3 pairs of spinnerets containing hundreds of microscopic tubes run to abdominal silk glands
 A scleroprotein secretion emitted as a liquid from the spinnerets hardens to form a silk thread
 Threads of spider silk are stronger than steel threads of the same diameter
 Kind of net spun varies among species
 Use nets for nest lining, sperm webs, egg sacs, bridge lines, warning threads, molting threads,
attachment discs, nursery webs and trapping of prey