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(phylum Nematoda)

Nonsegmented, generally colorless worms
Several parasitic roundworms infect humans.
Contain a pseudocoelom:
A “false” body cavity that is incompletely lined by mesoderm
Provides a space for internal organs and can serve as hydrostatic skeleton


Parasitic Roundworms

Ascaris – Intestinal roundworm
Produces over 200,000 eggs daily
Trichinella – Trichinosis
Humans can contract infection from eating rare pork infected with Trichinella.
Filarial worms – Heartworms
Causes fatalities in dogs
Wuchereria – Elephantiasis
Filiarial worm uses mosquitoes as a vector to transmit disease to humans.
Limbs may swell to a monstrous size.



(phylum Arthropoda)
Rigid, jointed exoskeleton composed of chitin
Must molt as they grow
Segmented, but some segments are fused into regions
Head, thorax, abdomen
Well-developed nervous system
Brain and ventral nerve cord
Eyes simple or compound
Variety of respiratory organs
Gills, book lungs, tracheae
Means “implying change”
Reduces competition because larvae require different food and environment than adults



They are hard, crusty exoskeleton containing calcium carbonate and chitin.
Most crustaceans have compound eyes and five pairs of appendages.
Majority of crustaceans live in marine and aquatic environments.
Decapods are the most familiar and numerous of crustaceans.
Shrimp, lobsters, crayfish, and crabs
Copepods and krill are small, free-living crustaceans that live in water and feed on algae.
Barnacles live on wharf pilings.



Thorax bears five pairs of walking legs.
Head and thorax fused into cephalothorax
Covered by nonsegmented carapace
Abdominal segments equipped with swimmerets
The respiratory system consists of gills.
Coelom is reduced.
They have an open circulatory system.
The nervous system is well developed.
The sexes are separate.
Green glands excrete metabolic wastes.



“Centipede” means hundred legs.
Each body segment has a pair of walking legs.
They live in moist environments.
The head includes paired antennae and mandibles.
Appendages on the first trunk segment are jaws that kill or immobilize prey.



Four thoracic segments each have one pair of legs.
Abdominal segments have two pairs of legs.
They have cylindrical bodies.
They have a tough chitinous exoskeleton.



Most insects live on land.
Body segments
Head bears sense organs and mouthparts.
Thorax bears three pairs of legs and sometimes one or two pairs of wings.
Abdomen contains most internal organs.
They live on land and breathe by tracheae.
Many insects undergo complete metamorphosis.
Body parts are completely reorganized.
Insects are an important food source for other animals and carnivorous plants.



Third pair of legs is for jumping.
First abdominal segment bears a tympanum.
Posterior region of exoskeleton of female has ovipositor.
Food is stored in crop before passing to gizzard.
The excretory system consists of Malpighian tubules.
The circulatory system consists of a slender, tubular heart.
They undergo incomplete metamorphosis.
Gradual change in form
Respiratory system begins with openings in exoskeleton called spiracles.
From spiracles, air enters tracheae.
Breathing through tracheae accounts for small size of insects



Live in terrestrial, aquatic, and marine environments
Include terrestrial spiders, scorpions, ticks, mites, horseshoe crabs, and sea spiders
All appendages attached to cephalothorax, none on abdomen
First pair (chelicerae) are used for feeding and defense.
Second pair (pedipalps) function in feeding or sensory systems.