Chapter 33: Invertebrates
Evolutionary Trends
• No/radial → Bilateral symmetry
• Diploblastic → Triploblastic
• No tissue → Tissues/organs
• Acoelom → Coelom/pseudocoelem
• No head → Cephalization
• Sessile → Motile
• Water → Land
Invertebrates
• Animals w/o backbones
• 95% of known animal species
• All animal phyla except for most of the chordates
Phylum Porifera
• Sponges
• Cell organization (no tissues or organs)
o Not diplo-/triploblastic
o Not proto-/deuterostome
• Acoelomate w/ no symmetry
• Sessile, suspension feeders
• Body Form
o Body – mesohyl layer (gelatinous region) in b/w 2 cell layers
o Amoebocyte – in mesohyl; digest, move food; secrete spicules (skeletal element)
o Choanocyte (flagellated collar cell) – create water current thru sponge
 Water enters thru pores & goes into spongocoel, & out thru osculum
Phylum Cnidaria
• Jellyfish, corals, sea anenomes, hydras
• Diploblastic w/ tissues
o Not proto-/deuterostome
• Acoelomate w/ radial symmetry
• Sac w/ gastrovascular cavity (digestive & circulatory)
• Single opening = mouth & anus
• Body Forms
o Polyp – sessile (non-moving)
o Medusa – motile
• Food?
o Cnidocyte – cell in tentacle that functions in defense & prey capture
o Nematocyst – organelle that ejects a stinging thread
Phylum Platyhelminthes
• aka Flatworms – flukes, planarian, tapeworms
• Marine, freshwater, & damp terrestrial habitats
• Acoelomate – dorsoventrally flattened & have 1 opening to gastrovascular cavity
• Gas exchange – across surface
• Triploblastic w/ bilateral symmetry
• Tissues & 1st organs
• Protostome development
• 1st w/ cephalization (head) = Anterior sensory organs
Phylum Nematoda
• aka Roundworms – trichinella, hookworm, heartworm, C. elegans
• Secrete a protective cuticle
• Water, soil, moist tissues of plants, body fluids & tissues of animals
• Alimentary canal (complete digestive tube w/ mouth & anus)
• Un-segmented w/ organs
• Protostome
• Triploblastic & bilateral symmetry
• Pseudocoelomate w/ hydrostatic skeleton
Phylum Annelida
• Earthworms, polychaetes, & leeches
• Bilateral segmented worms
• Protostome coelomates
• Have chaetae (NOT leeches) – chitin bristles for moving
• Triploblastic w/ organs
• Closed circulatory system
o Blood w/ hemoglobin
• Gas exchange across skin
• Hermaphroditic w/o self-fertilization
Phylum Mollusca
• Snails & slugs, oysters & clams, octopuses & squids, chitons
• Most marine, some fresh water & terrestrial
• Soft-bodied; most have hard CaCO3 (calcium carbonate)shell
• Body Parts
o Muscular foot – sensory & motor organs
o Visceral mass – digestive, excretive, & reproductive organs
o Mantle – surrounds visceral mass & secretes shell (if there is one)
o Radula – moveable toothed structure
• Triploblastic w/ bilateral sym.
• Alimentary canal
• Protostome coelomate
• Open circulatory system – blood pumped into open cavities (No capillaries)
• Four Classes
o Class Polyplacophora
 Chiton
o Class Gastropoda
 Slugs (no shell) & snails
 Undergo ‘torsion’
 Visceral mass rotates 180°; anus & mantle cavity end up above head
 Coiled shell (NOT from torsion) & radula
o Class Bivalva
 Scallops, oysters, clams, & mussels
 Shell divided into 2 halves
 Mantle cavity has gills for feeding & gas exchange
o Class Cephalopoda
 Octopus, squid, nautilus
 Carnivores – beak-like jaws w/ tentacles around it (modified foot)
 Well-developed sense organs; complex eye & brain
 Eject water from siphon to move
 Spray ink for defense
Phylum Arthropoda
• Over 1 mil. species
• Chitinous exoskeleton
• Segmented – head, thorax, & abdomen
• Open circulatory system (hemolymph, not blood)
• Triploblastic & bilateral
• Protostome coelomates
• Malpighian tubes – excretory organs
• Compound – many eye ‘units’
• Four Subphyla
o Subphyla Cheliceriformes
 Spiders, horseshoe crabs, scorpions, ticks, mites
 Chelicerae – claw-like feeding appendages
 Cephalothorax, abdomen, & 6 paired appendages

Spiders
 Chelicerae – fangs, secrete digestive enzymes, poison
 Pedipalp – front ‘pinchers’ for sensing, feeding, & reproduction
 Book lungs – extensive surface area for gas exchange
o Subphyla Myriapoda
 Terrestrial
 Millipede – 4 legs per segment
 Eat plant matter
 Centipede – 2 legs per segment
 Carnivores; venemous claws on front segment
o Subphyla Hexapoda
 Insects
 Abdomen, thorax, & head
 Wings – exoskeleton outcropping
 Several organ systems
 Diversified thru evolution of flight, feeding on gymnosperms, & expansion of
angiosperms
 Metamorphosis
 Complete
o Larval stage (maggot, grub, caterpillar) looks entirely different than adult
 Incomplete
o Nymphs (small young that resemble adults) go thru series of molts = full
size, fertile, wings
o Subphylum Crustacean
 Lobster, crab, crayfish, barnacle, pillbug
 Branched appendages specialized for feeding & locomotion
 Only arthropod w/ 2 pairs antennae
 Re-gain lost parts after a molt
 Most abundant marine arthropod
Phylum Echinodermata
• Sea stars, urchins, & cucumbers, sand dollars
• Deuterostomes w/ bilateral larva, radial adults
• Slow-moving or sessile marine
• External fertilization
• Regeneration w/ ring canal
• Thin epidermis over endoskeleton of hard calcareous plates
• Water vascular system – coelom
o Network of hydraulic canals branching into tube feet
o Function in locomotion, feeding, & gas exchange
Phylum Chordata
• Made of three invertebrate subphyla (lancelot, tunicate, hagfish) & vertebrates
• Share many features of embryonic development w/ echinoderms, but evolved separately for at least 500
million years