Invertebrates
A Survey of Invertebrates

Trends in Invertebrate
Evolution
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Common ancestors of multicellular animals had already evolved two distinct cell layers called germ layers, separated by a jelly-like middle layer

Germ Layers
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The ectoderm (outer layer) will develop into skin and other body coverings, glands, and nervous system tissues
The endoderm (inner layer) grows into tissues and organs of the digestive tract and respiratory system

Germ Layers
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Middle layers varies by animal
Sponges and jellyfish only have a primitive layer of jelly-like cells
Higher animals have an actual third cell layer called the mesoderm which develops into skeletal, muscle, and circulatory tissues, and reproductive system

Trends in Invertebrate
Evolution
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Second trend – existence of mesodermlined cavity called coelom
Provides space for organs to grow and function without being twisted or squeezed by body movements
Fluid within coelom helps to carry food, wastes, dissolved gases

Coelom
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Acoelomates = animals without a coelom, like the hydra
Pseudocoelomates = have a body cavity between mesoderm and endoderm, like roundworm
Coelomates = have a true coelom that houses digestive tract and organs, like annelids (earthworm)

Trends in Invertebrate
Evolution
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Third trend is the evolution of a body
plan, built of several compartments
Compartments called segments, allow for increase in body size
In higher animals, segments are specialized and more complex

Trends in Invertebrate
Evolution
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Many organisms also have symmetry
Radial symmetry = body parts repeated around an imaginary line drawn through center of the body
Bilateral symmetry = left and right sides are identical

Trends in Invertebrate
Evolution
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Organisms with bilateral symmetry also show cephalization, which means that they have a distinct front end and back end (anterior and posterior)

Invertebrate Phyla
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Phylum Porifera (sponges)
Phylum Cnidaria (cnidarians)
Phylum Platyhelminthes (flatworms)
Phylum Nematoda (nematodes or roundworms)
Phylum Mollusca (mollusks)
Phylum Annelida (annelids)
Phylum Echinodermata (echinoderms)
Phylum Arthropoda (arthropods)
Phylum Chordata (invertebrate chordates)

Phylum Porifera (sponges)
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Simplest multicellular animals
Asymmetric
Mostly independent, but can live together
Mostly marine, some in freshwater

Phylum Cnidaria (cnidarians)
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Jellyfish, corals, sea anemones, hydras
Mostly marine, few freshwater
Radial symmetry

Phylum Plathyhelminthes
(flatworms)
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Bilateral symmetry
Show cephalization
Mostly parasitic

Phylum Nematoda (nematodes
AKA roundworms)
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First to show a tube-like digestive system (mouth at one end, anus at the other end)
Some are microscopic, some can be a meter long

Phylum Mollusca (mollusks)
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Clams, snails, squids
Can live everywhere, even on land
Many have outer shells for protection
Sea snails secrete chemicals to avoid predators
Clams, mussels, oysters, scallops are bivalves
(double shells) and can shut when in danger
Squids and octopuses also emit dark ink to confuse predators

Phylum Annelida (annelids)
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AKA segmented worms
Many are detritus feeders
Live on land and in water

Phylum Echinodermata
(echinoderms)
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Star fish, sea cucumbers, sea urchins)
Live entirely in water
Spiny skin
Have five-part radial symmetry

Phylum Arthropoda
(arthropods)
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Most successful animals - largest and most diverse phylum
Jointed-leg animals
Exoskeletons
Divided into three groups

Phylum Arthropoda
(arthropods)
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Subphylum Chelicerata includes spiders and scorpions
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Named by mouth parts (chelicerae)
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Mostly carnivorous, can be posionous
Class Crustacea includes lobster, shrimp, crab
Class Insecta contains ½ of all arthropods

Phylum Chordata
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Invertebrate chordates
Link between invertebrates and vertebrates
Endoskeleton with notochord, to which muscles are attached
Ex: Sea squirt