Invertebrates
A Survey of Invertebrates
Trends in Invertebrate
Evolution
Common ancestors of multicellular
animals had already evolved two
distinct cell layers called germ layers,
separated by a jelly-like middle layer
Refer to chart on embryonic
development
Embryonic Development
Egg + sperm zygote
Zygote undergoes cleavage (mitosis)
to form a mass of cells
Mass develops or several days, hollow
space develops in the middle
At this point, it is called a blastula
Embryonic Development
Blastula further develops into gastrula,
at which time three germ layers are
formed
Germ layers will each develop into
different tissues in the animal
Germ Layers
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
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
Second trend – existence of mesoderm-
lined 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
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
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
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
Organisms with bilateral symmetry also
show cephalization, which means that
they have a distinct front end and back
end (anterior and posterior)
Invertebrate Phyla
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)
Simplest multicellular animals
Asymmetric
Mostly independent, but can live
together
Mostly marine, some in freshwater
Phylum Cnidaria (cnidarians)
Jellyfish, corals, sea anemones, hydras
Mostly marine, few freshwater
Radial symmetry
Phylum Plathyhelminthes
(flatworms)
Bilateral symmetry
Show cephalization
Mostly parasitic
Phylum Nematoda (nematodes
AKA roundworms)
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)
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)
AKA segmented worms
Many are detritus feeders
Live on land and in water
Phylum Echinodermata
(echinoderms)
Star fish, sea cucumbers, sea urchins)
Live entirely in water
Spiny skin
Have five-part radial symmetry
Phylum Arthropoda
(arthropods)
Most successful animals - largest and
most diverse phylum
Jointed-leg animals
Exoskeletons
Divided into three groups
Phylum Arthropoda
(arthropods)
Subphylum Chelicerata includes
spiders and scorpions
Named by mouth parts (chelicerae)
Mostly carnivorous, can be posionous
Class Crustacea includes lobster,
shrimp, crab
Class Insecta contains ½ of all
arthropods
Phylum Chordata
Invertebrate chordates
Link between invertebrates and
vertebrates
Endoskeleton with notochord, to which
muscles are attached
Ex: Sea squirt