Evolution of Animals
Fossil record of invertebrates is incomplete because soft-bodied animals are poorly preserved as fossils.
All animals probably evolved from Protists.
The classification of animals is based on the level of organization or number of germ layers, symmetry, type of coelom, body
plan, and presence or absence of segmentation.
The following evolutionary tree is based on these features and shows a possible evolutionary relationship between the
Levels of Organization
Three levels or organization: cell, tissue, or organ
One of the main events during animal development is the establishment of _____________________ layers.
If two germ layers (ectoderm and endoderm) are present, then the animal has the tissue level of organization.
If all three germ layers (ectoderm, endoderm, and mesoderm) are present, then the animals has the organ level of
Type of Body Plan
Two body plans are present in the animal kingdom:
_______________________ plan:
Incomplete digestive system with only one opening.
Ex: Jellyfish & planaria
____________________-within-a-tube plan:
Complete digestive system.
Two openings allows for specialization along the length of the tube.
Ex: Roundworms, earthworms, insects
Type of Symmetry
Animals can be asymmetrical, radially symmetrical, or bilaterally symmetrical.
________________________________ animals have no particular symmetry.
Radial symmetry means the animal is organized similar to a wheel.
Bilateral symmetry means the animal has definite right and left halves.
Bilateral symmetry leads to cephalization (brain and sense organs located an anterior end of animal).
Type of Coelom – for animals with bilateral symmetry
A true coelom (in coelomates) is an internal body cavity completely lined with mesoderm, where internal organs are found.
Allows organs to freely move, grow, and develop independently of the body wall
Fluid cushions and protects organs from shocks – in some acts as a skeleton
Allows for separation of digestion & circulation
Allows for increase in size & metabolic rate.
Coelomates are either protostomes or deuterostomes (explained a few slides later).
Ex: Annelids, mollusks, arthropods
Type of Coelom – for animals with bilateral symmetry
Pseudocoelomates have a body cavity incompletely lined with mesoderm.
No circulatory system – fluid in pseudocoelom transports oxygen & nutrients
Pressure of fluid inside also provides support as would a skeleton
More mobile, more complex reproductive & digestive systems.
They can store wastes for discharge out of the body.
Ex: Roundworms, rotifers
Type of Coelom – for animals with bilateral symmetry
Acoelomates have mesoderm but no body cavity.
Movement squeezes & distorts the body, restricting the flow of nutrients and other materials
Have no circulatory system. Must rely either on diffusion or on muscle contractions for the transport of
nutrients, respiratory gases, and waste products around the body. (Less efficient than heart & blood
vessels which develop in those with a coelom.)
Ex: Flatworms
Protostomes vs. Deuterostomes
During development, when the embryo resembles a tiny globe of cells, a small pucker develops on one side of the embryo.
This grows into a pocket, and allows some cells to migrate inside to form an additional layer of cells within the outer layer.
In the Protostomes, the mouth develops from the edge of this pocket; the anal opening develops later.
In the Deuterostomes, the reverse is true; the pocket edge develops into the anus, and the mouth is formed later.
Segmentation is the repetition of body parts along the length of the body.
Animals can be segmented or nonsegmented.
Segmentation leads to specialization of parts because the various segments can become differentiated for specific purposes.
Ex: annelids, arthropods, and chordates (includes vertebrates).
Phylum Porifera - Sponges
Meaning: Pore-bearing
Symmetry – none (asymmetrical)
Organization – cellular level
Acoelomates – NA
Body plan - NA
Habitat – fresh & salt water
PORIFERA:Life processes
Sponges are classified according to type of spicules
Support - spicules (act like bones for the sponge)
Diet - filter feeders – filter bacteria, protists, and sometimes small crustaceans
Feeding – Filaments trap food, Collar cells digest food (engulf food particles (endocytosis), digest them,
and pass them to amoeboid cells.
Movement:- Swim as larva- Sessile (permanently attached to a surface) as adults.
Response - no nervous system.
Excretion - through the osculum.
Respiration - take in oxygen as water passes through body – diffusion.
Internal transport - ameboid cells transport nutrients around the body from cell to cell
Reproduction –Hermaphrodites (make eggs & sperm).
Asexually by Budding: Produce internal buds called gemmules (cells containing ameboid cells, organic
molecules, & spicules) that can grow into new sponges when the conditions are more favorable
Regeneration: Growth of a whole organism from a fragment
Phylum Cnidaria
Named for – specialized stinging cells called cnidocytes (contain the stingers called nematocysts – these may contain
Symmetry – radial
Organization – have endoderm & ectoderm -tissue level
Acoelomates – NA
Non-segmented; sac body plan
Habitat – mostly salt water, hydra found in fresh water
Phylum Platyhelminthes
Meaning – flatworms
Symmetry - bilateral
Sac body plan; non-segmented; acoelomates.
Organization - 3 germ layers – endoderm, ectoderm, & mesoderm – organ level
Have organs for all life processes except respiration and circulation
Habitat – fresh or salt water, moist environments, inside host
Phylum Nematoda (Roundworms)
Bilateral symmetry
Organization - 3 germ layers – endoderm, ectoderm, & mesoderm – organ level
Pseudocoelomates (body cavity) – filled with fluid – space for organs
Tube-within-a-tube body plan – complete digestive tract with mouth and anus.
Habitat – fresh or salt water, soil, inside host (both plants & animals)
Phylum Annelida (segmented worms)
Symmetry - bilateral
Organization - 3 germ layers – organ level
Coelomates -have a body cavity – more complex organs
Tube-within-a-tube body plan – specialized organs in digestive tract
Segmented both externally, and internally by partitions called septa.
Habitat – fresh or salt water, soil
Phylum Mollusca
Meaning – soft-body
Coelomates (reduced in some)
Symmetry – bilateral
Organization – organ level
Tube-within-a-tube body plan
No segmentation
Found in fresh or salt water, and on land
Phylum Echinodermata
Habitat – salt water
Examples: Sea lilies, feather stars, sea cucumbers, brittle stars, sea urchins, sand dollars, & sea stars
Symmetry – radial as adult, larva is a free-swimming filter feeder with bilateral symmetry.
Organization – organ level
Coelomates; non-segmented
Tube-within-a-tube body plans
Phylum Arthropoda
Meaning – “jointed foot”
Symmetry – bilateral
Coelomates – complex internal organs
Tube-within-a-tube body plan
The most varied and numerous of animals (over 1 million identified, probably 30 million really exist!!)
Five Classes – Crustaceans, Insects, Arachnids, Millipedes, & Centipedes
Many exhibit social behavior, such as bees or ants.
Response: in addition to brain & nerve cord, head usually bears a pair of antennae, compound eyes, simple eyes, and in
some tympanum for the reception of sound waves.
Locomotion: thorax bears three pairs of legs and up to 2 pairs of wings.
Excretion - Malpighian tubules collect nitrogen waste, which is added to digestive tract; waste then passes out through anus.
Include terrestrial spiders, scorpions, ticks, and mites.
Ticks and mites are parasitic
2 body segments – cephalothorax & abdomen
The cephalothorax bears six pairs of appendages: the chelicerae and the pedipalps, and four pairs of walking legs.
Chelicerae contain fangs to deliver poison
Pedipalps sense or hold the prey