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General Biology 101
Chapter Outlines
Starr & Taggert
10th edition
Chapter 25:
Animals: The Invertebrates
The introduction of this chapter begins with a story of a how a little girl’s body plan has
design origins that are related to ancient animal lineages, in particular the later derived
primates, yet the complexity of animals extends even further back into geologic history.
The story of the Burgess Shale in western Canada is also briefly mentioned, which hilights some of the most primitive animals from the Cambrian era.
Section 25.1
Overview of the Animal Kingdom
Characteristics:
- Multi-celled
- Diploid chromosomes in body cells.
- Heterotrophs that ingest other organisms
- Require oxygen for aerobic respiration
- Reproduce sexually, though some forms also reproduce asexually
- Motile at some point in their life cycle.
- Life cycles include stages in embryonic development.
Different layers of tissues (important during developmental stages):
 Ectoderm (outer most)
 Mesoderm (middle layer)
 Endoderm (inner most)
Two main divisions of animals:
= Vertebrates – animals with a backbone for support and protection of nerve cord.
= Invertebrates – animals without a backbone
 There are more than 30 phyla of animals.
Body Symmetry
Radial – body parts arranged around a central axis like spokes in a wheel.
Radial animals live in water – can capture food drifting along from any direction.
Bilateral – body plan that can be divided in half i.e. front and back, or dorsal (top) and
ventral (bottom) or right and left sides.
Cephalization seen in animals with a head or forward end that searches for food in a
particular direction. Sensory parts and nerve cells concentrated towards the head end of
the animal.
Other features:
Gut – a sac projecting into the body or part of a tube through the body. Site of food
digestion.
 Saclike gut (one opening) e.g. jellyfish
 Tubular (two openings) e.g. vertebrates like dogs – considered to be a
“complete” digestive system.
Coelom – A body cavity between the gut and the body wall. Encloses organs and holds
them into position. Important because the presence of a coelom allows for greater
complexity in animals, greater size, more independent life style.
Peritoneum – a lining of a body cavity.
Segmentation – repeated series of body units e.g. segmented worms.
Section 25.2
Puzzles about origins
Animals are thought to have originated somewhere between 1.2 billion years ago and 670
million years ago (Precambrian) based on fossil tracks, burrows and microscopic
embryos.
Different hypotheses exist about where animals came from:
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Multi-nucleated protista that became compartmentalized.
Colonies of flagellated cells similar to Volvox, certain cells became specialists
with a division of labor.
Colonies flattened and began creeping on the seafloor.
Placozoan – plate like animal that is soft-bodied and marine.
Trichoplax adhaerans – 2 layered body plan with some specialist cells. Perhaps similar
to the very earliest of animals.
Section 25.3
Sponges
Phylum Porifera –
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Animals with no symmetry, no tissues and no organs.
Survivors since Precambrian
8000 species, but only 100 or so are fresh water
Home to many other animals e.g. shrimp and worms
Shape is flattened and sprawling, can be tube like or vase like too.
Different layers of cells but don’t have true tissues.
 Sharp glasslike spicules stiffen body and are a unique feature.
 Have collar cells that have flagella that direct movements of food through
pores and filter their food.
 Collars are food trapping devices.
 Reproduce sexually (making larvae) but some also reproduce asexually.
Section 25.4
Phylum Cnidarians
 Radial symmetry
 Tentacled
Scyphozoans – Jellyfish
Anthozoans – Sea anemones
Hydrozoans – Hydra
 Most forms are marine
 Have nematocysts i.e. stinging cells with barbed, harpoon like threads used in
hunting and for defense.
Body plan
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Medusa (bell shaped) – mouth underneath
Polyps (cup shaped) – mouth above
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Have epidermal tissues (first organized tissues seen in animal lineage)
Have a sac like gut
Have a nerve net – controls changes in movement and shape.
Hydrostatic skeleton (a fluid filled cavity which contractile cells like muscles
can act against for the purpose of movement).
Life cycle of cnidarians can include a sexual medusa stage followed by fertilized gametes
that unite to become a zygote. Then proceed to a planula stage that is a “creeping” stage
and then finally become a polyp and starts the cycle all over again when the medusa splits
loose from the polyp.
Many cnidarians form colonies e.g. Portuguese Man of War.
Section 25.5
Acoelomate Animals (having no coelomic cavity)
Organs are when there are two or more tissues that are in a patter and perform a specific
task.
Organ systems are composed of two or more organs working together that contribute to
the survival of the whole organism.
Flatworms are the simplest animals that demonstrate organ systems. Some are parasites
on other animals.
Characteristics of flatworms:
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Bilateral
Cephalization
Flattened body shape
Simple gut with some branching
Pharynx is muscular tube that is used in feeding.
Some are hermaphroditic i.e. have sex organs of both male and female.
Some can make clones of themselves.
Major groups of flatworms.
I.
Turbellarians – Planarians
Have a simple digestive system, ladder like nervous system, both ovaries and
testes and protonephridia (water regulation organs).
II.
Trematoda – flukes
Parasitic with both sexual and asexual life stages.
III.
Tapeworms
Parasites of vertebrate intestinal systems. Has a scolex “head” end for
attachment to the gut wall. No independent gut since they live bathed in predigested nutrients.
Proglottid segments behind the tapeworm are the reproductive structures; they
are hermaphroditic.
Section 25.6
Nematodes – Roundworms
May be the most abundant animals alive – they are literally in all habitats.
Characteristics:
 Cylindrical body
 Tapered on both ends
 Cuticle covering (protective and tough covering).
 Has a complex digestive system – through gut.
 False coelom – gut, and cavity around the gut but not lined with a peritoneum.
Some species of roundworms are parasitic, but most are free-living i.e. not parasitic and
they are harmless to other animals.
Important to humans because they cycle nutrients for communities i.e. they digest and
break down detritus.
Caenorhabditis elegans is important to researchers of inheritance. It was the first animal
to have its genetic sequence mapped.
Section 25.7
Rouge’s Gallery of Worms
Many species of worms can infect humans
Schistosomiasis – blood fluke (primarily in Asia)
Tape worms
Guinea worm (a sub-dermal roundworm)
Pinworms
Hookworms
Trichinella (a roundworm that is found in undercooked pork or some game animals).
Elephantitis – a roundworm that blocks lymph circulation and causes a deformed look to
the limbs especially.
Section 25.8
Rotifers
Characteristics
 Bilateral symmetry
 Cephalization
 False coelom – but full of many organs
 Most are fresh water
 Predators on microscopic bacteria and algae
 Have ciliated lobes on their head end for feeding – appears to be a rotating
wheel when viewed under the microscope.
 Usually found in ponds, lakes and some in marine environments.
 Important zooplankton form
Section 25.9 – Major Divergence
In late Precambrian there were two lineages of coelomates that occurred.
I.
Protostomes
Includes: mollusks, annelids, and arthropods
Demonstrate spiral cleavage in early embryonic development.
First opening to develop is the mouth.
II.
Deuterostomes
Includes: echinoderms and chordates
Early cleavage undergoes simultaneous parallel and perpendicular cell
divisions.
First opening to develop is an anus and second is the mouth.
Coelom forms as an out pocketing of the gut wall.
* This is relevant because the type of embryonic development affects body
plans in animals.
Section 25.10
A Sampling of Mollusks
Characteristics:
 Fleshy, soft bodies.
 Have a coelom
 Bilateral symmetry
 Many have shells (but not all)
 Presence of a mantle*
 Have internal gills with thin-walled leaflets.
 Many have a large foot for locomotion
 Radula – tongue like apparatus with a tooth to rasp away food.
 Most have eyes and tentacles.
*Mantle – A tissue that gives rise to calcium carbonate shells.
Groups:
I.
Gastropods – slugs and snails, limpets and nudibranchs.
Many have spiral shells.
II.
Chitons – dorsal shell with 8 plates
Usually slow-moving or sedentary
III.
Bivalves – Animals with 2 hinged shells e.g. clams, scallops and oysters
IV.
Cephalopods – squids and octopi have fast method of locomotion and highly
developed eyes and nervous systems.
Section 25.11
Evolutionary Experiments with Molluscan Body Plans
Torsion is a process of twisting the internal organs during development in gastropods i.e.
snails; results in the anus being near the mouth of the animal.
Adaptive (in a sense) in that it allows the snail to draw in its soft head part under the shell
to be protected.
Shell provides protection/armor.
In clams there are 2 siphon tubes that bring water in, and also cause it to be carried out
containing the wastes of digestion.
 Cephalopods include the squids, octopus and chambered sea nautilus, have
large brains relative to other mollusks and invertebrates.
 They can also discharge an inky fluid for protection and to confuse predators.
 They move by jet propulsion.
 Have a closed circulatory system.
Section 25.12
Annelids – Segments Galore
Annelids means “ringed” forms
Includes 15,000 species.
Characteristics:
 Bilateral
 Segmented
 Have setae – chitin reinforced bristles on the sides (except in leeches).
 Hydrostatic skeleton – fluid cushioned coelomic chambers.
 Contain nephridia, which controls water/body fluids composition.
 Have a brain and paired nerve cords.
 A closed circulatory system with multiple hearts.
Advantage of segmentation
- Partitions divide body into separate chambers each with a repeating set of
organs.
- Gut extends throughout
- Cuticle on the surface of the animal’s skin doesn’t prevent water loss though,
but there is some degree of protection.
Earthworms are scavengers.
One worm can eat its own weight equivalent of detritus each day.
The text describes in detail how earthworms move with respect to their interacting
muscles and bristles.
Polychaetes are many bristled, marine worms.
Section 25.13
Arthropod Diversity
Most prolific in terms of numbers i.e. numbers of species and the most diverse in terms of
their ability to inhabit different habitats.
Characteristics:
 Hardened exoskeleton
 Jointed appendages
 Fused and modified segments
 Specialized respiratory structures
 Efficient nervous system and sensory organs
 Division of labor in their life style.
- Exoskeleton is reinforced with chitin, proteins, surface waxes and calcium carbonate
deposits. This structure may be an adaptation for defense, and it also limits water loss.
Must be periodically shed/molted though to grow.
- Jointed appendages: cuticle is thinnest at the joints. Arthropod means “jointed foot.”
- Fused and modified segments: In most modern arthropods groups of segments are
fused/joined together to perform specialized functions.
An insect has three main regions called:
Head
Thorax
Abdomen
- Respiratory structures: Gills in aquatic forms. Tracheae tubes in land forms.
- Specialized sensory structures: complex eyes and antennae.
- Division of labor: with respect to different stages of development.
Metamorphosis is the body changing form from embryo to adult stages.
 Incomplete
 Complete
Section 25.14
The Spiders and their kin
Chelicerates – originated in shallow seas early in the Paleozoic, odd looking, almost
scorpion like crustaceans.
This group includes:
I.
II.
Aquatic forms e.g. horseshoe crab
Terrestrial forms e.g. spiders, scorpions, mites and ticks.
Ticks are parasites of vertebrates – sucking their blood for nutrients.
Spiders are very important because they are predators of many insect pests.
Characteristics:
 2 segments to their body
o Fore-body – includes the legs, pedipalps (sensory organs), and
chelicerae (which delivers the venom).
o Abdomen
 Some form spider webs for hunting purposes.
 Have an open circulatory system.
 Book lungs for respiratory structures.
Section 25.15
Crustaceans
Get their name because they have a hard external “crust” i.e. a very thickened
exoskeleton.
35,000 species
Some are food for humans
Most are marine forms (though there are a significant number that are fresh water).
Examples: Crabs, Lobsters, Barnacles, Pill bugs
Have numerous segments and legs
Diverse array of appendages including in some forms claws or feathery, food-collecting
appendages.
Copepods are aquatic grazers consuming phytoplankton; that play a very significant role
in aquatic food webs.
Section 25.16
Uniramians
Millipedes – have 2 pairs of legs per segment. Scavengers in the leaf litter.
Centipedes – have a flattened body, each segment has one pair of legs per each
segment. All species are fast, aggressive predators with fangs and venom glands.
Section 25.17
Insect Diversity
Characteristics:
 Segments fused into 3 distinctive regions.
= Head : with many feeding adaptive structures for handling food.
= Thorax: location of 3 pairs of legs and the wings (if present).
= Abdomen: holds the reproductive structures.
 Gut is complex with 3 sections i.e. fore, mid, and hind regions.
 Malpighian tubules – device that helps insect to get rid of wastes.
Waste product is called uric acid.
Most successful of insects are small and have wings.
Nymphs – are miniature forms of the adults. Molt between growing periods to grow
larger in size.
Pupae – larval form in some insects that undergo metamorphosis e.g. butterfly and moths.
Insects are important to us because they can destroy crops, food, and lumber.
Some transmit diseases e.g. malaria
Some are important pollinators of our food crops e.g. bees
Types of metamorphosis/transformation:
I.
II.
Incomplete – gradual, partial change from immature until the last molt.
Complete – go through various stages before final molt.
Egg  larvae  pupa  adult
Note: tissues of immature forms are destroyed and replaced before the adult emerges.
Section 25.18
This section hi-light some of the harmful insects and a few statistics about bites from
hazardous arthropods:
Examples:
Spiders – brown recluse and black widow
Mites – ticks included in this section and the mention of Lyme disease is made in
this context.
Scorpions – painful stings
Beetles – western corn rootworm is hi-lighted for an example of a crop-destroying
pest species.
Section 25.19
The Puzzling Echinoderms
Examples – sea stars, feather stars, sea urchins, sea cucumber, sea lilies, sea biscuits
(which is a type of sand dollar).
6000 different species
Characteristics:
 Marine
 Deuterostomes
 Coelomates
 Body has spines and plates made of calcium carbonate
 Have no brain – decentralized nervous system
 Water vascular system having tube feet for locomotion and feeding is unique
to echinoderms (some eviscerate their stomach externally to digest food).
 Demonstrate both bilateral and radial features.