What is an Animal?

Chapter 25 TURBO BLAST

Why are animals classified as one kingdom?

• All animals are:– Eukaryotic, multicellular– Able to move in specific ways to obtain food,

reproduce, and protect themselves• Most animals:– Have specialized cells/tissues (that form organs)

How do animals obtain food?• All animals are heterotrophic (must eat other

organisms for food)• Depend either directly or indirectly on autotrophs

for food• Animals hypothetically evolved in water (recall

what cells require for all biochemistry)• Some animals living in water do not move from

place to place, must filter food from water– Organisms that permanently attach to a surface

are called sessile– This is not possible for land animals— there is

very little suspended food in the air– Land animals need to use more oxygen and energy to

find food

How do animals digest food? Animals must ingest (take in) food– Once ingested, food must be digested• Some animals digest food within special cells• Other animals have a special internal digestion cavity

– Food is not always available for animals; when an animal encounters excess food, it can be stored as fat or glycogen to use as fuel when food is not available

• Ex: Flatworms (planarian) have only one opening to their digestive tract, called a pharynx

• Ex: Earthworms have a mouth (entrance) at one end and an anus (exit) at the other end of the tract

What are the functions of some animal cells?

• Most animal cells are specialized to carry out different functions– Sensing food– Finding mates– Identify predators/protect themselves – Light reception– Homeostasis– Movement

Development and Fertilization• Most animals reproduce sexually (males

produce sperm, females produce eggs)– Some animals do reproduce asexually

• Most animals develop from a fertilized egg cell called a zygote– Sperm (n) + egg (n) = zygote (2n)

• Fertilization may be internal or external for different species

• After fertilization the development of different animal species all have similar, genetially determined stages of development

Embryonic Development• The zygote (single fertilized cell) divides by

mitosis and cell division to form two cells– This process is called cleavage– Once cell division has occurred, the organism is

known as an embryoA: zygoteB: two cellsC: four cellsD: 8 cellsE: continued cell divisions…F: cell-covered, fluid-filled ball called a blastula

*blastula is formed early in development

Gastrulation• After blastula is formed, cell division continues– Cells from one side of blastula move inward to form a

gastrula• Gastrula: structure made up of two layers of cells with an

opening at one end(Picture how a potter might make a bowl from a round ball of clay)

– Layer of cells on outer surface of gastrula is called the ectoderm – eventually becomes skin and nervous tissue

– Layer of cells lining inner surface of gastrula is called the endoderm—eventually becomes digestive tract

How is the mesoderm formed?• In some animals, development of the gastrula

progresses until a layer of cells called the mesoderm forms – Mesoderm is the third cell layer found in

developing embryo, found between endoderm and ectoderm—eventually becomes muscles, circulatory, excretory, and respiratory systems

Which came first: the mouth or the anus?• When the opening in the gastrula develops into the

mouth, the animal is a protostome– (proto = first; stoma = mouth)– Ex: snails, earthworms, insects

• When the opening in the gastrula develops into the anus first, the animal is a deuterostome– (deutero = second; stoma = mouth)– Ex: starfish, fish, amphibians, birds, reptiles, and you

• Scientists hypothesize that protostomes appeared first in evolutionary history, followed by deuterostomes

• Determining if an animal is a protostome or deuterostome can help biologists identify its phylogeny

What changes occur during growth an development?• Embryonic development of animals holds clues to

animal evolutionary history• Cells in developing embryos continue to

differentiate and become specialized to perform different functions– Most animal embryos develop into juveniles (look like

smaller versions of the adult)– In some animals (insects, echinoderms), the embryo

develops inside an egg into a intermediate stage called a larva—usually doesn’t look like the adult• Sessile animals usually have free-moving larvae• Larvae continue to grow into juveniles, then to adults

Body Plans and Adaptations• Animal body structure is related to how the animal

moves around during its lifetime– Symmetry – describes how an animal’s body structures

are arranged • Asymmetry—no line can divide into two equal

halves– The bodies of most sponges consist of two layers of cells

(no formation of endoderm, mesoderm, or gastrula)

Body Symmetry Relates to Animal Movement

• Radial symmetry can be divided along any plane through a central axis into roughly equal halves (allows animals to move in 360⁰ and detect and capture prey from any direction)

– Animals with radial symmetry include: hydras, jellyfish, starfish, sea anemone, coral, sand dollars

– Animals with radial symmetry have muscle movement controlled by a neural net

Body Symmetry Relates to Animal Movement

• Bilateral Symmetry - can be divided down the animal’s length into similar right and left halves– Animals with bilateral symmetry move

forward, have more coordinated movement

– Anterior or head end has sensory organs (vision, olfactory, auditory) and a central nervous system

– Posterior is the tail end– Dorsal is the upper surface, which

looks different from the Ventral or lower surface

Bilaterally Symmetrical Animals All Develop from Three Embryonic Layers

• Recall the ectoderm, mesoderm, and endoderm cell layers - these give rise to specialized regions in animal body plans– Some bilaterally symm. Animals also have a fluid-

filled cavity called a coelom (coel = “cavity”) that provides internal support for movement

– Animals that lack a coelom are acoelomates– Animals that have a coelom partially covered in

mesoderm are pseudocoelomates– Animals with a coelom surrounded by mesoderm

are coelomates

• A coelom acts as leverage for muscle contractions• Think of the rigidity of a very-full water

balloon• Leads to flexibility of the organism

(bending from side to side, squeezing in sections, etc.)

• Acoelomates may have been the first organisms to evolve• Have a digestive tract that extends

throughout the body• Organs are embedded in the solid

tissues of the body• Pseudocoelomates have more complex

movement• Also have a one-way digestive tract

(mouth, middle section for food breakdown/absorption, anus)

• Coelomates have the greatest diversity• Specialized organs and organ systems

develop in the coelom (which cushions organs and allows them to grow & move within body cavity)

Body cavities led to greater animal diversity• As animals diversified, they adapted to life in different

environments– Some (like mollusks) developed hard shells to protect soft bodies– Some developed hardened internal support, such as spicules

(like sponges)– Some developed exoskeletons – hard coverings on the outside

of the body that provide a framework, protect soft tissues, prevent water loss, and provide protection from predators• These are rigid and must be shed for the animal to grow larger• Exoskeletons are often possessed by invertebrates, animals that lack a

spine (ex: crabs, beetles, spiders)

– Some developed endoskeletons – internal skeletons that protect internal organs and provide brace for muscles• Endoskeletons are made of calcium carbonate, cartilage, or bone

– (Starfish are invertebrates with endoskeletons, while fish, reptiles, amphibians, birds and mammals are vertebrates)

• A vertebrate is an animal with an endoskeleton and a backbone (spine)– All are bilaterally symmetrical

Invertebrate

Invertebrate with Spicules

Invertebrate with Endoskeleton

Invertebrate with Exoskeleton

Vertebrates with Endoskeleton and Spine

Animal Origins• Animals likely evolved from aquatic, colonial

protists– Traced back to late Precambrian fossil records– Bilateral symmetry appears much later, but all body

plans that exist today were present by the Cambrian Period 543 million years ago• These different body plans have diversified since then, but no

new body plans have arisen (based on fossil evidence)