Chapter 25 – What is an animal?
I.
Characteristics of Animals
a. eukaryotic – membrane bound nucleus
b. multicellular
c. reproduce
d. obtain food
e. protect themselves
f. specialized cells that form tissues and organs
A. Obtain Food
1. heterotrophs – consume food for energy
2. believes to evolve in water
3. sessile – permanently attached to a surface so don’t expend
much energy to obtain food
B. Animals digest food
C. Animal cells adapt
II.
Development of Animals
A. fertilization
1. most reproduce sexually
2. fertilization occurs when a sperm cell penetrates the egg
cell, forming a new cell called a zygote
3. may be external or internal
B. cell division
1. zygote divides by mitosis and cell division to form two
cells
2. once cell division occurs it is called an embryo
C. Gastrulation
1. a structure made up of two layers of cells with an opening
at one end
2. ectoderm – layer of cells on the outer surface of the
gastrula
3. endoderm – layer of cells lining the inner surface
D. Mesoderm
1. a third layer of cells between the ectoderm and the
endoderm
2. forms muscles, digestive system, circulatory system,
respiratory system
3. protosome – opening in the gastrula develops into the
mouth
4. See p. 678 Fig 25.5
III.
Body Plans
a. Symmetry – a term that describes the arrangement of body structures;
different kinds of symmetry enable animals to move about in different
ways
1. asymmetry (no symmetry) – irregularly shaped body with
no symmetry or asymmetrical; often sessile; ex. sponges
2. radial – can be divided along any plane into roughly equal
halves; enables an animal to detect and capture prey
coming toward it from any direction; typically has arms;
ex. jellyfish
3. bilateral symmetry – divided down its length into similar
right and left halves; only divided along one plane
a. anterior – head end
b. posterior – tail end
c. dorsal – back
d. ventral – belly
e. lateral – side
b. acoelomates – no body cavities
c. pseudocoelemates – a space that develops between the endoderm and
mesoderm
d. coelom – space completely surrounded by mesoderm
e. animal protection & support
A. exoskeleton – hard covering on the outside of the body that
provides a framework for support
B. invertebrate – does not have backbone
C. endoskeleton – internal skeleton covered by layers of cells and
provides support
D. vertebrate – with a backbone