29.2
Form and Function in
Invertebrates
1
ESSENTIAL QUESTION
HOW DO DIFFERENT
INVERTEBRATE PHYLA CARRY OUT
LIFE FUNCTIONS?
Feeding and Digestion
2
 Simplest animals digest food by intracellular
digestion
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food digested inside cells
Sponges ingest food in archaeocytes which pass into other cells
by diffusion
 More complex invertebrates use extracellular
digestion
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food broken down outside cells in a digestive cavity or tract
and then absorbed into the body
Mollusks, annelids, arthropods, echinoderms
 Flatworms and cnidarians use both processes
Patterns of Extracellular Digestion
3
 Simplest animals such as cnidarians and flatworms
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ingest and expel food through a single opening
Digested food then diffuses to cell throughout body
More complex animals digest food in a tube called a
digestive tract
One-way digestive tract is characteristic of roundworms,
annelids, mollusks, arthropods, and echinoderms
Food enters through mouth and waste leave through
anus
Specialization allows animals to process food more
efficiently
Respiration
4
 Respiratory organs have large surface area that are
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in contact with air or water.
Surfaces must be moist so diffusion to occur
More surface exposed to environment = greater
amount of gas exchange that can occur
Cnidarians and flatworms use diffusion
Aquatic invertebrates like mollusks use gills that
bring blood closer to surface for gas exchange
5
 Terrestrial invertebrates
 Snails have surfaces covered by mucous membranes which
reduces water loss
 Spiders have book lungs
 Insects have tracheal tubes and air enters through spiracles
Circulation
6
 Open circulatory system
 blood is partially contained within a system of blood vessels
 Blood comes into direct contact with tissues and is pumped
into system of sinuses that makes its way back to the heart
 Found in most mollusks (except octopus) and arthropods
 Closed circulatory system
 Heart or heart-like organ forces blood through blood vessels
 Blood stays in vessels and diffuses into tissues
 More efficient than open system
 Characteristic of larger more active animals
 Found in annelids and some mollusks (octopus)
Excretion
7
 Animals must get rid of a byproduct of cellular
respiration – ammonia
 Must also have way to control the amount of water
in tissues
 Aquatic invertebrates – ammonia diffuses from
body tissues into surrounding water
 Flatworms use a system of flame cells to eliminate
excess water
8
 Terrestrial invertebrates must conserve water while
removing wastes
 Animals convert ammonia into urea and is eliminated as
urine
 Some insects and arachnids use Malpighian tubules to
capture ammonia and excrete it as uric acid
Three trends in evolution of nervous system
9
 Centralization – nerve nets consist of nerve cells that
form a net-like arrangement throughout the body
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Flatworms have ganglia in their head
Cephalopods have ganglia organized into a brain that
coordinates the nervous system
 Cephalization – concentration of nervous tissue and
organs in one end of the body
 Specialization – the more complex animals the more
developed sense organs
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Flatworms have simple eyespots
Insects have compound eyes
Movement and Support
10
 Invertebrates have one of three kinds of skeletal
systems
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hydrostatic skeletons – skeleton based on a fluid-filled body
that supports the muscles; worms
Exoskeletons – external skeleton made of chitin which
supports muscles; arthropods
Endoskeletons- structural support inside of body made of
calcified plates; echinoderms
Sexual Reproduction
11
 Most invertebrates reproduce sexually during at least
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one part of their life cycle.
Sexual reproduction maintains genetic diversity by
allowing for new combinations of genes
Meiosis - fusion of haploid gametes during
fertilization to form diploid zygote
Zygote undergoes mitosis and develops into animal
by differentiation
Most invertebrates have separate sexes.
Some animals like the flatworm are hermaphrodites
Asexual Reproduction
12
 Zygote divides by mitosis after fertilization to grow
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into multicellular individuals.
Offspring are identical to adults
Involves diploid cells
allows animals to increase its numbers rapidly
Exact copy of original cell
Includes budding and mitosis (division in two)
Internal and External Fertilization
13
 External – eggs are fertilized outside of females
body
 Internal – eggs are fertilized inside of females body