Invertebrates : are animals that do not
have a backbone.
Most of the animals on earth are
invertebrates.
Invertebrates are are cold-blooded; their
body temperature depends on the
temperature of their environment.
Topics will be explained
•Features of Invertebrates
•Principles of Invertebrate Taxonomy
•Systematic position of selected Invertebrate
species
Features of Invertebrates
A-
Body symmetry
There are three types of body plan
symmetry.
1. Asymmetry :there are no planes of
regular symmetry.
In protozoans such as rhizopods
2. Radial symmetry: body
arranged in circle around a
central axis
• Found in sessile or floating
animals.
• Example : cnidarians and
echinoderms
3-
Bilateral symmetry:
body divides equally along one plane ,
which splits an animal into mirror-image
sides.
•
•
•
Animal have dorsal
and ventral surfaces ,
anterior and posterior ends
Found in freely
mobile animals
Includes the majority
of invertebrates
B- Cephalization:
concentration of sense organs and nerve
cells in the front of the organism ,
associated with the brain and the
formation of the head.
1. sponges: no nerve tissue
2. cnidarians: nerve net
3. worms and arthropods: groups of
nerves called ganglia
4. mollusks: brain
C- Germ layers :
•
Bilateral animals have three distinct layers
of tissue; radial animals have only two.
– both animal types have ectoderm and
endoderm
– bilateral animals have mesoderm
•
•
sponges and cnidarians: two cell
layers with a jellylike material
between them (diploblastic)
other invertebrates have 3 cell layers
(triploblastic )
D- Coelom : The main body cavity .
According to its presence or absence invertebrates
may be classified as :
a. Acoelomate: no body cavity, the body
volume is filled with mesenchyme and other
tissue ,e.g. Platyhelminthes
b. Pseudocoelomate: body cavity partially
lined with mesoderm , e.g. Nematoda
c. Coelomate: body cavity is fluid filled
and it is formed from mesoderm , e.g.
Annelida , Mollusca , Arthropoda .
E-
Metamerism
The arrangement of the organs of the body in
series of similar units along the longitudinal
axis of the body .
• It is most clearly in annelids , the body
divided both externally and internally
into number of segments( metameres )
F. Cleavage : The mitotic division of the
zygote that occurs immediately after
fertilization and produces a ball of smaller
cells (blastomeres )
• Patterns of cleavage :
• Holoblastic :the whole cell divides ,there
being little or no yolk .
• Meroblastic : part of the egg is involved in
the cleavage because of the presence of
large amount of yolk .
G- Specialized cells, tissues, and
organs:
1. Sponges and Cnidarians: little
internal specialization
2. Flatworms: simple organs
3. More complex invertebrates: organ
systems
H- Feeding and digestion
1. intracellular digestion: food is
broken down inside cells (lower
animals)
2. extracellular digestion:
food is digested and then enters the cells
– enables digestion of larger pieces of
food (higher invertebrates)
H- Respiration
1. diffusion through skin: many worms
2. gills: aquatic invertebrates (many mollusks
and arthropods)
3. Mantle cavity: snails
4. Book lungs: spiders
5. tracheal tubes:
many insects
I- Circulation
1. open circulatory system: blood does not
stay within vessels, empties into
sinuses; examples: arthropods, most
mollusks
2. closed circulatory system: blood
remains within vessels examples:
annelids, some mollusks
J- Movement and support
1. hydrostatic skeleton: fluid-filled body cavity
supports the muscles; examples: annelids,
jellyfish
2. exoskeleton: skeleton is outside the body
;examples: arthropods
• Taxonomy (systematics ) :branch of biology
concerned with naming and classifying of the
diverse forms of life .
• Binomial System of nomenclature : method of
naming plants and animals devised by Carlous
Linnaeus .
• Each species is given a two part name made up
of a generic name and specific name .
• For example : in the species Musca
domestica
Musca Is the generic name
domestica Is the specific name
 both generic and specific names are
printed in Italics .
The generic name has a capital initial
letter and the specific name a small letter
.
Protozoan protists
Porifera (sponges)
Cnidaria (coelentratrates)
Platyhelminthes (flatworms)
Nematoda ( round worm )
Annelida (segmented worm)
Mollusca
Arthropod
a
Echinodermata