The document defines various biological terms related to animal development and symmetry. It describes processes like cleavage, gastrulation, and germ layer formation. Key terms include acoelomate, bilateral symmetry, blastopore, blastula, cephalization, coelom, coelomates, determinate cleavage, deuterostomes, diploblastic, ectoderm, endoderm, eumetazoan, gastrulation, germ layers, grade, Hox gene, indeterminate cleavage, mesoderm, parazoan, protostomes, pseudocoelomates, radial cleavage, radial symmetry, spiral cleavage, and triploblastic. Diagrams accompany many definitions

2. ACOELOMATE
Lacking a
fluid filled
body cavity
Organisms showing
acoelomate formation
include the platyhelminthes
(flatworms, tapeworms etc.),
the cnidarians (jellyfish and
allies), and the ctenophores
(comb jellies)

3. BILATERAL SYMMETRY
n bilateral
symmetry (also
called plane
symmetry), only
one plane, called
the sagittal
plane, will divide
an organism into
roughly mirror
image halves

4. BLASTOPORE
an opening into
the
archenteron
during the
embryonic
stages of an
organism.

5. BLASTULA
hollow sphere of
cells formed
during an early
stage of
embryonic
development in
animals. The
blastula is created
when the zygote
undergoes the
cell division
process known as
cleavage.

6. CEPHALIZATION
an evolutionary
trend, whereby
nervous tissue,
over many
generations,
becomes
concentrated
toward one end of
an organism. This
process
eventually
produces a head
region with
sensory organs

7. COELOM
fluid-filled cavity
formed within
the mesoderm
of some
animals.
coelom of an annelid

8. COELOMATES
A coelomate
animal is
basically a set
of concentric
tubes, with a
gap between
the gut and the
outer tubes.
An earthworm is a
coelomate

9. DETERMINATE CLEAVAGE
the form of cleavage
in most
protostomes. It
results in the
developmental fate
of the cells being
set early in the
embryo
development. Each
cell produced by
early embryonic
cleavage does not
have the capacity
to develop into a
complete embryo.
A Caribbean Reef Squid, an
example of a protostome.

10. DEUTEROSTOMES
Deuterostomes
are
distinguished
by their
embryonic
development;
in
deuterostomes,
the first Sea cucumbers and
opening (the other echinoderms
blastopore) are deuterostomes.
becomes the
anus,

11. DIPLOBLASTIC
Pertaining to a
condition in
which there are
two primary
germ layers,
such as
ectoderm and
endoderm. Diploblastic animals
have two germ
layers: an outer
ectoderm and an
inner endoderm.

12. ECTODERM
The "ectoderm"
is one of the
three primary
germ cell
layers in the
very early
embryo

13. ENDODERM
Endoderm is one of
the three primary
germ cell layers in
the very early
embryo. The other
two layers are the
ectoderm (outside
layer) and
mesoderm
(middle layer),
with the
endoderm as the
intermost layer

14. EUMETAZOAN
a clade comprising all
major animal groups
except sponges
Anything but sponges
are eumetazoan

15. GASTRULATION
a phase early in the
embryonic
development of
most animals,
during which the
single-layered
blastula is
reorganized into a
trilaminar ("three-
layered") structure
known as the
gastrula - it is, in
other words, the
formation of the
gut.
Stages of gastrulation

16. GERM LAYERS
a group of cells,
formed during
animal
embryogenesis
Germ layers are particularly
pronounced in the vertebrates;
however, all animals more complex
than sponges (eumetazoans and
agnotozoans) produce two or three
primary tissue layers

17. GRADE (PHYLOGENETIC)
a group of species
united by
morphological or
physiological
traits, that has
given rise to
another group
that differs
markedly from the
ancestral
condition, and is Fish represent a grade,
thus not inasmuch as they have
considered part of given rise to the land
the ancestral vertebrates. In fact, the
group. three traditional classes
of fish (Agnatha,
Chondrichthyes and
Osteichthyes) all
represent evolutionary

18. HOX GENE
group of related In the late 1940s, Edward Lewis
genes that began studying homeotic
determine the mutation on Drosophila
basic structure melanogaster which caused
and orientation bizarre rearrangements of
of an organism. body parts. Mutations in the
genes that code for limb
development can cause
deformity or lead to death. For
an example, mutations in the
Antennapedia gene cause legs
instead of the antenna to
develop on the head of a fly.
These mutants sometimes
occur in wild populations of
flies, and it was these mutants

19. INDETERMINATE CLEAVAGE
A cell can only be The sea urchin is a
indeterminate if it has a deuterostome
complete set of
undisturbed
animal/vegetal
cytoarchitectural
features. It is
characteristic of
deuterostomes - when
the original cell in a
deuterostome embryo
divides, the two
resulting cells can be
separated, and each one
can individually develop
into a whole organism.

20. MESODERM
one of the three
primary germ cell
layers in the very
early embryo. The
other two layers
are the ectoderm
(outside layer)
and endoderm
(inside layer), with
the mesoderm as
the middle layer
between them.

21. PARAZOAN
Parazoans are
sponges

22. PROTOSTOMES
Protostomes are
animals in
which the
blastopore
becomes the
mouth
A Caribbean Reef
Squid, an example
of a protostome.

23. PSEUDOCOELOMATES
A pseudocoelomate is Roundworms are
any invertebrate examples of
animal with a three- pseudocoelomates
layered body and a
pseudocoel. The
coelom was
apparently lost or
reduced as a result of
mutations in certain
types of genes that
affected early
development. Thus,
pseudocoelomates
evolved from
coelomates.

24. RADIAL CLEAVAGE
Radial cleavage is Sea urchins have
characteristic of radial cleavage
the
deuterostomes,
which include
some vertebrates
and echinoderms,
in which the
spindle axes are
parallel or at right
angles to the
polar axis of the
oocyte.

25. RADIAL SYMMETRY
These organisms
resemble a pie
where several
cutting planes
produce roughly
identical pieces. An
organism with
radial symmetry
exhibits no left or
right sides. They
have a top and a
bottom (dorsal and
ventral surface)
only.

26. SPIRAL CLEAVAGE
Spiral cleavage is Mollusks have spiral
characteristic of cleavage
protostomes. A
developing embryo has
spiral cleavage if as it
undergoes cell division
(cleavage) and changes
from a four-cell embryo
to an eight-cell embryo,
the cells divide at sli ght
angles to one another,
so that the none of the
four cells in one plane
of the eight-cell stage is
directly over a cell in the
other plane.

27. TRIPLOBLASTIC
condition of the Flatworms are
blastula in which triploblastic
there are three
primary germ
layers: the
ectoderm,
mesoderm, and
endoderm. The
germ layers
form during
gastrulation of
the blastula.