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1.Compare and contrast the protostomes and deuterostomes.
Protosomes
Blastopore becomes mouth & 2
becomes anus
Deuterosomes
nd
opening
Blastopore becomes anus & 2nd opening
becomes mouth
Coelom formed via schizocoely
Coelom formed via enterocoely
spiral, determinate cleavage at 4 —> 8
cell division
radial, indeterminate cleavage at 4 —> 8
cell division
circulatory system primitively dorsal and
nervous system primitively ventral
circulatory system primitively ventral and
circulatory system primitively ventral
Phyla Mollusca, Annelida, Arthropoda and
others
Phyla
Echinodermata,
Chordata and other
Deuterostome
Protostome
Future
intestine
Blastopore Mouth
Hemichordata,
Future
Anus
Future
mouth
Future
intestine
Blastopore
2 Radial cleavage
Gut
Blastocoel
Blastocoel
Mesoderm
Pocket
of gut
Coelom
Mesoderm
4 Regulative embryo
4-cell
embryo
1 blastomere
excised
4-cell
embryo
1 blastomere
excised
2
normal
larvae
2. Describe the enterocoel hypothesis of coelom origin and discuss
the implications.
Enterocoelous development is the stage of embryological development of
deuterostomes in which the coelom forms. The stage starts with the gastrula; as the
archenteron forms, pockets of migrating cells also form, creating another layer
between the endoderm and ectoderm, the mesoderm. These pockets gradually
expand to form the coelom.
Enterocelous development is advance in comparison to schizocoelous development
due to the formation of the archenterons.
3.
Why was the development of a true coelom such an important step in
the evolution of higher animals?
A true coelom is a fluid-filled body cavity formed from mesodermal tissue. It lies
between the outer body wall of epidermal tissue and the gut or digestive tract.
Evolutionary advantages of coelom can be outlined as follows :
1.
The coelomate body plan is a “tube within a tube”. Because this tube is filled
with fluid, it allows fluid circulation, even in primitive animals that lack
circulatory systems.
2.
Fluids (like water) are relatively incompressible. The fluid-filled coelom can
therefore act as a type of rigid skeleton, or hydrostatic skeleton. The
muscles now have something solid to push against.
3.
The coelom allows for an open digestive tract, with a mouth at one end, an
anus at the other, and this tract can be increased by coiling within the coelom
so that it is many times longer than the animal itself.
4.
Animals like flatworms, on the other hand, with one opening into a hollow
cavity, are limited in how fast they can eat, digest, and excrete.
5.
A coelom allows digestion independent of movement. Gut action need no
longer depend on the muscular contractions generated by the animals
movements.
6.
There is more space for the internal organs to develop, especially the gonads,
and large numbers of eggs and sperm can be stored in the coelom as well.
7.
And finally, the combination of a coelom and bilateral symmetry opens up an
entirely new evolutionary pathway, in which parts of the body can be adapted
to perform special functions. This new pathway, which has ultimately given
insects dominion over all other life, is called segmentation.
4. What are the major characteristics of the phylum Cnidaria?
1.
Multicellular, metazoan animals which are radially symmetrical about an oral aboral axis.
2.
3.
4.
Two types of body forms, medusae and polyps.
The animals have reached the tissue level of organisation.
The bodies are derived from two embryonic cell layers (diploblastic), the
ectoderm and endoderm which are separated by a third gelatinous layer called
the mesoglea (this is not the same as “Mesoderm”).
The main body cavity, the gastrovascular cavity has a single opening that
serves as both the mouth and anus.
5.
6.
7.
8.
9.
10.
Support of the organisms is generated either through a calcareous exoskeleton
or through a rudimentary hydrostatic skeleton.
The inner, outer or both surfaces are possess special stinging cells, cnidocytes.
Locomotion and co-ordination are generated by epithelio-muscular cells and a
primitive nerve net.
The animals have no excretory, circulatory or respiratory systems.
Reproduction is by asexual budding or sexual reproduction.
5. What is a possible consequence of a one-way digestive tract?
Cellular specialization in one way digestive tract allows ingested food to be
processed along a one way passage, with unassimilated matter being
ejected out the anus. It does not return and passed back out the oral
opening as observed in cnidarians and most platyhelminthes
6. What criteria are used to divide the animal kingdom into phyla?
Phylum is the second highest taxonomic classification for the kingdom
Animalia. It lies in between kingdom level and class level. Each kingdom is
subdivided into Phylums which are in turn differentiated into classes of
organism.
Phylum category is recognized on the basis of a fundamental body plan and
secondly organisms of one phyla evolved from a common ancestry.
7.
Compare and contrast the degree to which members of the phyla
Porifera, Cnidaria and Platyhelminthes exhibit nervous control of
body functions.
Porifera
Nervous system is absent
Cnidaria
Nervous system organized as a “nerve net”, but lacking a brain.
Platyhelminthes
Nervous system consists of :
a) “brain” and sensory organs located on anterior portion (cephalization)
b) “eyespots” are sensitive to the presence of light
c) lateral nerve cords connected by transverse nerves forming a “nerve ladder”