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Characteristics of Animals
Nutrition
Plants vs. Fungi
Plants: are autotrophic eukaryotes that generate organic molecules through
photosynthesis
Fungi: are heterotrophic but grow next to or on their food, and release
enzymes that digest food outside of their bodies
Animals
Unlike plants, animals cannot construct all of their own organic molecules so in
most cases they ingest them- either by eating other living organisms or by eating
nonliving organic material.
Unlike fungi, most animals use enzymes to digest their food only after they have
ingested it
So, basically
• Animals ingest living or nonliving material
• Only secret enzymes after ingestion
Also,
Some animals are:
• Herbivores - eat plants or Carnivores - eats meat, or both. Why? Animals don’t sort food,
(ex. Lions and bunnies)
• Omnivores - Mix of both
• Detritivores- eat things in decay ex. earth worms, important aspect of ecosystems
(heterotrophs) that obtain nutrients by consuming detritus, by doing so they contribute to
decomposition and the nutrient cycles. Many species of bacteria, fungi and protists, unable
to ingest discrete lumps of matter, instead live by absorbing and metabolizing on a
molecular scale
• Parasites - Not in omnivores, specialized mode of life, it’s very specific to it’s host
Cell Structure and Specialization
Cell structure
Animals are multicellular. They lack support of cell walls. Instead, animal bodies are held
together by structural proteins, most abundant being collagen. Animals have 3 unique
types of intercellular junctions - tight junctions, desmosomes, and gap junctions that
consist of other structural proteins.
Specialization
Two distinct specialized cells:
• Muscle cells- organized into muscle tissue, responsible for movement
• Nerve cells- organized into nervous tissue, responsible for impulse conduction
Also,
- Not all animals have true tissues and organs or sexual reproduction (can be lost)
- Definition of Eukaryotic and Multicellular = Not limited to the animals, plants, and
fungi
Reproduction
Most reproduce sexually, many are capable of asexual reproduction.
Asexually
• Considered for the more primitive phylum
• Usually takes place through parthenogenesis (where fertile eggs are produced
without mating, or in some cases through fragmentation)
The diploid stage usually dominates the life cycle. Only haploid part is sex cell,
organisms are haplobiontic diploid
Most of the time, the small flagellated sperm fertilizes a larger, nonmotile egg forming a
diploid zygote.
Early Embryonic development in animals
a. The zygote undergoes cleavage -(mitotic cell divisions without cell growth between
division cycles).
b. Cleavage leads to formation of a multicellular stage called a blastula- (a hollow ball)
c. Following a blastula stage is gastrulation- (many layers of embryonic tissue develop into
adult body parts)
d. The resulting stage is called a gastrula
* Note: Gastrulation: a rearrangement of the embryo in which one end of the embryo
folds inward, expands, and eventually fills the blastocoel, producing layers, the (ecto +
endoderm)
* Belzer said: In most other groups, the blastula undergoes more complicated
rearrangement. It first invaginates to form a gastrula with a digestive chamber, and two
separate germ layers - an external ectoderm and an internal endoderm. In most cases, a
mesoderm also develops between them. These germ layers then differentiate to form
tissues and organs.
Life cycles in many animals also include at least one larval stage. a Larva is a sexually immature
form of an animal that is morphologically distinct from the adult stage, it usually eats different
food, and may even have a different habitat than the adult, as in the case of a tadpole (larva) of a
frog. Larvae eventually undergo metamorphosis, a resurgence of development that transforms
the animals into an adult.
Also,
In sponges, blastula larvae swim to a new location and develop into a new sponge
Clues to the origin of animals
Colonial Hypothesis
Based on the idea that the ancestral animals were heterotrophic colonial flagellates . In essence,
the colonial flagellates were free swimming hollow balls of eukaryotic cells. The colonial
flagellate probably resulted in choanoflagellate or collar cells found in the choanocyte cells of
modern sponges.
How?
Starts with a colonial protist, which is an aggregate of identical cells. Then, it increases to a
hollow sphere of unspecialized cells, then the beginning of cell specialization then a sort of
infolding and finally, left with a gastrula like proto-animal. (outer-layer cells could be used for
locomotion, and the inner layer of cells for digestion and reproduction)
Important animal characteristics:
Biologists consider body symmetry, cephalization, number of germ layers, type of body cavity
and embryonic development important in the classification of animals.
A group of animal species that share the same level of organizational complexity is
known as a grade. Grades not the same as clades. An organism can by polyphyletic.
Traits that define a grade are called a body plan,
Grade vs. Clade
Grade = level of adaptational organisms of similar grade due to similar adaptations due to
convergence.
Clade = a group descended from one common ancestor, a genetic lineage
Body Symmetry
Most sponges lack symmetry altogether, so animals can be classified by lack of symmetry.
Among the animals that do, symmetry can have different forms:
1. Radial Symmetry- The form of a flowerpot. (Ex. sea anemones) The parts radiate from the
center, any slice through the central axis divides the animal into mirror images
2. Bilateral Symmetry- The form of a shovel. (ex. lobster) Has a left and right side. Only one cut
divides the animal into mirror image halves. (ex. humans and arthropods)
A Bilateral animal has
• a dorsal (top) side
• ventral (bottom)
• left and right side
• and an anterior (head) end with a mouth, and a posterior (tail) end **Most bilateral
animals have sensory equipment concentrated at the anterior end, along with a central
nervous system (“brain”) in the head- an evolutionary trend known as cephalization
The symmetry of an animal generally fits its lifestyle. Radial animals are sessile (living attached
to a substrate) or planktonic (free swimming). Bilateral animals generally move actively from
place to place. Their CNS enables them to coordinate complex movements (crawling, burrowing,
flying or swimming)
Tissues ( Germ layers )
True tissues are collections of specialized cells isolated from other tissues by
membranous layers.Sponges lack true tissues. In all other animals, the embryo becomes
layered through gastrulation, as development progresses, these concentric layers called
germ layers, form the various tissues and organs of the body.
Ectoderm- the germ layer covering the surface of the embryo, gives rise to the outer
covering of the animal and, in some phyla, to the central nervous system.
Endoderm - the innermost germ layer, lines the developing digestive tube or archenteron,
and gives rise to the lining of the digestive tract (or cavity) and organs derived from it
(ex. liver, and lungs of vertebrates)
Animals that have only 2 germ layers are said to be diploblastic. (jellies, corals,
cnidarians, comb jellies)
Others with 3 layers (3rd being the mesoderm- between the ecto and endo derm)
Are said to be triploblastic. In triploblasts the mesoderm forms the muscles and
most other organs between the digestive tract and the outer covering of the animal.
Triploblasts include all bilaterally symmetrical animals (flatworms, arthropods and
vertebrates)
Body Cavities
Most triploblastic animals have body cavities - a fluid filled space separating the
digestive tract from the outer body wall.
This body cavity is also known as a coelom. True coelom forms from tissue derived from
mesoderm. The inner and outer layers of tissue that surround the cavity connect dorsally
and ventrally and form structures called mesenteries that suspend the internal organs.
Animals that posses a true coelom are known as coelomates.
Some triploblastic animals have a body cavity formed from the blastocoel, rather from
the mesoderm. This is called a pseudocoelom, and animals that have one are
pseudocoelomates. Currently, true coeloms and pseudocoelom have been gained or lost
multiple times in evolution, so thats why the terms refer to grades, not clades.
Basically:
• Coelomates- (ex. annelids) have a true coelom, a body cavity completely
lined by the mesoderm
• Pseudocoelomates- (ex. nematodes) have a body cavity lined only
partially by mesoderm
• Acoelomates- (ex. flatworms) lack a body cavity
Modes of Development
Protostome and Deuterostome
Both distinguished by many features such as:
Cleavage
Protostomes have a spiral cleavage- in which the planes of cell division are diagonal to the
vertical axis of the embryo. Smaller cells lie in the grooves between the larger, underlying cells.
This determinate cleavage of some animals with this pattern rigidly casts
(“determines”) the developmental fate of each embryonic cell very early. A cell
isolated at the four cell stage from a snail for example, forms an inviable embryo
that lacks many parts.
Deuterostomes have a radial cleavage. Planes are either parallel or perpendicular to the vertical
axis of the egg. The tiers of cells are aligned, one directly above the other.
Most animals with deuterostome development are further characterized by indeterminate
cleavage- meaning that each cell produced by early cleavage divisions retains the capacity to
develop into a complete embryo. (ex. if a sea star embryo is isolated at the four cell stage, each
cell will form a larva) IN humans, the indeterminate cleavage of the zygote makes identical twins
possible. This characteristic also explains the developmental versatility of the embryonic “stem
cells”
Coelom formation
In gastrulation, the developing digestive tube of an embryo initially forms as a blind pouch, the
archenteron. As the archenteron forms in protosome development, initially solid masses of
mesoderm split and form the coelomic cavity, this pattenr is called schizocoelous development
(Greek Schizein: to split).
In Deuterosome development, the formation of the body cavity is described as enterocoelous: the
mesoderm buds from the wall of the archenteron and its cavity becomes the coelom
Fate of the blastopore
Blastopore- the indentation that during gastrulation leads to the formation of the archenteron.
After the archenteron develops, a second opening forms at the opposite end of the gastrula.
Ultimately, the blastopore and this second opening become the two openings of the digestive
tube. (mouth and anus)
In protostome development, the mouth generally develops from the first opening.
In deuterostome development, the anus generally develops from the first opening, and the mouth
second..
Phylogeny
- Modern phylogenetic systematics is based on the identification of clades, which are
monophyletic sets of taxa as defined by shared derived characters unique to those taxa and their
common ancestor.
- A clade might be defined by key anatomical and embryological similarities that are
homologous.
Points of agreement
1. All animals share a common ancestor - tree is monophyletic.
2. Sponges are basal animals.- sponges branch from the base of both animal trees. They exhibit a
parazoan (meaning “beside the animals”) grade of organization. Tissues evolved only after
sponges diverged from other animals. Recent studies believe sponges are paraphyletic
3. Eumetazoa is a clade of animals with true tissues. Every animal but sponges belong in the
clade of eumetazoans “true animals”. Basal members of the eumetazoan clade include
Cnidaria and Ctenophora. These guys are diploblastic (2 germ layers) and have radial
symmetry
4. Most animal phyla belong to the clade Bilateria.- Bilateral symmetry is a shared derived
character that helps to define a clade (and grade) containing the majority of animal phyla
called bilaterians.
5. Vertebrates and some other phyla belong to the clade Deuterostomia. The name deuterostome
refers not only to an animal development grade but also to a clade that includes vertebrates