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For its first four billion years, earth was populated, initially, only by prokaryotic organisms, and later by very simple metazoan life forms. This was the Precambrian Period, and there were no rabbits.
But about 540 million years ago, the diversity of marine animals suddenly expanded dramatically, an event known as the Cambrian Explosion or (less dramatically) the Cambrian Radiation.
Recall that the (relatively) rapid diversification of life forms from a single ancestor is known as adaptive radiation.
The Cambrian Explosion saw the evolution of modern animal phyla--and even more that are now extinct--from simple, ancestral forms.
Molecular clock studies suggest that...
Fossil evidence does not go back that far (only about 550 million years), but that doesn't mean the ancestors didn't exist at an earlier time.
- the last common ancestor of Animalia and Fungi existed approximately a billion years ago.
- the last common ancestor of all animals existed between 800-675 million years agoM
Eumetazoans are defined by gastrulation and the presence of true embryonic germ layers (endoderm, ectoderm, mesoderm) that develop into true tissues.
Eumetazoans can be broadly divided into two main groups, based on body similarity and number of germ layers:
- radially symmetrical and diploblastic OR
- bilateral symmetrical and triploblastic
I. Radially Symmetrical Animals
These diploblastic organisms are among the simplest of modern animals.
In the radially symmetrical animals we first see...
- true plane of symmetry: radial (sometimes modified as biradial
- true tissues
- ectoderm (becomes the epidermis in the adult)
- endoderm (becomes the gastrodermis in the adult)
- mesogloea (gelatinous layer between epidermis and gastrodermis)
- Note: When mesogloea contains cellular components, it is known as mesenchyme. A few, more derived cnidarians have mesenchyme.
These simplest of all animals are of uncertain evolutionary affinity. They are little more than a jelly-like plate of interdependent cells exhibiting the beginnings of radial symmetry.
The name of the taxon derives from the Greek knide meaning
The Cnidarian Bauplan
- diploblastic--having endoderm and ectoderm
- radial symmetry
- no coelom or other body cavity (only a simple gut)
- primary body axis is oral --> aboral (no head or tail)
- Many species are dimorphic: alternating forms of polyp (asexually
reproducing, feeding form) and medusa (sexually reproductive form)
- Unique cells called cnidoblasts (or cnidocytes) containing stinging capsule, the
- Larva is a ciliated gastrula called a planula. In sessile forms,
this little guy settles on the substrate and develops into the fully
formed adult polyp.
Major Groups of Extant Cnidarians:
1. Hydrozoa - polyp and medusa phases alternate
- Physalia (Portuguese Man o' War)
2. Scyphozoa - medusa is the dominant phase; polyp is reduced
- various species of jellyfish
3. Anthozoa - sea anemones,
sea pens, etc. (polyp is dominant stage; medusa is reduced or absent.)
mouth, but no anus
The "comb jellies" are so named because of paired rows of cilia along the axis of their bodies that beat in waves to propel the animal through the water (slowly, but surely).
They are suspension feeders, with a pair of long, sticky tentacles that they drape out into the water to capture microscopic plankton and detritus. Every now and then, the ctenophore wipes its tentacle along its mouth to remove the food particles captured there.
Their tissues are colorless and translucent, with the same refractive index as water. This makes them quite cryptic in their natural habitat, and well concealed from predators. But to the human eye, they are spectacular in motion.
Cnidarians and Ctenophores are beautiful!
The bilaterally symmetrical animals are a vast assemblage of animals that exhibit bilateral symmetry: in longitudinal section, the two halves of the body form mirror images.
Flatworms comprise the (probably polyphyletic) taxon Platyhelminthes (literally "flat" (platy) "worm" (helminth).
Molecular data suggest that some members of a large group of free-living flatworms, the Acoela, are actually extremely early offshoots of the very first bilaterally symmetrical ancestor.
In these primitive bilaterians, we see the origin of:
primordial gut precursor (By definition, a true intestine must be lined with
epithelial cells. In these, it is not.
the ingestive structure is a syncytium that creates vacuoles around ingested food
a short, sometimes eversible, pharynx leads to the syncytium/vacuole
no circulatory, respiratory or excretory systems
no nerve ganglia or brainlike structure
cephalization vague: slightly more nervous tissue at one end
the only sense organs are statocysts (gravity detector) and sometimes a light detecting ocellus (plural = ocelli)
hermaphroditic, but no gonads: produce both eggs and sperm from mesenchymal cells
Mysterious and ancient.
- true bilateral symmetry and cephalization
- more complex true tissues
- ectoderm (becomes the epidermis in the adult)
- mesenchyme (celluar mesogloea)
B. Turbellaria - Free-living (non-parasitic) triploblastic flatworms, a.k.a. "planarians"
C. Trematoda - Flukes (all species parasitic)
D. Cestoda - Tapeworms (all species parasitic)
The Platyhelminth Bauplan
Flatworms DO have:
- integumentary system
Parasitic forms (flukes and tapeworms) have a protective tegument--external to the epidermis--formed by nonciliated, cytoplasmic extensions of mesenchymal cells.
- digestive system (simple gut; no anus)
- reduced in Trematoda
- secondarily lost in Cestodes
- nervous system
- well developed in Turbellarians
- reduced in Trematodes
- even more reduced in Cestodes (Why?)
- sense organs
- chemoreceptors flaplike extensions of the
head called "auricles"
- photoreceptors are ocelli
- statocysts (gravity sensing cells) near the cerebral ganglion
- muscular system
- well-developed in turbellarians and flukes, vestigial in cestodes
all forms are hermaphroditic
- simple, tubular protonephridial system composed of anucleate flame cells attached to collecting tubules concentrate nitrogenous waste.
They do NOT have:
- skeletal system
- circulatory system
- respiratory system
- immune, lymphatic or endocrine systems
Turbellaria - The Planarians (free-living flatworms)
The planarians are the most cephalized flatworms, as they live free in the environment. Their nervous systems and other organisms are the best developed of the flatworms.
Trematoda - The Flukes
The flukes are entirely parasitic, and often have complex life cycles.
Their sensory systems and other organ systems are reduced, as compared to the free-living planarians. (why?)
(click on the photo for a required link!)
- definitive host - organism in which the adult parasite resides
- intermediate host - organism in which various larval/developmental
stages of the parasite exists until they is passed to the definitive
host and metamorphose into the adult
Most species of parasite are relatively host-specific, but some can
inhabit more than one different species of definitive host.
In many life cycles, more than one intermediate host is required for
the full life cycle to be completed.
Transmission of parasites (or any pathogen, for that matter) may be
- horizontal - from one individual to another via direct or environmental contact
- vertical - from parent to child, at or before birth
Cestoda - The Tapeworms
The tapeworms are entirely parasitic, and have the most reduced organ systems of all the flatworms. Nervous system is vestigial, and digestive system is secondarily lost. Transport of nutrients, oxygen, and waste takes place across the integument.
The head of the animal is the scolex, from which sprout segments called proglottids. Each early proglottid contains both ovaries and testes (tapeworms are hermaphroditic), and older proglottids contain hundreds of eggs which are shed with the host's feces.
Let's look at some pretty flatworms.