Flatworms and nearly all other animals from here on are bilaterally
symmetrical (right and left halves are mirror images).
The front
end of such animals usually forms a distinct head.
However,
flatworms still have a single all-purpose cavity with only one opening.
Three germ layers:
The animals we will study from here on are triploblastic, meaning that they have three germ layers instead of just two.
The third layer, or mesoderm, is a middle layer, sandwiched between the ectoderm and endoderm. Triploblastic animals
also have many more distinct tissue types than are present in diploblastic animals such as Cnidaria:
- Ectoderm gives rise to skin, shells and other protective coverings, and many sense organs and nerves.
- Mesoderm gives rise to muscles, blood and blood vessels, reproductive organs, organs of excretion, and
muscular linings that surround many other organs including the gut.
- Endoderm gives rise to the inner lining of the gut, including digestive and respiratory organs.
In particular, notice that triploblastic animals have muscle tissue that permits them to make coordinated movements.
Bilateral symmetry:
Animals other than sponges, Cnidaria, and Ctenophora all have
bilateral symmetry, which means that their right and left halves are mirror
images of one another. My right hand, held up to a mirror, looks just like my left hand. My ribs, my eyes,
my legs, and nearly all my internal organs are bilaterally symmetrical.
Bilateral symmetry probably evolved when animals with muscle tissues began to move forward in a consistent direction.
Imagine an animal like Hydra (phylum Cnidaria, class Hydrozoa) moving consistently
along an axis line going through its body. Movement would be made easier if its body became elongated
along the axis of movement, and most simple, bilaterally symmetrical animals have elongated, wormlike bodies.
If such an animal creeps along the bottom, the water above is very different from the sediment below, so natural
selection favors certain structures (like muscles for movement or touch-sensitive sense organs) on the bottom
and different structures (like visual organs to look out for predators) on the top. Selection also favors sensory structures
and feeding structures in front (where the animal encounters new things) rather than behind, and the accumulation of sensory structures,
feeding structures, and some kinds of weapons at the front end results in the buildup of a head, a process called
cephalization.
As the animal moves forward, however, any desirable stimulus (like food) or any danger (like a predator) that is off to the right
is just as likely to be off to the left, so whatever is favored by selection on the right side is equally favored on the left, and vice versa.
The result of such selection pressures over time is a body that is elongated along the axis of movement, with sense organs and feeding
structures concentrated in front, and with right and left sides similar to one another across a plane of symmetry running down the axis.
The simplest triploblastic animals with bilateral symmetry are the flatworms, belonging to the phylum Platyhelminthes
(platy- means "flat", and helminth means "worm").
Phylum Platyhelminthes (flatworms): Bilaterally symmetrical animals
with a flat body; dorsal (top) and ventral (bottom) surfaces
differ; no circulatory system needed because every part of body is near a
surface.
Anterior (front) end differs from posterior (hind) end.
Sense organs and brain are concentrated at the front end (cephalization)
to form a head.
A single all-purpose gastrovascular cavity, as in the Cnidaria.
The single opening functions as both mouth and anus,
still subject to the "eating from your toilet"
inefficiency: anything taken in as food may include material previously discarded as waste.
A simple, ladder-like nervous system, more concentrated at the head end.
Simple excretory tubules (flame cells whose beating cilia resemble a
flickering flame).
Three germ layers: ectoderm (outer epidermis);
endoderm (lining of gut); mesoderm (a loose mesenchyme
in flatworms). Acoelomate (no body cavity).
Many flatworms can regenerate missing parts following injury.
- Class Turbellaria: Mostly free-living; digestive tract and sense organs
still present; mouth often in middle of ventral surface.
- Class Trematoda: Small, parasitic worms (flukes) with small, oval
bodies; digestive tract simple; mouth at anterior end. The Chinese liver fluke (Clonorchis) is a well-studied species,
but blood flukes of the genus Schistosoma infect more people worldwide (mostly in poor tropical countries with unsafe
water supplies) and is the second most prevalent parasitic infection in the world (after malaria).
- Class Cestoda: Highly degenerate internal parasites (tapeworms)
with greatly reduced digestive tract, nervous system, and sense organs.
Parasitism:
The flukes (Trematoda) and tapeworms (Cestoda) are parasitic, meaning that they live in symbiosis with another species
and cause harm to their hosts. These worms are internal parasites, living inside the bodies of their hosts.
- Advantages of parasitism:
- The host generally provides a constant environment, free from environmental extremes, and free from the dangers of predators.
- Food is supplied by the host (usually by the host's own tissues, which is why parasites cause harm to their hosts)
- Disadvantages of parasitism:
- Parasites can be dislodged, so they evolve hooks, suckers, and other adaptations to hold on tightly, or else they burrow deep into host tissues.
- *Hosts evolve defenses (like an immune system) to protect them from parasites. Some parasites try to evade these defenses by frequently
changing their genes or their surface proteins, but this is difficult and costly.
* More often, parasites evolve adaptations to minimize
the harm that they do or the pain they cause, by entering the host painlessly and inconspicuously, but, above all, by evolving smaller and smaller
body sizes and by minimizing their food requirements (meaning the food energy that they take from their host).
* The best long-range strategy is for the parasite to return to the host some benefit, which minimizes the selective advantage for the host species to get rid
of the parasite. Eventually, this strategy may lead to a mutualism in which the internal symbiont (no longer a parasite)
confers a net benefit to the host (it does more good than harm), a condition which favors hosts that maintain the internal symbiont
instead of getting rid of them. For example, symbiotic wood-digesting protists allow termites to derive carbohydrate energy from the
cell walls (cellulose and lignin) that the termites chew and the protists digest.
- The greatest disadvantage of parasitism is that the parasite's habitat is the host's mortal body. Parasites must therefore evolve complex
lifestyle adaptations to transmit their offspring into the body of another host individual. Parasite life cycles are for this reason often very
complex, involving multiple life stages and often several intermediate hosts (see the accompanying illustrations). Because the chances of
survival and transmission through such an unlikely chain of events is low, parasites must reproduce by shedding eggs or larvae in large numbers,
usually in the millions. Advanced parasites (like tapeworms) may lose most of their other organs but always keep a complex set of
reproductive parts and sometimes become little more than a bag of reproductive organs.
- Degeneracy (evolved simplicity and loss of organs) among parasites:
- Over time, most parasites evolve to become much simpler than their ancestors— they lose organs that they don't need,
such as digestive organs (if food can simply be absorbed from their hosts) or sense organs and a nervous system (if their environment is
dependably maintained by the host and they have no need to be on the lookout for food or for danger). Loss of unneeded organs also
helps parasites become small and minimize their metabolic needs.
Flatworms, etc.
Related phyla, also without body cavities:
- Phylum Mesozoa: Small, marine parasites with very few cells.
- Phylum Acoela: Small, simple, bilateral animals similar to
flatworms, but with no gut or digestive tract. Formerly included
in the Platyhelminthes, but many zoologists now regard them as the most
primitive of bilateral animals.
- Phylum Rhynchocoela (Nemertea): "Proboscis worms," with a long, barb-tipped proboscis
(or "evert") that can be protruded as a weapon or withdrawn (inverted, like the finger of a glove)
when not in use.
- Phylum Gnathostomulida: Small worms; outer epidermal cells each have
a single cilium; mouth with paired, cuticle-hardened jaws.
FURTHER EVOLUTION OF BILATERAL ANIMALS
Bilateral animals above the flatworm stage evolved a complete "assembly-line"
digestive tract running from
mouth to anus. Most also evolved body cavities.
FROM THIS POINT ON, all remaining phyla share several important derived features:
- A complete "assembly line" digestive tract (mouth to anus).
- Some type of body cavity, either a pseudocoel (a persistent blastocoel) or a
true coelom (surrounded with mesoderm throughout).
"Assembly line" digestion: Nearly all animals above the flatworm level
have a complete digestive tract, with a separate entrance (mouth, in front) and
exit (anus, in the rear). This allows food to be processed in stages, in the manner
of an assmbly line, with different regions or organs specialized for different
sequential steps or for different nutrients (such as proteins in one region and carbohydrates in another).
A tubular digestive system, with a separate entrance (mouth) and exit (anus) also solves the problem of re-ingesting one's wastes,
because wastes discarded from the anus are left behind as the animal moves forward.
Evolution of body cavities: Fluid-filled body cavities, whatever their
origin, are useful:
- in support, as a hydrostatic skeleton
- in burrowing, where inflation of the body cavity can swell and
anchor part of the body, or else wedge forward and push sediment aside.
Because of their usefulness, body cavities have evolved many times,
independently, and are often constructed differently in different phyla:
- Some animals have a pseudocoel, lined with both endoderm and
mesoderm, derived from persistence of the blastocoel cavity.
- Other animals have a true coelom, lined with mesoderm throughout.
This may be either an enterocoel, derived from outpouching of the
gut (as in starfish), or a schizocoel, arising within the mesoderm
by splitting (as in mammals).
Differences in the structure of the coelom are useful in distinguishing many
phyla, but are a poor guide to relationships among phyla because body cavities
have evolved repeatedly and independently.
Analysis of RNA sequences allows scientists to divide bilateral animals into:
Protostomes—
bilateral animals in which early cleavages are spiral and
determinate, and in which the mouth forms early
from the blastopore.
Protostomes are further divided into:
Lophotrochozoa, containing the Mollusca,
Annelida, Bryozoa, etc.; and
Ecdysozoa, contining
the Nematoda, Arthropoda and several smaller phyla.
Deuterostomes—
bilateral animals whose early cleavages are radial and
indeterminate, and whose mouth forms
at the other end from the blastopore.
Animal family tree
Preview: Further Evolution of Bilateria
Protostomes are bilateral animals sharing the following traits:
- The opening to the embryonic archenteron becomes the mouth
(protostome means "first mouth").
- Spiral cleavage, introducing an asymmetry in the 8-celled
stage; the top 4 cells are rotated clockwise or counterclockwise
with respect to the lower 4 cells.
- Determinate cleavage, meaning that the cells destined to
form the front left portion of the animal lose the ability to form
structres on the right or the rear. (The fate of each cell is thus determined as early as the third cleavage.)
Deuterostome animals (considered later) have the opposite traits.
Phylogeny and classification of bilateral animals: Studies of
ribosomal RNA sequences show evidence that bilateral animals evolved
in three large groups (the first two are protostomes):
- Lophotrochozoa: A large group that includes annelid worms,
mollusks, and bryozoa, characterized in some cases by a ciliated
feeding organ called a lophophore and in other cases by a
ciliated larval stage called a trochophore.
- Ecdysozoa: A group that includes the two largest phyla,
Arthropoda and Nematoda, characterized by a hard outer covering
that must be shed periodically during growth, using steroid hormones
(ecdysones) to control the molting process.
- Deuterostomes, including the chordates and echinoderms.
REVIEW:
Study guide and vocabulary
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Index
Syllabus
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