Flatworms are acoelomate: they lack a body cavity. The rest of the phyla we
discuss do have a body cavities; first, Pseudocoelomates:
Pseudocoelomates: phylum Nematoda
Nematodes are the most successful of several pseudocoelomate phyla
• 80-100,000 describes species, probably > 500,000 total
• Found in nearly all habitats (marine, freshwater, terrestrial, free living,
and parasitic)
• Abundant and very ecologically important.
• All are very similar in body plan:
All fundamental bilaterian traits: TRIPLOBLASTIC, ORGAN
SYSTEM level of organization (more complex than flatworms),
BILATERAL SYMMETRY
• Motile and cephalized -- supporting system is hydrostatic and
viscoelastic
• External cuticle -- protection, support, etc. -- a major key to the success
of this group; molted about 4 times during its maturation
• Internal cavity: pseudocoelom, formed from blastocoel
Pseudocoelomates: phylum Nematoda
Pseudocoelomates: phylum Nematoda
Internal cavity: pseudocoelom, formed from blastocoel early in
development:
Internal cavity: pseudocoelom, formed from blastocoel early in
development:
Pseudocoelomates: phylum Nematoda
Pseudocoelomates: phylum Nematoda
Internal cavity: pseudocoelom, formed from blastocoel early in
development:
Adult body plan of nematodes, with pseudocoelom, no mesodermal tissue
around gut. Other major features:
ectoderm
mesoderm
• Eutely (constant cell number)
to varying degrees -- makes
them important ‘model animals’
pseudocoel
Gut (no
muscle)
Gut lumen
Cross-section
Chaenorhabditis elegans:
adult worm has 959 somatic
cells, plus some reproductive
cells (gonads)
Pseudocoelomates: phylum Nematoda
Pseudocoelomates: phylum Nematoda
Adult body plan of nematodes, with pseudocoelom, no mesodermal tissue
around gut. Other major features:
ectoderm
mesoderm
• Eutely (constant cell number)
to varying degrees -- makes
them important ‘model animals’
pseudocoel
Gut (no
muscle)
• Complete gut with mouth and
anus
Basic body plan of adult
nematode:
Gut lumen
Cross-section
Functional attributes of nematode body plan:
• Cuticle provides leverage for muscle action, and protection; must be
molted to grow
• no circulatory system, but pseudocoelom provides some convective
transport (especially when ‘stirred’ by movement)
• complete gut is a major advance: it allows continuous-flow food
processing (one-way); also permits specialization of gut regions.
• incomplete gut requires batch processing
• no muscle in gut, no gut motility. Food ‘pushed’ through by pharynx
action.
Pseudocoelomates: phylum Nematoda
Pseudocoelomates: phylum Nematoda
Nematodes are extremely important ecologically
Nematodes are extremely important ecologically
• scavengers, predators, parasites (in nearly every habitat, often in very
large numbers -- millions per cubic meter of soil)
• scavengers, predators, parasites (in nearly every habitat, often in very
large numbers -- millions per cubic meter of soil)
• Many are parasites on plants (including crop plants; causative agents of
some very important crop diseases).
• Many are parasites on plants (including crop plants; causative agents of
some very important crop diseases).
• A few parasitize humans: pinworm, hookworm, Trichinella spiralis
(causes trichinosis).
• A few parasitize humans: Ascaris -- which infects about 25% of humans
worldwide
Adults in intestine
Trichinella cysts in human muscle
Eggs shed in
feces
(eaten)
(DON’T eat raw pork!!!)
Crawl up trachea to
esophagus and
swallowed
Ascaris lumbricoides cluster passed from a person
(cluster is about 4 inches wide)
Pseudocoelomates: phylum Nematoda
Nematodes are extremely important ecologically
• scavengers, predators, parasites (in nearly every habitat, often in very
large numbers -- millions per cubic meter of soil)
• Many are parasites on plants (including crop plants; causative agents of
some very important crop diseases).
• A few parasitize humans: filaria such as Wuchereria (mosquito
transmission, causes elephantitis)
Elephantitis: severe,
chronic (permanent)
lympho-edema
(swelling)
Wuchereria in blood smear
Juveniles penetrate gut wall;
migrate in blood to lungs
Eucoelomates -- animals with a ‘true’ coelom: the ‘classic’ phylogeny from
morphology and development:
The alternate molecular-based view of protostomes, with two main lineages:
Ecdysozoans (those that molt) and Lophotrochozoa
Eucoelomate protostomates
The pseudocoelomate body plan has a complete gut but
lacks muscle tissue in the gut (no inherent gut motility).
In eucoelomates the coelom is lined with mesoderm,
providing muscle in the gut to power gut motility
Coeloms and their mesodermal lining develop in two distinct
ways. First we consider protostomates:
• spiral determinate cleavage
• coelom forms by schizocoely: a block of mesoderm splits to
form an internal cavity
Eucoelomate protostomates
Coelom formation by schizocoely:
Eucoelomate protostomates
Coelom formation by schizocoely:
Eucoelomate protostomates
Coelom formation by schizocoely:
Eucoelomate protostomates
Coelom formation by schizocoely:
At the end of this
process, the internal
cavity is completely
lined with mesoderm
(definition of a true
coelom)
Mesoderm layer
around the gut leads to
muscle tissue, lets gut
process food more
effectively.
Eucoelomate protostomates
Coelom formation by schizocoely:
Eucoelomate protostomates: phylum Arthropoda (‘jointed feet’)
Eucoelomate protostomates: phylum Arthropoda (‘jointed feet’)
BY FAR the most successful animals in terms of numbers
of species and ecology:
• > 1 million described species (mainly insects); probably 520 times as many that are not yet described (along with
many fossil forms)
• Found in essentially all habitats, usually in great numbers
• diverse in form and function
Triploblastic eucoelomate protostomes, with
segmentation
Like nematodes, they have an external cuticle
Eucoelomate protostomates: phylum Arthropoda (‘jointed feet’)
Key features of arthropod body plan:
• Reduced coelom in adults
• open circulation (hemocoel)
• Hard external skeleton (exoskeleton); also called cuticle, made
largely of chitin (acetylglucosamine polymer) mixed in a complex way
with protein and varying degrees of mineralization
• jointed appendages (often highly modified) on segments
• segmentation (metamerism: serial repetition of body parts in
segments). As in annelids, segments contain subdivisions of organ
systems (muscles, nerve nets, etc.)
• Tagmosis (grouping--sometimes fusion--of segments into
specialized units). In many groups, tagmosis can obscure the
underlying segmentation.
Eucoelomate protostomates: phylum Arthropoda (‘jointed feet’)
Eucoelomate protostomates: phylum Arthropoda (‘jointed feet’)
Exoskeleton is key to arthropod success. Three functional attributes:
Exoskeleton is key to arthropod success. Three functional attributes:
1. Movement requires flexible joints in appendages (hence, “jointed
feet”). Joints are thin, flexible regions of exoskeleton (the rest of the
exoskeleton is usually thick ‘armor’).
2. Exoskeleton provides support as well as protection. Helps
arthropods move on land without buoyant support of water.
•
Exoskeleton also reduces evaporation (another advantage on
land)
Exoskeletons preadapted early arthropods (which were aquatic) to
invade land; they were among the first land animals.
Jointed legs, protection, and support are advantages of exoskeletons.
• Muscles are connected across joints in antagonistic pairs (one to extend, one to flex)
• Hard exoskeleton provides good muscle anchorage and effective movement;
attachment arrangement provides variable leverage (fast movement or slower but
powerful movement).
But, there is a big problem:
Exoskelton is largely non-living and it cannot grow.
Therefore, it must be molted (shed) in order for animal to get
bigger.
Eucoelomate protostomates: phylum Arthropoda (‘jointed feet’)
3. How to grow with an exoskeleton: molting process
Eucoelomate protostomates: phylum Arthropoda (‘jointed feet’)
3. How to grow with an exoskeleton: molting process
• arthropods grow in bursts
• Many arthropods go through a number of
molts as they progress from larvae to adults
• often change form (metamorphosis) and
size
Molting may limit maximum size,
especially in terrestrial forms
(without skeleton, animal is
mostly helpless and cannot
support itself -- too large and it
collapses. So giant ants and
crabs wouldn’t work well….
Eucoelomate protostomates: phylum Arthropoda (‘jointed feet’)
This is about as big as terrestrial arthropods get (some fossil forms
were a bit bigger)
Eucoelomate protostomates: phylum Arthropoda (‘jointed feet’)
This is about as big as aquatic arthropods get (some fossil forms
were a bit bigger)
Eucoelomate protostomates: phylum Arthropoda (‘jointed feet’)
Multiple repeats of body segments (with appendages): metamerism.
Tagmosis (segment fusion and modification) is pronounced in
several arthropod classes.
• Tagmosis creates
specialized body regions
comprised of multiple fused
segments
Eucoelomate protostomates: phylum Arthropoda (‘jointed feet’)
Arthropod taxonomy is complex and controversial; most prevalent
view is of 4 major subgroups:
TRILOBITES: (extinct; extremely abundant in Paleozoic, and as
fossils); many segments but little specialization of appendages
and relatively little tagmosis.
• most commonly, body is
divided into three regions:
head, thorax, abdomen (as
in insects)
• Many appendages become
modified or lost
Eucoelomate protostomates: phylum Arthropoda (‘jointed feet’)
CRUSTACEANS: diverse, primarily aquatic, a few terrestrial
(crabs, lobsters, shrimp, barnacles, isopods, etc).
• About 40,000 species; abundant
and important in marine and
freshwater ecosystems.
• Most go through a very different
larval stage before
metamorphosing to adults.
• Many appendages that are
highly specialized (gills, legs,
claws, mandibles, antennae,
feeding nets, etc.)
• Appendages are biramous; they
occur as pairs, often with different
functions; in crustaceans, usually
legs and gills
Eucoelomate protostomates: phylum Arthropoda (‘jointed feet’)
• Most crustaceans show
considerable tagmosis; often
there is a fusion of head and
thorax into a carapace
• Barnacles are a highly derived
form that is sessile, living within a
protective shell. Don’t look like
most crustaceans but have
typical jointed arthropod legs,
used as feeding nets.
Eucoelomate protostomates: phylum Arthropoda (‘jointed feet’)
UNIRAMIANS: uniramous
appendages; extraordinarily
successful; millions of species
(mostly insects).
• Very diverse: centipedes, millipedes, insects; crawling, swimming,
burrowing, flying, parasitic. Often highly specialized (especially mouth
parts).
• Only invertebrate lineage to have evolved flight (probably several times in
different insect orders).
• Social insects (bees, ants, termites) have the most complex social
systems of all animals--including humans.
Eucoelomate protostomates: phylum Arthropoda (‘jointed feet’)
CHELICERATES: spiders, scorpions, horseshoe crabs, etc.
Very successful (about 65,000 species, and very ancient.
• two major regions (tagmata), anterior (cephalothorax) and abdomen
• usually 4 pairs of legs on anterior and two pairs modified as mouth parts.
The appendages used as mouth parts are very claw-like (chelicera) and (in
spiders) are connected to venom glands.
• Life cycles usually simple; eggs hatch into miniature adults (no larval
stage in most species).
• A major “technology” of spiders is use of silk (for webs, safety lines, etc.).
Eucoelomate protostomates: phylum Arthropoda (‘jointed feet’)
Uniramian body form is either