Download Bilaterally Symmetrical Worms

4. Bilaterally Symmetrical Worms
4. Bilaterally Symmetrical Worms
• Marine worms, and organisms
hereafter, kick it up a notch
exhibiting bilateral symmetry
(just like humans).
• Instead of addressing front and
back, we will discuss anterior,
posterior, dorsal and ventral.
• Also pick up a CNS!
• Bilateral symmetry, and all
that goes with it, results in
enhanced motion and
sophisticated behaviors.
4. Bilaterally Symmetrical Worms
• Anterior – head with brain or
accumulation of nerve cells
and sensory organs like eyes
• Posterior – bottom/backside
• Dorsal – back
• Ventral – Belly
• Bilateral symmetry allows
organisms to actively pursue
prey and develop sophisticated
behaviors
A. Flatworms: Phylum Platyhelminthes
•Simplest flatworms – has CNS
•Simple brain – aggregation of nerve cells
•Several nerve cord running from brain through length of body
•Nervous system coordinates movement of well-developed muscular system.
A. Flatworms: Phylum Platyhelminthes
•Gut has an opening
•Have a middle layer of tissues, in developing embryos it is called mesoderm
•Mesoderm gives rise to muscles, reproductive system and other organs
•20,000 known species
•Turbellarians are the most common – striking color patterns
• Live in or on oysters, crabs, and other invertebrates
A. Flatworms: Phylum Platyhelminthes
Flukes (trematodes)
• Largest group (6,000 species)
• All parasitic
• Feed on tissues, blood, or intestinal contents
• Adult flukes live in vertebrates
• Larvae may inhabit invertebrates like snails and clams
or vertebrates like fish
• Larva must be eaten by a vertebrate
• Common in fish, seabirds, and whales
A. Flatworms: Phylum Platyhelminthes
Tapeworms (cestodes)
• Parasitic
• Have long body with repeating units
• Head attaches to walls of gut by suckers
and hooks
• Lack gut or mouth
• Absorb nutrients from host across body
wall
• Found in invertebrates and vertebrates
• Longest recorded in sperm whale was 50
feet!!
B. Ribbon Worms: Phylum Nemertea
•More complex than flatworms
•Complete digestive tract (mouth, gut, anus)
•Have a circulatory system
•Proboscis – long flesh tube to capture prey
• Everted in cavity above mouth
•Feed on worms and crustaceans
•900 species, mostly marine
•Found in all oceans, most common in shallow temperate
waters
B. Ribbon Worms: Phylum Nemertea
•Some nocturnal, some brightly colored found under rocks at low tides
•Elastic, and proboscis may extend to a meter or more beyond body length
•Longest invertebrate – reaches 100 ft!!
C. Nematodes: Phylum Nematoda
• Roundworm numbers are staggering in
the marine world, yet hardly ever seen
• Many are parasitic and found in most
marine organisms.
• Adapted to live in sediment or tissues
of other organisms
C. Nematodes: Phylum Nematoda
• Typically pointed at both ends
• Anus lies within body cavity filled with fluid that
transports nutrients
• Layer of muscle in the tough but flexible body
pushes and squeezes against the fluid which acts as
hydrostatic skeleton that provides support and aids
in locomotion
C. Nematodes: Phylum Nematoda
• 10,000 to 25,000 species and estimates that half a million
still undiscovered
• Anisakis and few related inhabit seals and dolphins
– Larvae found on flesh of many types of fish and may infect humans
who eat raw or poorly cooked fish
– Often larvae are vomited or coughed up without further
complications
–
Sometimes penetrate into walls of stomach or intestine causing
symptoms similar to ulcers
D. Segmented Worms (phylum Annelida)
• 20,000 spp. known as Annelids
• First example of segmentation.
• Gut through all segments and lies in cavity known
as coelom
– Coelom fluid filled and divided by partitions
D. Segmented Worms (phylum Annelida)
• Segments acts as hydrostatic skeleton contracting
in sequence by muscles in body
• Efficient crawlers and burrowers
i. Polychaetes (class Polychaeta)
•Body segments of most have a pair of flattened extensions or parapodia
• Parapodia have stiff sharp bristles (setae)
•Have closed circulatory system
•Muscular contractions help circulation of blood
•In small organisms, oxygen in water can easily move across body wall to all
tissues
•Large organisms have gills on parapodia
• Parapodia - One of the fleshy paired appendages that function in
locomotion and breathing
• Gills - thin wall extensions of body that have capillaries where gas
exchange takes place
Sandworm Nereis
i. Polychaetes (class Polychaeta)
•Have planktonic larval stage – trochophore
•Biologists compare larvae stage of polychaetes with other invertebrates to
examine evolutionary relationships
•Body 5-10cm
•Many crawl and hide under rocks
•Have several pairs of eyes and other sensory organs
•Use proboscis with jaws to capture prey
•Others are deposit feeder
i. Polychaetes (class Polychaeta)
•Many live in temporary or permanent tubes made from mucus, proteins, bits of
seaweed, cemented mud particles, sand grains, or tiny fragments of shells
• Tube dwelling have reduced parapodia
• Have tentacles which have cilia and mucus that catch organic particles
and move them to their mouths
• Some use feathery tentacles covered with cilia to capture, sort, and
transport particles
• Some of calcium carbonate tubes built on rocks and other surfaces
This feather-duster worm, Sabella melanostigma, is a perfect example
of beauty, form and function.
i. Polychaetes (class Polychaeta)
•Some are ptanktonic and use flat parapodia to swim
•The Palolo worm break off & posterior half swims up to surface to spawn
• Timed with phases of the moon
• Some places people with gather worms for food
•Live on external surface of sea stars & sea urchins, in burrows of invertebrates,
and shells of hermit crabs
Beard worms (pogonophorans)
•Highly specialized lack gut and mouth
•Tuft (thousands of long tentacles) absorb nutrients in dissolved water
•Some have symbiotic relationship w/ bacteria
• Bacteria use the nutrients to manufacture food for the worm
•Mostly restricted to deep water
•4-7 feet
•Related vestimentiferans grow larger and found at hydrothermal vents
ii. Oligochaetes (subclass Oligochaeta)
•Small and found in mud and sand feeding on detritus
•Lack parapodia
iv. Leeches (class Hirudinea)
•Blood sucking found attached to fish and other invertebrates
•Distinguished by suckers and no parapodia
E. Peanut Worms (phylum Sipuncula)
•Soft unsegmented bodies
•Burrow in muddy bottoms, rocks, corals, and empty shells
•All are marine living in shallow waters
•Deposit feeders
•Long anterior portion of body contains mouth and set of small lobes or
branching tentacles
•Tentacles can be pulled into remaining body, become compact
•1-35 cm in length
F. Echiurans (phylum Echiura)
•Soft segmented sausages buried in mud or in coral
•Look like peanut w/out retractable spoon-like or forked proboscis
•Deposit feeders that use proboscis to gather organic matter
•Urechis caupo lives in U-shaped tubes in mud