4 Bilaterally Symmetrical Worms 4 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 -10 cm • 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