Bilateral Symmetry

Bilateral Symmetry• Most animals show bilateral symmetry, the

arrangement of body parts such that there is only one way to cut the body to produce two identical halves

• Bilaterally-symmetric animals have a front (anterior), end (posterior), back (dorsal), and belly (ventral) surface

Bilateral Symmetry: the Flatworms

• Bilateral symmetry promotes actively moving organisms, permits streamlining, favors the formation of a central nerve center, and contributes to the concentration of a head region where nervous tissue and other sensory systems are located

• The simplest bilaterally-symmetric animals are the flatworms (phylum Platyhelminthes)

Platyhelminthes (Flatworms)

• Flatworms are the simplest animals in which tissues are organized into real organs and organ systems

• Marine flatworms include the turbellarians

Opening evolution’s can of worms

• Many advancements in organization and structure occurred in marine worms

• The digestive tract and circulatory system appeared in ribbon worms (Phylum Nemertea)

• Segmentation appeared in the annelids, or segmented worms (Phylum Annelida)– Series of repetitive segments– Allows different regions of the body to develop

differently for different uses

Marine Annelids: the Polychaetes

• Almost all marine annelids are polychaetes (class Polychaeta), which are also known as “bristle worms” for their stiff and often sharp bristles stemming from each of their repeated segments– Bristles replaced by gills in many species– Planktonic larvae stage

Marine Annelids: the Polychaetes

• Many polychaetes live in temporary or permanent tubes– Singular or aggregations

• Tube-dwelling polychaetes are suspension feeders, using feathery tentacles to trap and capture suspended material

How about this heat?!!?

• One of the most remarkable polychates is the Pompeii worm

• Found only at hydrothermal vents in the Pacific Ocean– Temperatures as high as 80°C (176°F)!– “Fleece-like” covering of bacteria on their backs;

the worms secrete mucus to feed the bacteria and in return, are protected by a degree of insulation on their backs

Polychaetes are pretty!

Christmas Tree Worm Free-living polychaete

Marine Annalids: the Beard Worms

• Beard worms, or pogonophorans are highly specialized annelids (class Pogonophora)

• Pogonophorans lack a mouth and a gut• A tuft of long tentacles absorb nutrients in the

water and give these annalids their common name

• Found at hydrothermal vents; symbiotic associations with bacteria

Riftia pachyptila

http://bioweb.uwlax.edu/bio203/s2007/rossing_jaco/introduction__the_worm_under_inv.htm

Phylum Mollusca• Molluscs (Phylum Mollusca) are soft-bodied

organisms enclosed in a CaCO3 shell

• Very successful; more species of Molluscs in the ocean than of any other animal group

• Their body is covered by a mantle, a thin layer of tissue that secretes the shell

Phylum Mollusca

• Molluscs have a ventral, muscular foot and a scraping tongue, or radula

• Most Molluscs belong to one of three groups:–Class Gastropoda (snails, limpets, abalone,

and nudibranchs)–Class Bivalvia (clams, mussels, oysters, etc.)–Class Cephalopoda (squid, octopus,

cuttlefish, and nautilus)

Phylum Mollusca, Class Gastropoda

• The Gastropods (Class Gastropoda) are the largest and most varied group of Molluscs

• Gastropod means “stomach footed”; a typical gastropod is a coiled mass of organs enclosed by a dorsal shell, which rests on the foot– Have head and sensory

structures on head– Operculum: trap door,

pulls in snail to shell



• Most gastropods use their radula to scrape algae from rocks (e.g., periwinkles, limpets, and abalone)

• Others are carnivorous and use their radula to drill into the shells of bivalves and other gastropods

• Nudibranchs, or sea slugs, are gastropods that have lost their shell altogether

Phylum Mollusca, Class Bivalvia

• Bivalves (Class Bivalvia) include clams, mussels, oysters, and scallops

• The body of bivalves is laterally compressed (flattened sideways) and consists of 2 hinged valves that are mirror images of one another


• In bivalves, the mantle forms a thin membrane that lines the inside surface of the shell

• This creates a mantle cavity, within which the entire body of the bivalve lies


• Strong muscles, the adductor muscles, are used to close the valves

• Two siphons, an incurrent and an excurrent siphon, draw water into and out of the mantle cavity, respectively

• Since many clams burrow into the sediment, these siphons allow the clam to feed and breathe

www.marinebio.net/marinescience03ecology/mfunder.htm


• Not all bivalves are burrowers; mussels secrete strong byssal threads to attach to rocks and other surfaces

• Oysters cement themselves to hard substances including other oysters!

• Scallops are unattached and can swim for short distances by rapidly ejecting water from the mantle cavity and flapping their valves

http://upload.wikimedia.org/wikipedia/commons/thumb/b/bc/Mytilus_with_byssus.jpg/400px-Mytilus_with_byssus.jpg

Phylum Mollusca, Class Cephalopods

• Cephalopods (Class Cepahlopoda) are a group of molluscs that include squid, nautilus, cuttlefish and octopus

• They are the most highly evolved of all molluscs; nearly all are agile swimmers with a complex nervous system

• Cephalopods have a reduction in or loss altogether of the shell


• Cephalopods (which means “head-footed”) have heads surrounded by a foot modified into arms and tentacles, usually equipped with suckers that are used to capture prey

Phylum Mollusca, Class Cephalopoda

• Largest of all Molluscs• All have a radula, beak-like structure• Cephalopods have large eyes, with acute

vision, extremely sensitive to slight details and impressions


• Cephalopods pump water through the siphon, or funnel into their mantle cavity

• By forcing water out of the mantle cavity through the siphon, cephalopods swim by means of “jet propulsion”

• By changing the direction of the siphon, cephalopods can move backward or forward, or can remain motionless in one place

Jet Propulsion

Water enters mantle cavity

Water exits from funnel or siphon

Direction of water out of siphon

Direction of cephalopod motion

Nautilus: Order Nautilida

• Nautilus are called “living fossils”; only 6 species remain of an extinct superfamily• Survived relatively unchanged for millions of years• Gas-filled chambers aid in buoyancy

• Paper nautiluses are actually pelagic octopus; named for the paper-thin eggcase in females

Squids: Order Teuthoidea

• Squids are better adapted for swimming than octopuses

• Squids have two triangular fins modified for swimming

• Elongated body• Squids have eight arms &

2 tentacles, all with suckers which circle the mouth

http://cas.bellarmine.edu/tietjen/images/Image32.gif

Long & retractable

Squids: Order Teuthoidea

• Suckers only at broadened tips on tenacle• The shell is reduced to a stiff pen embedded in the upper surface of the mantle

Chromatophores

• Cephalopods control their skin pattern and color by way of chromatophores

• Chromatophores are pigment-containing and light-reflective cells used for camouflage

• Inside the chromatophore, pigment granules are enclosed in an elastic sac– To change color, the sac is distorted by way of

muscular contraction

Cuttlefish: Order Sepiida

• Cuttlefish are among the most intelligent invertebrates

• Cuttlefish have a calcified internal shell called a cuttlebone, which aids in buoyancy– Chambered, gas-filled

• Called “chameleons of the sea” for their astonishing ability to change skin color at will

Octopus: Order Octopoda

• One of the most iconic cephalopods is the octopus

• Octopuses have 8 long arms (“octo” means 8) and no shell

• Common bottom dwellers, they are efficient hunters– Bite their prey (mostly shellfish)

with a pair of beak-like jaws– Ink sac emits a dark cloud of fluid

to deter predators

You will never be as cool as a cephalopod

• Mimic octopus http://www.youtube.com/watch?v=H8oQBYw6xxchttp://www.youtube.com/watch?v=ygh1-ul6E94

• Vampire squid from hellhttp://www.youtube.com/watch?v=S3CJIKKSUpg

www.islandream.com/sangalakigallery7.htm http://www.itsnature.org/sea/other/colossal-squid/ www.flickr.com/photos/maxcdc/3615629745/

…or as smart???

Most cephalopods display color

changes correlated with particular behaviors and

moods

Octopus are known to use

tools, and predict World Cup champions!

Octopuses and cuttlefishes have a remarkable capacity for

learning

Giant nerve fibers rapidly conduct

impulses allowing cephalopods to capture prey or

escape at amazing speeds

Paul the Psychic Octopus

Documents

Bilateral Symmetry