• Good examples of ADAPTIVE RADIATION(evolution of a variety of species from a common
ancestor; their appearance may differ from each other and from their ancestor; each specie is adapted
to a particular niche).
THIRD largest phylum of animals
• Take the TRIPTOBLASTIC(Organisms formed from THREE cell layers: endoderm, ectoderm and
mesoderm) characteristic of Platyhelminthes one step further by adding a BODY CAVITY.
• The body cavity makes them EUCOELOMATES(i.e. ‘Coelomate’: organisms that have a true
coelom with a mesoderm lining the whole body cavity; Pseudocoelomates have a mesoderm adjacent
only to ectodermal tissue).
• Flatworm gliding across a substrate using CILIA (cellular hairlike locomotory structures that
consist of an extension of the plasma membrane surrounding the 9+2 organization of
microtubules; unlike flagella, cilia are numerous and are shorter on the cell surface) on the
VENTRAL(in animals with bilateral symmetry, it is the lower surface of animals that glide on
their substrate and the front if the animal walks upright) and secreting protective
SPICULES(needlelike deposits of shell) on the DORSAL(in animals with bilateral
symmetry, it is the upper surface of the animal in crawling animals and the back if it walks
upright) surface. The spicules fuse to form a shell and the underlying tissue that secrets them
is called the MANTLE(Thin sheetlike, membranous extension of the visceral mass that forms
two flaps of skin; secretes shell on dorsal side and the space between the two flaps of skin is
called the mantle cavity)
• as Exchange:
• Increase SA underneath the overhang of the shell, i.e. the mantle cavity and we get the GILLS.
The Gills are combined with a RADULA(unique feeding structure that looks like a tongue with teeth on it
and works like a file by grating food off the substrate).
• xamples of Molluscs:
• Octopus enough intelligence to learn and be considered a pet
• Snail 1. CLAMS (bivalves)
Example of ADAPTIVE radiation
Laterally compressed and most organs of the VISCERAL MASS(one of the 3 parts of a molluschead,
foot and visceral mass It consists of fleshy part of the mollusc and contains the main organ systems of the
mollusc) are embedded in the FOOT.
Due to later compression the DORSAL shell folds in half and covers and protects the left and right sides of
the animal HIDDEN inside.
Strong ABDUCTOR MUSCLES(muscle that moves part of body toward the central midline; in bivalves
the muscle closes the bivalve shell) contract to close the shell if there as any perceived threat; the
ELASTICITY of the shell’s HINGE keeps it open when the muscle relaxes.
• Filter feeders
a. Water is drawn into the mantle cavity by cilia that cover the CTENIDIA(molluscan gills that
often have additional features other than respiration)
b. As the water passes through, particulate food in the water is trapped.
c. Some cilia drive water across the gill and OUT of the EXCURRENT SIPHON (opening
that expels water that is clean of food and oxygen AWAY from the clam’s body) OR other cilia
pass the food down to the edge of the gills where MUCOUS is added to strengthen the food.
d. Cilia propel the food to the LABIAL PALPS(control amount of food digested) and into the
The gills are not so large for because of a high oxygen demand, but because they are
involved in FEEDING; they are still involved in respiration but their size is due to
• They NO radula; they also NO distinct head and NO concentration of the sensory and nervous system
that is found in most biradially symmetric animals.
• The absence of those characteristics makes clams HIGHLY DERIVED i.e. have ADVANCED
CHARACTERS(traits that an animal has that are NOT ancestral to the taxon; appear later in the
evolutionary history of the taxon and can NOT be found on the ancestor to the group) form of mollusc.
• Not all clams and their relatives are sessile; SCALLOP is capable of swimming by opening and closing
the halves of the shell.
2. SNAILS (gastropoda) Unusual Molluscs
BILATERAL SYMMETRY(one plane of symmetry passing through the longitudinal axis that creates
two identical halves) modified by TORSION(unusual 180º twisting of the gastropod body –NOT shell–
which caused asymmetric body plan and Ushaped alimentary tract; and the then posterior anus and
mantle cavity now on the anterior head; some gastropods can change the position of the anus).
Advantage of torsion:
may have been advantageous by allowing snails a faster and more effective retreat into their protective
by preventing the mantle cavity from filling up with dust the ancestral snail “kicked up” as it sped across the
To young snail larvae swimming in a sea of other ciliated larvae?
Snails and their relatives are the MOST ABUNDANT molluscs.
Impact of torsion on how snails function is tremendous.
Usually every bilaterally symmetric organism has an organ on the left side paired with another on the right
side BUT in snails, the torsion causes some organs to be on the outside of the bend and others to be
trapped on the inside; examples of organs trapped on the inside may be METANEPHRIDIA(excretory
osmoregulatory organ; consists of a ciliated funnel, nephrostome connected to tubules that lead to the
external nephridiopore. The nephrostome collects coelomic fluid and all of its contents to produce URINE
i.e. waste products), CTENIDIA and GONADS(reproductive organ tat produces gametes i.e. eggs or
sperm, by meiotic REDUCTION of the chromosome content) and since they are trapped on the inside they
are REDUCED or LOST creating an ASYMMETRIC animal.
**The spiral shell of the snail is NOT due to torsion, but is the most efficient way to compact the visceral
Torsion causes those organs to be lost, which loses organ systems AND the twist in the visceral mass
means that as the animal grows the visceral mass grows HIGHER and HIGHER above the underlying
muscular foot.SOLUTION : wind it up in a SPIRAL; the winding up of the viscera, including the overlying
mantle, is what creates the SPIRAL shells. **torsion is NOT the spiraling shell of the snail,
but instead the shape of the shell is the result of torsion of the body cavity; the
shell itself is not rotated!**
3. CEPHALOPODS (squids and octopods)
• Marine predators that have legendary size (big)
• Their nervous system and sensory structures are very like the human’s. • They have elongated their bodies in the DORSAL direction, which put the posterior part of the
foot right next to the anterior which makes them HIGHLY EFFECTIVE PREDATORS; they are
• Being an active predator requires ADVANCED nervous system and sensory structures to be
able to trap prey and differentiate them from mates.
• Include a well developed BRAIN with EYES that is ANALOGOUS(refers to structures that
do NOT have the same evolutionary origins BUT have same function; e.g. wings of a bat and
insect are analogous because both are used to fly but they do not come from the same
ancestral origins) to humans i.e. they have the same function but obviously humans and
squids/octopods do not come from the same ancestor.
• TETRANEURAL NERVOUS SYSTEM = unlike most protostomes which have a single
pair of nerve cords that run the length of the body, molluscs have a PAIR of NERVES that
supply the visceral mass and asecond pait to supply the foot; FOUR major nerve cords and the
nervous system is TETRANEURAL; apical brain and SUBESOPHAFEAL (below the
esophagus) ganglia have fused to form a ring around the esophagus and TWO pairs of nerve
cords attach to the brain.
• Being an active predator means that they need a circulatory system to SUPPLY OXYGEN to
the MUSCLES involved in locomotion and other tissues.
• An OPEN CIRCULATORY SYSTEM (blood flows into vessels/tubes NOT connected to
each other by small capillaries but instead enters into the HEMOCOELprincipal body
cavity in molluscs and arthropods, remnant of the blastocoel; forms part of
open circulatory system; true coelom is usually reduced to a cavity
surrounding the heart, the pericardial cavity OR gonads) before returning to the
heart) is NOT efficient enough in these molluscs so it is replaced by a CLOSED SYSTEM
with EXTRA hearts to pump blood through the gills.
Gills are modified to assure an adequate supply of oxygen.
Instead of using cilia to move water across the gills, the MANTLE CAVITY which contains the gills is
ENLARGED and becomes MUSCULAR.
Contraction and Relaxation of the mantle’s musculature allows water to be PULLED into and FORCED out
it through the funnel.
This allows for effective AERATION of the gills and creates a unique JET PROPULSION (how cephalopods
move up and down) of the squids and their relatives. This would have not been possible if the mantle secreted spicules which fused into a shell; so shell is now
reduced to a supporting rod or pen OR missing completely.
• Between Annelida and Molluca:
o Pericardial cavity and heart: a true COELOM (true body cavity completely lined by
mesoderm which forms the peritoneum(membrane lining cavity of abdomen and abdominal
organs); animals with true
coelom are referred to as
eucoelomates) exists in both
ANNELIDS and MOLLUSCS and
arises by SCHIZOCOELY(a
true body cavity that forms by
schizocoelus splitting of the
mesoderm); a true coelom is an
example of a 2º body cavity and
the coelom is lined with a
of cells covering the outer surface of an organism/lining the free surface of internal vessels,
cavities and ducts). When the blood vessels forms the two layers of the epithelium come
together with a space between which makes the lumen of the blood vessel. This results in
closed blood vessels in an OPEN CIRCULATORY SYSTEM(closed blood vessels,
pumping hearts, which have similar origin based on the wall of the coelom). The coelom is
restricted to a fluidfilled sac surrounding the heart i.e. the PERICARDIAL
CAVITY. Most of the circulatory system is a HEMOCOEL (principle body cavity in
molluscs, remnant of the blastocoel; forms part of the open circulatory system found in these
animals) that bathes the tissues and extends into the foot.
• Mantle and Calcareous shell
• Ventral muscular foot
• Tetraneural nervous system SYMPLESIOMORPHIES:
• ANNELIDA 03/07/2013
• Segmented body plan
• Take eucoelomate body plan a step further.
• Instead of having only one coelom, their body cavity and its associated structures are DUPLICATED
and arranged end to end in a LINEAR sequence.
• This gives a SEGMENTED appearance which each segment being SERIALLY
HOMOLOGOUS(homology in an animal due to the repetition of components of an organ system in
each body segment) to the segment adjacent to it i.e. METAMERIZATION(division of body into
identical units i.e. METAMERES, repeated down the length of the animal on a longitudinal axis; each
metamere contains IDENTICAL structures to the adjacent metameres).
• ncestrally each METAMERE was a copy of the one adjacent to it and structures from the
MESODERM(third cell layer that develops in the gastrula between the ecto and endoderm n a
triptoblastic animal. Mesoderm develops into muscle, connective tissues and bones as well as blood
and other components) and its overlying ECTODERM were duplicated in each segment. Each segment
was thought of as a completely independent organism with its OWN excretory system, circular and
longitudinal muscles to work the fluid filled coelomic fluid as a HYDROSTATIC
SKELETON(formed from fluid filled cavity surrounded by a body wall that has muscles oriented in
different directions i.e. circular and longitudinal. Muscular contractions maintain the rigid form or change
the shape of the organism) and a GANGLION (mass of nerve cells that forms an enlarged knotlike
structure) to COORDINATE events in the metamere. There were paired METANEPHRIDIA to filter
coelomic fluid and remove METABOLIC WASTES(biochemical waste products of metabolism
which include ammonia, CO2 and water) as well as paired GONADS which appeared in the
SEPTAL(Sheets of tissue that separate two compartments or cavities) wall of each metamere.
• Structures derived from the ENDODERM(innermost layer of cells that forms the digestive tract and
other associated organs; the ectoderm and endoderm form the two primary GERM [can develop into
new one] layers on an animal) are NOT metamerically arranged.
The COMPLETE GUT(alimentary tract has both an anal and oral opening; this allows for linear
processing of ingested food with specialized regions in the gut for grinding, mixing, and digesting food
under different conditions) was a major event for animals BUT this was a potential problem from
METAMERIC animals where some segments adjacent to others were undergoing digestion while
others may have completed itSOLUTION: Development of CLOSED CIRCULATORY
SYSTEM(blood flows in vessels/tubes connected to each other by capillaries) AND
METAMERISM ensured that nutrients were evenly distributed throughout ancestral annelid. ANNELIDA 03/07/2013
• The isolated HYDROSTATIC SKELETONS created by METAMERIC organization that allows
annelids to succeed. Different parts of the body can NOW contract and change shape without affecting
any other segment because they are independent of each other, so basically the dimater and shape of
one segment can change for e.g. in EARTHWORMS or even the left and right side could change
their shapes independent of each other, just like marine worms. New form of LOCOMOTION
developed and this was a successful body plan for annelids.
• Adapted to live in TERRESTRIAL environment.
• Ancestral annelids, have lateral PARAPODIA(paired, lateral, unjointed appendages of polychaete
worms; they have variety of shapes and appearances depending on their roles in LOCOMOTION and
RESPIRATION in different polychaete worms) and SETAE(bristles, hairlike structures, usually made
of CHITIN that extend from the bodies of arthropods and annelids).
• Earthworms have LOST their PARAPODIA, and the # of SETAE is reduced.
• No prominent appendages on the earthworm’s head and those changes are related to the
BURROWING habit of this species.
• By CONTRACTION of the CIRCULAR and LONGITUDINAL muscles of each metamere, the
earthworm can change its shape and burrow into the soil and instead of just pushing into the soil they
also FEED on the soil. Ingested dirt which includes ORGANIC DIGESTIBLE material passes
down the length of the DIGESTIVE TRACT and becomes the earthworm’s FOOD.
TWO unique changes to the digestive tract:
i. TYPHLOSOLE(invagination of the gut wall in earthworms) which increases surface
area for absorption of nutrients. ANNELIDA 03/07/2013
ii. CALCIFEROUS GLANDS – since the initial stages of digestion are ACIDIC and
small stones in ingested food dissolve and release CALCIUM, those glands manage the
excess calcium so that it does not kill the worm.
STEPS of digestive system(consists ofpharynx, esophagus, the crop, intestine and
gizzard ) :
1. Food such as soil enters the earthworm’s mouth where it iswallowed by the pharynx.
2. Then, soil passes through the esophagus, which has CALCIFEROUS GLANDS that
release calcium carbonate to rid the earthworm’s body ofexcess calcium .
3. Then, food moves into the crop where it is STORED.
4. Food, eventually moves into the gizzard, which uses STONES that the earthworm eats to
GRIND the food completely.
5. Then, the food moves into the intestines where GLAND CELLS release fluids to aid in
the digestive process.
6. Blood vessels in the intestinal walls ABSORB the digested food and TRANSPORT it to the
rest of the body.
Earthworms have an exterior CLITELLUM(specialized fused metameres in oligochaetes (i.e. organisms
with few bristles) that secretes mucous during mating) and are MONOECIOUS(at a point in the life
cycle where the organism carries BOTH female and male reproductive structures).
During mating the clitellum secretes the mucous which HOLDS adjacent worms together while the sperm is
Then, the clitellum produces a SLEEVE of mucous and as the sleeve moves forward, eggs and sperm are
released into it.
The sleeve then slides off at the ANTERIOR end and seals itself up to form a COCOON filled with
FERTILIZED eggs and GOOD for the baby worms when they hatch.
• Earthworms do not have lungs.
• breathe through their skin.
• Oxygen and carbon dioxide pass through the earthworm’s skin by diffusion.
• For diffusion to occur, the earthworm’s skin must be kept moist. ANNELIDA 03/07/2013
• Body fluid and mucous is released to keep its skin moist.
• Earthworms therefore, need to be in damp or moist soil.
• This is one reason why they usually surface at night when it is possibly cooler and the
“evaporating potential of the air is low.”
• Earthworms have developed the ability to detect light even though they cannot see.
• They have tissue located at the earthworm’s head that is sensitive to light.
• These tissues enable an earthworm to detect light and not surface during the daytime where they could
be affected by the sun.
• Earthworms are more ADVANCED than their marine cousins because of the. WHY?
o Loss of PARAPODIA and SETAE
o Have permanent GONADS internally in a restricted area
o REGIONAL SPECIALIZATION of internal systems; visible in circulatory system with
FIVE prominent HEARTS.
o GONADS no longer found in each segment like in ancestral form; now RESTRICTED to
only a few segments.
• CLOSED circulatory system. i.e. circulated blood exclusively through vessels.
• THREE main vessels that supply the blood to organs within the earthworm:
1. aortic arches function like HUMAN HEART; FIVE pairs which are responsible for
pumping blood into dorsal and ventral blood vessels.
2. dorsal blood vessel carry blood TO the FRONT of the earthworm’s body.
3. ventral blood – carry blood TO the BACK of the earthworm’s body.
• ANNELIDA 03/07/2013
2. MARINE WORM – NEREIS
• ANCESTRAL CHARACTERISITCS(character shared by ALL members of a taxonomic group
and use to define the unique nature of the group; character may be modified or even disappear in some
members of the taxon) can be seen in Nereis.
• PRIMITIVE CHARACTERS vs. characters that make the animal a PREDATOR.
• PRIMITIVE CHARACTERS(traits that an animal has that are ancestral to the taxon)
• PREDATORY CHARACTERS:
o Sensory tentacles and CIRRI (thin filamentous structures) in the head along with EYES; do
NOT form images.
o To capture prey the PHARYNX(region of digestive tract between the MOUTH and the
ESOPHAGUS; in most animals it is MUSCULAR and FORCES food into digestive tract that
lies behind it) is EVERTED i.e. inside out, exposing TWO jaws that PIERCE the prey and
DRAG it into the alimentary tract as the pharynx is RETRACTED.
• TWO types of marine worms:
a. ERRANT(mobile, actively moving organisms) that are active swimmers and
b. SEDENTARY(somewhat inactive) that live in burrows/ tubes and are FILTER
FEEDERS(feeding mechanism by most sessile organisms like clams, oysters,
corals and sponges; they feed on particles or small organisms that are strained out
of water by circulating them through its system).
• All annelids have a METAMERIC body and ancestrally each METAMERE WAS identical to the next.
• Marine annelids like NEREIS have PAIRED:
o NO gonads
• Why no GONADS? No permanent gonads; form from the mesoderm of the septal walls when it is
breeding season. ANNELIDA 03/07/2013
• Each segment has pair of BILOBED PARAPODIA with CHITINOUS (structure composed of
chitin; complex carbohydrate composed of linearly arrNacetylglucosamine units;
characteristic of cell wall of fungi and outer cuticle of arthropods) SETAL(bristles; hairlike structures
that extend from the body usually made of chitin and are common in annelids and arthropods) hairs
that are characteristic to the phylum.
• The longitudinal muscles in this marine worm are arranged in FOUR distinct blocks; this
arrangement of the muscles AND the separation of the left and right side of the COELOM (true body
cavity completely lined by the mesoderm which forms the peritoneum –serum producing membrane
lining the cavity of the abdomen and covering abdominal organs–.Animals with the true coelom are
known as EUCOELOMATES) by the peritoneum allows the unique locomotion. NEMATODA 03/07/2013
Nematoda – PSEUDOCOELOMATE example
• NSEGMENTED cylindrical bodies
Minute small wormlike animals.
• E.g. ASCARIS i.e. roundworm PARASITIC
• Their body plan is referred to as PSEUDOCOELOMATE(animals that have a body cavity that is
NOT completely lined by a mesoderm; no apparent common ancestor to the group so no longer called
ASCHELMINTHES) and this body plan is shared by 8 other phyla.
• PSEUDOCOELOMATES have:
• PSEUDOCOELOM (body cavity NOT completely lined with a mesoderm; mesoderm ONLY
associated with the ECTODERMAL surface but NOT endoderm; REMNANT of the
PROTONEPHRIDIA(osmoregulatoryexcretory structure found in some invertebrates; also
called a lamecell , the tubule is closed at its DISTAL i.e. distant from center of body; beating
of internal CILIA pull water across the cell membrane and then PROPELS it down the tubule)
• OUTER CUTICLE(nonliving, non cellular, outer layer of an organism secreted by the
underlying epidermis; common in variety of animals including nematodes, annelids and
arthropods; presence of the cuticle prevents the presence of CILIA.)
• In unusual situations, every organism in a species has the EXACT SAME # of cells i.e.
EUTELY(a.k.a. cellular constancy; eutelic organisms have some tissues and organs
composed of FIXED # of cells, or nuclei, in the adult, and the # is speciesspecific; often
associated with animals that have SPECIALIZED for MINIATUREIZATION); zoologist at that
time were sure that those eutelic animals must have evolved from a COMMON ancestor.
• ASCHELMINTHES phylum which was the name given to PSEUDOCOELOMATES was
shattered due to the new CLADISTIC(method of classifying organisms based on primitive
and derived characteristics; the resulting arrangement of organisms reflect evolutionary
relationships between the taxa) approach. WHY?