AP
Biology
Notes Inverts II
Bilateria:
Phylum Platyhelminthes(Flatworms):
·
There are
about 20,000 species of flatworms living in marine, freshwater, and damp terrestrial habitats.
They also include many parasitic
species, such as the flukes and tapeworms.
·
Flatworms have thin
bodies, ranging in size from the nearly
microscopic to tapeworms over 20 m long.
·
Flatworms and other
bilaterians are triploblastic, with a middle
embryonic tissue layer, mesoderm,
which contributes to more
complex organs and organs systems and to true muscle
tissue.
· While flatworms are
structurally more complex than cnidarians
or ctenophores, they are simpler than
other bilaterans.
·
Like cnidarians and
ctenophores, flatworms have a gastrovascular
cavity with only one opening (and
tapeworms lack a digestive system
entirely and absorb nutrients across their
body surface).
Flatworms are divided
into four classes: Turbellaria, Monogenia, Trematoda, and Cestoidea.
Turbularians
are nearly
all free-living (nonparasitic) and most are marine.
·
Planarians,
members of the genus Dugesia, are
carnivores or
scavengers in unpolluted ponds and streams.
Planarians and
other
flatworms lack organs specialized for gas exchange and
circulation.
Their flat shape places
all cells close to the
surrounding water and fine branching of the digestive
system
distributes food throughout the animal.
Nitrogenous wastes
are
removed by diffusion and simple ciliated flame cells
help
maintain osmotic balance.
Planarians move using
cilia on the
ventral epidermis, gliding along a film of mucus they secrete.
A planarian has a head
with a pair of eyespots to detect light
and lateral flaps that function mainly
for smell.
The planarian
nervous
system is more complex and centralized than the
nerve net of cnidarians.
Planarians can learn to
modify their
responses to stimuli.
Planarians can reproduce
asexually
through regeneration.
Monogeneans (class
Monogenea) and the trematodes (class Trematoda) live as parasites in or on other
animals.
Many have suckers for
attachment to their host.
A tough covering protects
the parasites.
Reproductive organs
nearly fill the interior of these worms.
·
Trematodes parasitize a
wide range of hosts, and most
species have complex life cycles with alternation
of sexual
and asexual stages.
Many require an
intermediate host in
which the larvae develop before infecting the final hosts
(usually a vertebrate) where the adult worm lives.
The blood fluke Schistosoma
infects 200 million people,
leading to body pains, and dysentery.
Most monogeneans are
external parasites of fish
Tapeworms (class
Cestoidea) are also parasitic:
The adults live mostly in
vertebrates, including humans.
Suckers
and hooks on the
head or scolex anchor the worm in the
digestive tract of the host.
A long series of proglottids,
sacs of sex organs, lie posterior to the scolex.
Tapeworms
absorb food
particles from their hosts.
Mature proglottids,
loaded with thousands of eggs, are released from the
posterior end of the
tapeworm and leave with the host’s feces.
In one type of cycle,
tapeworm eggs in contaminated food or water
are ingested by intermediary hosts,
such as pigs orcattle.
The eggs
develop into
larvae that encyst in the muscles of their host.
Humans
acquire the larvae
by eating undercooked meat contaminated with cysts.
The larvae develop into
mature adults within the human.
Phylum
Rotifera: Rotifers are pseudocoelomates with jaws, crowns of cilia, and complete
digestive tracts
·
Rotifers, with about
1,800 species, are tiny animals
(0.05 to 2 mm), most of which live in
freshwater.
Some live
in the sea or
in damp soil.
·
Rotifers have a complete
digestive tract with a separate
mouth and anus.
Internal organs lie in
the pseudocoelom,
a body cavity that is not completely lined with mesoderm.
The fluid in the
pseudocoelom serves as a hydrostatic skeleton.
Through the movements of
nutrients and wastes dissolved in
the coelomic fluid, the pseudocoelom also
functions as a
circulatory system.
·
The word rotifer,
“wheel-bearer,” refers to the crown of cilia
that draws a vortex of water
into the mouth.
Food particles drawn
in
by the cilia are captured by the jaws (trophi) in the pharynx and
ground up.
· Some rotifers exist only
as females that produce more females from unfertilized eggs, a type of parthenogenesis.
Other species produce two
types of eggs that develop by parthenogenesis.
One type forms females
and the other forms degenerate males that survive just long enough to fertilize
eggs.
The zygote forms a
resistant stage that can
withstand environmental extremes until conditions
improve.
Phyla: Bryozoans, phoronids, and brachiopods are coelomates with
ciliated tentacles around their mouths
·
These phyla are known as
the lophophorate animals,
named after a common structure, the lophophore.
The
lophophore is a
horseshoe-shaped or circular fold of
the body wall bearing ciliated tentacles
that surround
and draw water toward the mouth.
In addition to the
lophophore, these three phyla share a U-shaped
digestive tract and the absence
of a head.
Bryozoans
(“moss animals”) are colonial animals that superficially resemble mosses.
·
In most species, the
colony is encased in a hard exoskeleton.
The lophophores extend
through pores in the exoskeleton.
Almost
all the 5,000
species of bryozoans are marine.
In the sea, they
are
widespread and numerous sessile animals,
with several species that can be
important reef builders.
Phoronids
are tube-dwelling marine worms ranging from 1 mm to 50 cm in length.
·
Some live buried in the
sand within chitinous tubes.
They
extend the
lophophore from the tube when feeding and pull
it back in when threatened.
There are about 15
species of
phoronids in two genera.
Brachiopods,
or lamp shells, superficially resemble clams and other bivalve mollusks.
·
However, the two halves
of the brachiopod
are dorsal and ventral to the animal, rather
than lateral as
in clams.
Brachiopods live attached
to the substratum by a stalk.
All of the 330 extant
species of brachiopods are marine.
Phylum
Nemertea: Proboscis worms are named for their prey-capturing apparatus
·
The members of the Phylum
Nemertea, proboscis
worms or ribbon worms, have bodies much like that
of
flatworms.
However, they have a
small fluid-filled
sac that may be a reduced version of a true coelom.
The
sac and fluid
hydraulics operate an extensible proboscis
which the worm uses to capture prey.
Proboscis worms
range in
length from less than 1 mm to more than 30 m.
·
Nearly all of the more
than 900 species are marine, but a few
species inhabit fresh water or damp soil.
Some are active
swimmers,
and others burrow into the sand.
Proboscis worms
and
flatworms have similar excretory, sensory, and nervous
systems.
·
However, nemerteans have
a complete digestive tract and a
closed circulatory system in which the blood is contained
in
vessels.
Phylum
Mollusca: Mollusks have a muscular foot, a visceral mass, and a mantle
·
The phylum Mollusca
includes 150,000 known species of diverse forms,
including snails and slugs,
oysters and clams, and octopuses and squids.
Most mollusks are marine,
though some inhabit fresh water, and some
snails and slugs live on land.
Mollusks are soft-bodied
animals, but most
are protected by a hard shell of calcium carbonate.
·
Slugs, squids, and
octopuses have reduced or lost their shells
completely during their evolution.
·
Despite their apparent
differences, all mollusks have a similar body
plan with a muscular
foot (typically for locomotion), a visceral
mass
with most of the internal organs, and a mantle.
·
The mantle, which
secretes the shell, drapes over the visceral mass and
creates a water-filled
chamber, the mantle cavity, with the
gills, anus,
and excretory pores.
Many mollusks feed by
using a straplike rasping
organ, a radula, to scrape up food.
·
Most mollusks have
separate sexes, with gonads located in the
visceral mass.
However, many
snails are
outcrossing hermaphrodites.
·
The life cycle of many
marine mollusks includes a ciliated
larvae, the trochophore.
This larva is also found
in marine
annelids (segmented worms) and some other lophotrochozoans.
The basic molluscan body
plan has evolved in various ways in the eight classes of the phylum.
·
The four most prominent
are the Polyplacophora (chitons), Gastropoda (snails and slugs),
Bivalvia
(clams, oysters, and other bivalves), and Cephalopoda (squids, octopuses, and
nautiluses).
Polyplacophora
·
Chitons are marine
animals with oval shapes
and shells divided into eight dorsal plates.
Chitons use their
muscular foot to grip the rocky
substrate tightly and to creep slowly over the
rock
surface.
Chitons are grazers that
use their radulas
to scrape and ingest algae.
Gastropoda
·
Most of the more than
40,000 species in the Gastropoda are marine,
but there are also many freshwater
species.
Garden snails and slugs
have
adapted to land.
· During embryonic
development, gastropods undergo torsion
in which
the visceral mass is rotated up to 180 degrees, such that the anus and
mantle cavity are above the head in adults.
·
Most gastropods are
protected by single, spiraled shells into which
the animals can retreat if
threatened.
While the shell is
typically conical,
those of abalones and limpets are somewhat flattened.
·
Most gastropods use their
radula to graze on algae or
plant material.
Some species are
predators.
In these species,
the
radula is modified to bore holes in the shells of other
organisms or to tear
apart tough animal tissues.
In the tropical
marine
cone snails, teeth on the radula form separate poison
darts, which penetrate and
stun their prey, including fishes.
·
In place of the gills
found in most aquatic gastropods, the lining
of the mantle cavity of terrestrial
snails functions as a lung.
Bivalvia:
·
The class Bivalvia
includes clams, oysters, mussels, and scallops.
Bivalves have shells
divided into two halves.
The two parts are
hinged
at the mid-dorsal line, and powerful adductor muscles
close the shell tightly to
protect the animal.
When the shell is
open,
the bivalve may extend its hatchet-shaped foot for digging
or anchoring.
·
The mantle cavity of a
bivalve contains gills that are used for feeding and gas exchange.
·
Most bivalves are
suspension feeders, trapping fine particles in
mucus that coats the gills.
Cilia convey the
particles to the mouth.
Water flows into the
mantle cavity via the incurrent siphon, passes
over the gills, and exits via the
excurrent siphon.
·
Most bivalves live rather
sedentary lives.
·
Sessile mussels secrete
strong threads that tether them to
rocks, docks, boats, and the shells of other
animals.
Clams
can pull themselves
into the sand or mud, using the muscular
foot as an anchor.
Scallops can swim in
short bursts to avoid
predators by flapping their shells and jetting water out
their
mantle cavity.
Cephlalopoda
Cephalopods use rapid
movements to dart toward their
prey which they capture with several long
tentacles.
Squids and octopuses use
beak-like jaws to bite their prey
and then inject poison to immobilize the
victim. A mantle
covers the
visceral mass, but the shell is reduced and internal
in squids, missing in many
octopuses, and exists externally
only in nautiluses.
Fast movements by a squid
occur when it
contracts its mantle cavity and fires a stream of water through
the excurrent siphon.
The foot of a cephalopod
(“head foot”)
has been modified into the muscular siphon and parts of the
tentacles and head. Cephalopods have an
active, predaceous
lifestyle. Unique among mollusks,
cephalopods have a closed
circulatory system to facilitate the movements of
gases, fuels,
and wastes through the body.
They have a
well-developed
nervous system with a complex brain and well-developed sense
organs. The ancestors of
octopuses and squids were probably
shelled mollusks that took up a predaceous
lifestyle. The loss
of the shell
occurred in later evolution.
6. Phylum
Annelida: Annelids are segmented worms
·
All annelids (“little
rings”) have segmented bodies.
Annelids live in
the sea,
most freshwater habitats, and damp soil.
The coelom of the
earthworm,
a typical annelid, is partitioned by septa, but the digestive tract,
longitudinal blood vessels, and nerve cords penetrate the septa and run
the
animal’s length.
·
The digestive system
consists of a pharynx, an esophagus, a crop,
a gizzard, and an intestine.
The closed circulatory
system carries
blood with oxygen-carrying hemoglobin through dorsal and ventral
vessels that are connected by segmental vessels.
·
In each segment is a pair
of excretory tubes, metanephridia,
that remove wastes
from the blood and coelomic fluid.
Wastes are discharged
through exterior pores.
·
A brain-like pair of
cerebral ganglia lie above and in front of the pharynx.
A ring
of nerves around
the pharynx connects to a subpharyngeal ganglion.
·
Earthworms are
cross-fertilizing hermaphrodites.
Some earthworms can also
reproduce asexually by fragmentation followed by regeneration.
·
The phylum Annelida is
divided into three classes: Oligochaeta, Polychaeta, and Hirudinea.
·
Each segment of a
polychaete (“many setae”) has a pair of paddlelike or ridgelike parapodia
(“almost feet”) that function in locomotion. Each parapodium has
several chitinous setae.
In many polychaetes, the
rich blood vessels in the parapodia function as gills. Most polychaetes are
marine.
Polychaetes include
carnivores, scavengers, and planktivores.
The majority of leeches
inhabit fresh water, but land leeches move through moist vegetation.
The evolutionary
significance of the coelom cannot be overemphasized.
The coelom provides a
hydrostatic skeleton that allows new and diverse modes of locomotion.
It also provides body
space for storage and for complex organ development.
The coelom cushions
internal structures and separates the action of the body wall muscles from those
of the internal organs, such as the digestive muscles.
Segmentation allows a
high degree of specialization of body regions.