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Annelids
Annelids
• Phylum Annelida – “little ring” – segmented body
• Round worm-like animal that has a long, segmented
body
• Sizes range from .5mm to 3m
• True coelom lined with mesoderm
• “tube-within-a-tube” digestive tract – mouth to anus
• Segments separated by internal walls celled septa. Most
segments are identical, some modified
Anatomy of an Earthworm
Anus
Setae
Body segments
Gizzard Crop
Dorsal
blood vessel
Clitellum
Mouth
Brain
Ganglion
Circular muscle
Longitudinal
muscle
Nephridia
Ganglia
Ring
vessels
Reproductive
organs
Ventral
blood vessel
Anatomy of an Earthworm
Feeding in Annelids
• Filter feeders to predators
• Pharynx – very muscular, may be armed
with jaws (predators and herbivores), may
be sticky (mucus, detritus feeders), act like
a pump (deposit feeders and parasites)
• Mucus bag – filter feeders
• Feather-like structures – filter feeders
• Pharynx – esophagus – crop (storage) –
gizzard (ground up) – intestine
Circulation in Annelids
• Closed circulatory system – blood contained
within blood vessels
• Blood moves toward the head (dorsal vessel)
• Blood moves away from the head (ventral
vessel)
• Ring vessels in each segment connect dorsal to
ventral vessels
• Vessels act as “hearts” which help pump blood
through the system
Respiration in Annelids
• Gills – aquatic
• Skin – must stay moist, secrete cuticle
(earthworm)
Excretion in Annelids
• Solid wastes pass through the anus
• Waste from cellular metabolism eliminated
by nephridia
Response in Annelids
• Brain and several nerve cords
• Ventral nerve runs entire length of body
• Sense organs most often found in polychaetes
– True eyes that see shapes, Statocysts, chemical
receptors, sensory tentacles, vibration sensors
• Defense, runaway and hide, a few fight with jaws
(sandworm)
– Marine fireworms have irritating bristles
Movement in Annelids
• Muscles
– Longitudinal – lengthwise – make worm
longer and shorter
– Circular – make worm fatter and skinnier
Reproduction in Annelids
• Most reproduce sexually
• External (broadcast) spawners – worms
swarm to surface to spawn by millions
• Some hermaphrodites – exchange sperm;
clitellum secretes mucus ring containing
eggs and sperm which forms cocoon
(earthworm)
Groups of Annelids
• Three classes of Annelids
– Class Oligochaete
– Class Polychaete
– Class Hirudinea
Class Oligochaetes
• Few “bristles” – few setae
• Earthworms and tubifex worms
• Deposit feeders, eat dirt and produce
“castings” – aerate and fertilize the soil
• Tropical earthworms produce castings
18cm long and 2cm in diameter
• Soil or freshwater
Examples of Class Oligochaetes
Class Polychaetes
• Many “bristles”
• Paired, paddle-like, appendages tipped
with bristles (setae) (sea mouse)
• Live in all sorts of marine habitats
• Some free-living, some tube-builders
• May be brightly colored, iridescent, or
luminescent
Examples of Class Polychaetes
Class Hirudinea
• Most parasitic, some carnivorous, most
freshwater, leeches
• 6cm to 30cm long, two suckers, one at
each end
• Penetrate skin by use of proboscis or
sharp jaws
• Produce secretions that prevent clotting
and anesthetizes wound
• Can swallow ten times its weight
Examples of Class Hirudinea
Comparing Flatworms, Roundworms, and Annelids
CHARACTERISTIC
FLATWORMS
ROUNDWORMS
ANNELIDS
Shape
Flattened
Cylindrical with tapering
ends
Cylindrical with tapering
ends
Segmentation
No
No
Yes
Body cavity
Acoelomate
Pseudocoelomate
Coelomate
Digestion and
excretion
Gastrovascular cavity with
one opening only; flame
cells remove metabolic
wastes
Tube-within-a-tube
digestive tract; opening at
each end; metabolic
wastes excreted through
body wall
Tube-within-a-tube
digestive tract; opening at
each end; nephridia
remove metabolic wastes
Respiration
Through skin; no
respiratory organs
Through skin; no
respiratory organs
Through skin; aquatic
annelids breathe through
gills
Comparing Flatworms, Roundworms, and Annelids (Continued)
CHARACTERISTIC
FLATWORMS
ROUNDWORMS
ANNELIDS
Circulation
No heart, blood vessels,
or blood
No heart, blood vessels,
or blood
Blood circulated through
blood vessels in closed
circulatory system
Response
Simple brain; nerve cords
run length of body;
eyespot and other
specialized cells that
detect stimuli
Several ganglia in head
region; nerve cords run
length of body; several
types of sense organs
Well-developed nervous
system with brain and
several nerve cords; many
sense organs
Movement
Gliding, twisting,
and turning
Thrashing
Forward peristaltic
movement
Reproduction
Sexual (hermaphrodites);
asexual (fission)
Sexual (primary males and
females)
Sexual (some are
hermaphrodites; some
have separate sexes)
Mollusks
Phylum Mollusca
• Phylum Mollusca
– 100,000 species
– Most share trocophor larval stage (freeswimming larval stage)
– Defined as soft-bodied animals that have an
internal or external shell
Anatomy of a Clam
Stomach
Coelom
Shell
Heart
Nephridium
Adductor muscle
Mouth
Anus
Excurrent
siphon
Adductor
muscle
Incurrent
siphon
Gills
Mantle cavity
Intestine
Mantle cavity
Foot
Form and Function in Mollusks
• Body Plan – three main parts: foot,
visceral mass, mantle
– Foot – movement, mouth, and associated
feeding structures
– Visceral mass – contains all organs
– Mantle – thin layer of tissue that covers most
of the mollusk body – contains shell glands
The Mollusk Body Plan
Squid
Snail
Shell
Mantle cavity
Foot
Clam
Early
mollusk
Gills
Digestive tract
Feeding in Mollusks
• Herbivores, carnivores, or filter feeders
• Radula – found in snails and slugs – rasp-like
tongue
– Herbivores scrape food
– Predators drill shells or “dart” food
• Cephalopods – beaks
• Clams, oysters, scallops – filter feeders use gills
• Octopi use siphon (tube-like structure) to trap
plankton
Respiration in Mollusks
• Gills in mantle cavity in aquatic species
• Modified mantle in terrestrial species
Circulation in Mollusks
• Open circulatory system – blood flows
through vessels and “sinuses” (open
spaces) (clams and snails)
• Closed circulatory system – blood always
flows inside vessels (cephalopods)
Excretion in Mollusks
• Remove ammonia with use of tubeshaped organs called nephridia
Response in Mollusks
• Simple to very complex nervous systems
(scallops, octopi)
• Cephalopods, highly evolved eyes and
brains
Movement in Mollusks
• Mucus in snails and slugs
• Jet propulsion in octopi
Reproduction in Mollusks
• In most mollusks, sexes are separate,
broadcast spawners
• Cephalopods, internal fertilization
• Some gastropods are hermaphroditic
Groups of Mollusks
• Three classes of mollusks
– Class Gastropoda
– Class Bivalvia
– Class Cephalopoda
Class Gastropoda
• “stomach foot”
• Snails, slugs, abalones, nudibranchs
• Some snails have operculum (hard disk on
foot that forms a “door” when inside shell)
• Nudibranchs feed on cnidarians; utilize
nematocysts for their own defense
• Bright colors mean bad taste or even
poison
Examples of Class Gastropoda
Class Bivalvia
• “two shell”
• Clams, oysters, scallops
• Most are sessile
– Epifaunal – on top of bottom (oyster, mussels)
– Infaunal – in bottom (clams)
• Some are motile – scallops can move by
flapping shells and have eyespots (Ocelli)
Scallop Swimming
Examples of Class Bivalvia
Class Cephalopoda
•
•
•
•
•
•
•
“head foot”
Squid, octopus, cuttlefish, nautilus
Predators, beak for mouth
Seem to be intelligent
Move quickly via jet propulsion
Little or no shell
Highly developed eyes and brains
Examples of Class Cephalopoda
Nautilus Swimming
Ecology of Mollusks
• Fried calamari
• Shipworms
• aquaculture
Comparing the Three Major Groups of Mollusks
MOLLUSK GROUP
SHELL
FOOT
EXAMPLES
Gastropods
Shell-less or
single-shelled
Muscular foot
located on ventral
side and used for
movement
Snail, slug, sea
hare, nudibranch
Bivalves
Two shells held
together by one
or two muscles
Burrowing species
have muscular foot.
Surface-dwelling
species have either
no foot or a
“reduced” foot.
Clam, oyster,
mussel, scallop
Cephalopods
Internal shell or
no shell
Head is attached to
a single foot. The
foot is divided into
tentacles or arms.
Octopus, squid,
cuttlefish, nautilus