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Chapter 37: Mollusks and Annelids
37-1 Phylum Mollusca
37-2 Phylum Annelida
37-1 Phylum Mollusca
I. Characteristics of Mollusks (“SOFT-body”~ 112,000 species)
• COELOMATES, aquatic AND terrestrial, and 3 CLASSES of mollusks
(some shelled, some unshelled).
(1) Trochophore (aquatic larval SHARED among mollusks AND annelids)
• Zygote  ciliated larvae, propels through water, FREE-LIVING.
II. Body Plan of Mollusks
• Divided into “head-foot” AND “visceral mass;” covered by MANTLE,
(SECRETES shell).
(1) Head-Foot
• Contains MOUTH, GANGLIA (head), and LARGE FOOT (locomotion).
(2) Visceral Mass (contains INTERNAL organs)
• Contains heart AND organs for digestion, excretion, and reproduction.
NOTE: The COELOM is limited to a SPACE around the heart.
(3) Mantle & Mantle Cavity (outer-covering OVER visceral mass)
• Tissue produces a CaCO3 shell; CAVITY is between mantle AND visceral
mass.
(4) Ganglia (connected by TWO pairs of long nerve cords)
• Control locomotion, feeding, and process sensory information (light,
touch, chemicals).
(5) Radula (FEEDING adaptation, diversified)
• In MOST mollusks, a tongue-like strip of ABRASIVE tissue with teeth
for scraping, piercing, or cracking.
III. Class Gastropoda (“STOMACH-foot”, mobile, ~ 90,000 species)
• Snails, abalones, conchs, slugs, nudibranchs; most have a SINGLE shell,
(some have lost their shells through evolution).
(1) Torsion (DEVELOPMENTAL process of gastropod LARVAE)
• Visceral mass TWISTS around the HEAD region, resulting in a mantle
cavity, gills, and anus (makes a space to WITHDRAW head if threatened).
(2) Hemolymph (OPEN-circulatory fluid of gastropods & bivalves)
• Collected, pumped, and released DIRECTLY into the tissues
(Cells are directly bathed WITHOUT vessels).
(3) Hemocoel (OPEN fluid-filled SPACE or blood-cavity)
• Hemolymph drains BACK to the heart from hemocoel.
(A) Snails (eyes on the ENDS of modified tentacles)
• Terrestrial (hermaphrodites, mantle cavity acts as a LUNG requiring
MOIST air); Aquatic (separate sexes, respiration through GILLS)
Critical Thinking
(1) Land snails are hermaphrodites. Of what advantage is this
characteristic to the land snail?
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(B) Other Gastropods (slugs, nudibranchs, pteropods)
• Moisture is VITAL to exchange gases (slugs); Nudibranchs—naked gills;
Pteropods—foot is modified for SWIMMING.
IV. Class Bivalvia (e.g., clams, mussels, scallops, etc…)
• SESSILE lifestyle, 2-shells on a muscular hinge, and LACK a head region.
Critical Thinking
(2) Marine clams and other aquatic mollusks reproduce by releasing sperm
and eggs into the water. How might this process affect the reproductive
success of these mollusks? Would you expect aquatic mollusks to release
many sperm and eggs or only a few?
(1) Adductor Muscles
• HINGE muscles open AND close shell for feeding, mating, and protection.
(A) Clams (marine (EXTERNAL) vs. freshwater (INTERNAL) reproduction
• Bivalves that INHABIT mud or sand and feed as FILTER-feeders.
(1) Incurrent Siphon & Excurrent Siphon (Pair of hollow, fleshy tubes)
• Draw IN and circulates OUT water after nutrients and wastes have been
EXCHANGED.
Critical Thinking
(3) Many clams have very long incurrent and excurrent siphons. For
example, the siphons of a glam called the geoduck, Panope generosa, may
exceed one meter in length. What do you suppose could be the adaptive
advantage of such long siphons? (Keep in mind the habitat of most clams)
(B) Other Bivalves
• Oysters (permanently sessile), scallops (move by BACKWARD propulsion),
shipworm (ingests DRIFTWOOD, broken down by symbiotic gut bacteria).
Critical Thinking
(4) Humans value pearls for their luster and color, features that are of no
significance to an oyster. Furthermore, making a pearl consumes resources
that an oyster could use for other purposes, such as strengthening its
shell. Given these facts of what advantage is it to an oyster to
manufacture a pearl?
V. Class Cephalopoda (“HEAD-foot”, MOST evolved mollusks)
• Shells ARE flexible or LOST for a free-swimming, predatory lifestyle
(e.g., octopuses, squids, cuttlefishes, and chambered nautiluses)
(1) Chromatophores
• PIGMENT cells show a sudden change in COLOR, used with camouflage,
mating, and intimidation.
(A) Squids (cephalopods with TEN tentacles)
• 2 LONGER tentacles for capturing prey, 8 shorter to assist with feeding;
can release a cloud of INK to deter predators.
NOTE: The RECORD for the LARGEST invertebrate is the GIANT squid,
Architeuthis, which can reach a length of 60 feet and way more than 3.5
tons)
(B) Octopuses and Chambered Nautiluses
• Cephalopods with 8 tentacles (octopus), and ONLY cephalopod that has
RETAINED its external gas-chambered SHELL (nautilus).
37-2 Phylum Annelida (“Little Rings”)
I. Characteristics and Classification of Annelids (~15,000 species)
• Bilateral segmented worms, coelomates (i.e., different areas can contract
OR expand independently; also, DUPLICATION of some organs)
Critical Thinking
(5) A mutation results in the birth of an earthworm that lacks moisturesensing cells in its skin. Explain why this earthworm may be less likely to
survive than one with such sensory cells present.
(1) Setae (used to CLASSIFY annelids into 3 CLASSES)
• External BRISTLES help with TRACTION and reproduction.
(2) Parapodia (found among POLYCHAETES)
• FLESHY protrusions ON SURFACE of most annelids, SOME with setae.
II. Class Oligochaeta (“FEW bristles”)
• Inhabit soil OR freshwater and LACK parapodia with FEW setae
(e.g., Lumbricus terestricus, the earthworm)
(A) Structure and Movement
• LONGITUDINAL muscles squeeze FLUID into adjacent segments
(PULLS posterior FORWARD).
Critical Thinking
(6) For quite a while after an earthworm is cut in half, both halves will
continue to move about, and both will retract if they are touched. What do
these observations suggest about the role the brain has in coordinating
these movements?
(B) Feeding and Digestion (of earthworm)
• Ingest SOIL into a PHARYNX and GRINDING soil in GIZZARD.
Critical Thinking
(7) Clams are aquatic and earthworms are terrestrial. Nevertheless, the
feeding methods of clams and earthworms are basically similar. Explain
how this could be so.
(1) Esophagus (a.k.a. the pharynx)
• Moves SOIL from MOUTH to CROP.
(2) Crop (soft and expandable)
• Soil is STORED before moving into GIZZARD.
(3) Gizzard (thick and muscular, follows crop)
• Soil is MASHED UP into a brown PASTE of nutrients.
(4) Typhlosole (infoldings of the INSTESTINAL wall)
• Increases SURFACE AREA for ABSORPTION of nutrients from paste.
(C) Circulation (CLOSED-circulatory system; blood ALWAYS in vessels)
• Blood travels through VENTRAL blood vessel (to posterior) and
RETURNS through a DORSAL blood vessel (to anterior).
(1) Aortic Arches (5 pairs of muscular tubes or “hearts”)
• Link dorsal AND ventral blood vessels; contractions FORCE blood through
circulatory system.
(D) Respiration and Excretion (i.e., skin AND nephridia)
• GAS exchange and WASTE removal BOTH occur through SURFACE of
SKIN, which MUST be kept MOIST.
(1) Nephridia (excretory tubules, found in pairs)
• WASTES, excess water  REMOVED through PORES on ventral surface.
(E) Neural Control (cerebral ganglion AND ventral nerve cord)
• Nerves BRANCH from ganglia to carry messages to MUSCLES.
(F) Reproduction (earthworms—hermaphroditic)
• Press their VENTRAL surfaces together with bodies in opposite directions.
(1) Clitellum (also HOLDS the two worms in place)
• Secretes a MUCUS coat (receptacle) for SPERM, creating a COCOON of
fertilized eggs.
(2) Seminal Receptacle (works with “receptacle” of other worm)
• POUCH where sperm is stored BEFORE released,
(NOTE: FERTILIZED EGGS are laid in a PROTECTIVE case until hatching).
(3) Chitin (rigid material, polysaccharide)
• Made by clitellum  protective COVERING for eggs until they HATCH.
III. Classes Polychaeta and Hirudinea
• Polychaetes (NUMEROUS setae, antennae, mouthparts), MOST live as
free-swimming marine PREDATORS.
• Hirudinates (NO setae or antennae, mouthparts specialized for feeding),
most live as freshwater PARASITES; 300 species of LEECHES.