Chapter 37: Mollusks and Annelids

37-1 Phylum Mollusca

37-2 Phylum Annelida

I. Characteristics of Mollusks (“SOFT-body”~ 112,000 species)• COELOMATES, aquatic AND terrestrial, and 3 CLASSES of mollusks

(some shelled, some unshelled).

37-1 Phylum Mollusca

(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)

(1) Land snails are hermaphrodites. Of what advantage is this characteristic to the land snail?

Critical Thinking

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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.

(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?

Critical Thinking

(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.

(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)

Critical Thinking

(B) Other Bivalves• Oysters (permanently sessile), scallops (move by BACKWARD propulsion), shipworm (ingests DRIFTWOOD, broken down by symbiotic gut bacteria).

(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?

Critical Thinking

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).

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)

37-2 Phylum Annelida (“Little Rings”)

(5) A mutation results in the birth of an earthworm that lacks moisture-sensing cells in its skin. Explain why this earthworm may be less likely to survive than one with such sensory cells present.

Critical Thinking

(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).

(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?

Critical Thinking

(B) Feeding and Digestion (of earthworm)• Ingest SOIL into a PHARYNX and GRINDING soil in GIZZARD.

(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.

Critical Thinking

(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.