MAST451 - Unit 2 Exam

Phylum Mollusca - Body Plan

• Hypothetical Ancestral Mollusc (HAM)

Major features of the Hypothetical Ancestral Mollusc (HAM)

1) Bilateral Symmetry

2) Lack of Segmentation

3) Body organization into:

• head, foot, visceral mass, mantle

Phylum Mollusca - Main Body Regions

• Head

• Foot

• Visceral mass

• Mantle

Phlyum Mollusca - Head

• Mouth w/ radula (radula not always present)

• Sensory structures (eyes, tentacles)

• Ganglia of nervous system (clusters of nerve cell bodies)

Phylum Mollusca - Foot

• muscular for crawling, burrowing, clinging

• in-sessile bivalve → secretes attachment material to hard substrate

• in cephalopods → modified into tentacles, arms, funnel (if present)

Phylum Mollusca - Visceral Mass

• bulk of body, contains:

  • gut

  • digestive glands

  • excretory organs

  • pericardial cavity

  • gonads

Phylum Mollusca - Mantle

• tissue covering visceral mass

  • secretes shell, typically hangs as posterior fold creating mantle cavity (gills + anus)

Molluscan Function: Feeding

Radula:

• common feeding structure

• ribbon of chitin + protein

• 2 rows of chitinous teeth (point backwards, worn away, replaced)

• odontophore below radula/cartilage + muscle (odontophore + radual = buccal mass)

Molluscan Gas Exchange & Circulation - Ctenidium (Ctenidia)

Gill

• usually paired, hang in mantle cavity

• covered w/ cilia to move water through cavity

• Osphradia (sensory)

• Exhalant water include waste + gametes

Molluscan Gas Exchange & Circulation - Development of Transport System

• Development of Transport System for Body Fluid

  • lacking in Porifera, Cnidaria, Ctenophora, Platyhelminthes

  • circulatory system allows for more efficiency

    • ^ size + ^ activity

Molluscan Gas Exchange & Circulation - Heart w/ 2 Atria

• which receives oxygenated blood from gills

• one ventricle which receives atrial blood + pumps to body via 1 or 2 arteries

Molluscan Gas Exchange & Circulation - Circulatory Systems

• Open Circulatory System

  • blood leaves vessels

  • ventricle → aorta → smaller arteries → sinuses → veins → gills → atria → ventricle….and so on..

  • blood = hemolymph

• Closed Circulatory System

  • blood is always in vessels

  • heart ventricle → aorta → smaller arteries → capillaries → veins → gills → heart atria → heart ventricle…and so on..

Molluscan Gas Exchange & Circulation - Blood

Blood often contains proteins that bind to O₂ to CO₂

• which increases solubility of these in blood and magnifies diffusion gradients

Hemocyanin

• copper group

• pale blue when bound to O₂

Molluscan Gas Exchange & Circulation - With Counter Current Exchange

• fully deoxygenated blood meets increasingly oxygenated water, equilibrium never reached

Molluscan Gas Exchange & Circulation - Without Counter Current Exchange

• fully deoxygenated blood meets fully oxygenated water, an equilibrium reached

Molluscan Shell

• Periostracum (outer organic layer)

  • cross-linked proteins

  • added at mantle edge

• Prismatic (think + calcareous)

  • below periostracum

  • laid by mantle epidermis

  • prisms of calcium carbonate (calcite) at right angles to periostracum

• Nacreous (inner calcareous layer)

  • thin sheets by mantle epidermis

  • protein/sugar, surrounded crystals (aragonite)

The Molluscan Groups - 7 Classes

1) Class Monoplacophora

2) Class Polyplacophora (chitons)

3) Class Aplacophora

4) Class Scaphopoda (tusk shells)

5) Class Gastropoda (snails, slugs, nudibranchs)

6) Class Bivalvia (clams, oysters, mussels, scallops)

7) Class Cephalopoda (squid, octopodes, nautilus, cuttlefish)

Class Gastropoda

• Largest mollusc class w/ 40,000 species

• “Stomach Foot”

• Marine, freshwater, terrestrial

• Mostly benthic, but some sessile + pelagic

Gastropod - Modifications

1) Dorso-ventral elongation of body

• visceral mass sits atop the muscular foot

2) Shell as protection

• predation + dessication

• body can quickly withdraw

• columellar muscle attach body to shell

• left or right handed

3) Torsion

4) Gas Exchange

Gastropod - Left or Right Handed Shell

• “sinistral” = left

  • aperture opens to left of columella

• “dextral” = right

  • aperture opens to right of columella

Gastropod - Modifications TORSION

• counter-clockwise twisting of the visceral mass by 180 degrees during early development

• 3 consequences:

  • 1. mantle cavity (ctenidia + anus)

  • 2. digestive tract is now looped

  • 3. Nervous system is twisted into figure 8

What is the adaptive significance of torsion in gastropods?

• Torsion is larval adaptation

• Larval head is vulnerable + would be the last part withdrawn into untorted shell

• Torsion brings mantle cavity anteriorly, for more rapid head withdraw

Gastropod - Modifications GAS EXCHANGE

• H₂O into animal by siphon, over gills, into mantle cavity

• counter-current exchange

• oxygenated blood to heart

• deoxygenated blood from hemocoel (fluid-filled internal body cavity, for open circulatory system)

Class Bivalvia - Modifications

1) Two-piece shell

2) Lack of cephalization

3) Absence of radula or teeth (crystalline style (rod-like structure found in digestive tract) grinds food against gastric shield)

4) Large mantle cavity (mantle: secrete shell, form mantle cavity, form siphons)

5) Sedentary, burrowing life style

• foot reduced in mussels, scallops, oysters, which don’t burrow

6) Respiration + Feeding

• food collection is done by labial palps (function similar to arms and push food towards esophagus)

Class Bivalvia - Two-piece Shell

• 2 valves held together by an elastic ligament

• “teeth” near hinge which interlocks + keeps valves from slipping

• typically 2 adductor muscles (anterior + posterior)

  • contracts = shell closes

  • relaxes = ligaments spring shell open

Class Cephalopda - Modifications

1) Head merged with foot

2) Large size (Largest invertebrate)

3) High Activity

4) Shell is reduced

5) Skin with chromatophores

6) Cephalization + sensory systems

Cephalopods - Head merged with foot

• cephalopod = “head foot”

• mouth + eyes are part of head, arms + tentacles + funnel are derivatives of foot

• funnel is moving water in and out of mantle cavity (propulsion + gas exchange)

Cephalopods - High Activity

• jet propulsion (squid, cuttlefish)

• crawling (octopus)

• closed circulatory system (No Counter Current Exchange)

  • low O₂ carrying capacity

Cephalopods - Shell is reduced

• squid: internal pen (gladius = sword-shaped internal structure, made of chitin)

• cuttlefish: internal shell, reduced

• octopodes: highly reduced

  • nautilus have external shell

  • siphuncle = calcified tube (thread-like tube, used to control buoyancy

Cephalopods - Skin with Chromatophores

• cells with pigment

• muscles expand and contract chromatophores, changing skin color and pattern

• nervous control

• skin color influences: predator avoidance, competition, matin displays

Cephalopods - Cephalization + Sensory Systems

• highly advanced

• learning + memory

• excellent eyes → convergent to vertebrate eye (sight-based predators)

• bioluminescence in deep sea species

Phylum Annelida

• segmented worms

• smaller then 1 m to > 2m

• +12,500 species

Phylum Annelida - Characteristics

• Setae or Chaetae

• High degree of Metamerism

• Segmentation by transverse septa

• Metanephridia

Phylum Annelida - Setae

• Setae or Chaetae

  • chitinous bristles on each segment

  • locomotion (creates friction)

  • Class Polychaeta = many setae

  • Class Clitellata:

    • Subclass Oligochaeta = many setae (earthworms)

    • Subclass Hirudinea = no setae (leeches)

Phylum Annelida - High Degree of Metamerism

• Serial repetition of the body + organ systems into similar body segments

• Arranged in a linear series along the anterior - posterior axis

• Some segments fuse to form one specialized segment

  • (EX: Prostomium = fused anterior segments forming head)

Phylum Annelida - Segmentation by Transverse Septa

• Septa form from mesoderm (peritoneum)

• Present in larva, adult, or both

• Septa divide the coelom into discrete compartments (each with its own coelomic fluid)

• Hydrostatic skeleton is created by coelomic fluid

Advantages of a Coelom + Metamerism in Annelids

• faster/more efficient burrowing + movement

• independen movement of segments

  • some can bulge, others can narrow

• Anterior ▬ Posterior = circular muscles contract, segment narrows/elongates

• Anterior ▮ Posterior = longitudinal muscles contract, segment widens/shortens

Phylum Annelida - Metanephridia (excretory organs)

• some wastes are excreted at the body wall

• most waste is voided at the nephridia

• ciliated funnel (nephrostome) draws in coelomic fluid (non-selective)

• selective reabsorption into capillaries

  • aids in osmoregulation

• active secretion into neph. tubule

• final urine at nephridiopore

Metanephridium

• nephridial tubule that opens into the coelom by a ciliated funnel (nephrostome) and exits the body wall by a nephridiopore

Annelids: Major Taxa

• Class Polychaeta (30% of Annelid species)

  • marine polychaetes

  • deep sea tube worms

• Class Clitellata

  • Subclass Oligochaeta (earthworms)

  • Subclass Hirudinea (leeches)

    • Echiurans (spoon worms)

    • Sipunculids (peanut worms)

Class Polychaeta : Characteristics

1) “Parapodia” extend from body segments

2) Setae (Chaetae) project from parapodia

3) Head (Prostomium)

Polychaeta Characteristics: Parapodia

“Parapodia” extend from body segments

• fleshy, paddle-like appendages of body wall

• Each parapodium is biramous (2 parts)

  • w/ dorsal lobe (notopodium)

  • w/ ventral lobe (neuropodium)

• active, benthic crawlers have well-developed parapodia setae

• burrowers have reduced parapodia

Polychaeta Characteristics: Setae (Chaetae)

Setae project from parapodia

• chitinous

• locomotion mostly

  • some w/ hooks used to grip tube/burrow walls

• defense

  • toxins released when broken

Polychaeta Characteristics: Head (Prostomium)

• w/ sensory structures

• eyes, antennae, palps

Class Polychaeta: Function Biology

• Diversity of feeding strategies

  • Raptoral feeders, Deposit feeders, Filter feeders

• Respiration

• Blood + Circulation

• Excretion + Osmoregulation

• Reproduction

Polychaeta Biological Function: Raptoral Feeders

• various inverts, algae, dead animals

• capture prey w/ eversible pharynx

  • w/ jaws (2 or more)

• jaws seize food + pharynx retracts

Polychaeta Biological Function: Deposit Feeders

• non-selective or selective

  • non-selective = apply mouth to substrate + consume mud/sand

  • selective = organic matter selected from between sand/mud particles + conveyed to mouth (ciliated grooves)

Polychaeta Biological Function: Filter Feeders

• head w/ specialized feeding structures to collect plankton + direct to mouth (ciliated grooves)

Polychaeta Biological Function: Respiration

• most w/ gills on body surface

• many pump seawater through burrow

• parapodia actas as gills

  • sedentary w/ gills extending from burrow opening

    • Hemoglobin is often the blood respiratory pigment

Polychaeta Biological Function: Blood + Circulation

• closed circulatory system

• blood flows by body peristalsis

  • vessel contraction in some (no true hearts)

  • posterior to anterior (dorsal blood vessel)

  • anterior to posterior (ventral blood vessel)

Polychaeta Biological Function: Excretion + Osmoregulation

• metanephridia in most

• more primitive species without blood vascular system w/ protonephridia

Polychaeta Biological Function: Reproduction

• sexual + gonochoristic

• gametes produced by peitoneum + released into coelom to mature

• “Epitoky” = morphological transformation in advance of reproduction

• Fertilization is external in sea water

• Trochophore larvae

Class Polychaeta: Family Siboglinidae

• ~150 sp. marine, mostly deep sea

• gas exchange through cephalic lobe

• live in chitinous tube

• no septa

• trophosome = symbiotic microbes (sulfur/methane oxidizing bacteria), lacks digestive tract (feeds on DOM or carbon from symbionts)

Class Clitellata: Subclass Oligochaeta

• ~3500 sp. 65% marine

• clitellum = thickened anterior region, swollen by mucous producing glands

  • cocoon production + copulation

• simultaneous hermaphrodites w/ testes + ovaries

Oligochaete: Characteristics

1) Well-developed segmentation

2) No parapodia

• single chaetae

• peristalsis helps movement/burrow

3) Prostomium is simple (not elaborate)

Class Clitellata: Subclass Hirudinea

• ~600 sp. most freshwater or terrestrial

• derived from oligochaete

• no parapodia, prostomium appendages

• lack chaetae

• reduced segmentation

  • coelom w/ mesenchyme

• suckers (anterior + posterior) used for locomotion

Class Echiura (Spoon Worms)

• ~160 sp.

• shallow, sand/mud

• 2° loss of segmentation

• extensible, muscular proboscis

  • brain located inside

  • ciliated for deposit feeding

• anal sacs + metanephridia

  • process coelomic fluid + pass to rectum

• gonochoristic

Class Sipuncula (Peanut Worms)

• ~350 sp.

• shallow water

• mud/sand, vacant shells, tubes

• “introvert”

  • anterior structure fully retracts into body

  • mucous-covered tentacles for deposit feeding

• no chaetae, no segmentation

• no circulatory system BUT blood respiratory pigment in hemerythrin = coelomic fluid

• “urns” clusters of cells, collect waste in coelomic fluid, release at body wall or metanephridia

Phylum Arthropoda

• “jointed foot”

• Largest - 75% of described animals

• Marine, freshwater, terrestrial

• ~1 million sp.

Two phyla related to Arthropoda

1. Onychophora

2. Tardigrada

Arthropoda: Similarities w/ Onychophora

• ~100 sp.

• Annelid-like:

  • non-jointed appendages

  • one pair of excretory organs per segment

  • muscle

• Arthropod features:

  • some striated/skeletal muscle

  • trachea-like gas exchange

  • mouth appendages

  • reduced coelom (hemocoel)

Arthropoda: Similarities w/ Tardigrada

• ~900 sp.

• “Water bears”

• interstitially in marine but most live in freshwater

• similar body plan to arthros:

  • lack jointed appendages

  • no calcified cuticle

  • not perfect protostomes

  • cryptobiosis (metabolic activity stops in response to extreme environmental stress)

Arthropoda: Characterisitics

1) Chitinous exoskeleton

2) Jointed, segmented appendages

3) Striated muscle + neural control

4) Ecdysis

Arthropoda: Chitinous Exoskeleton

• support, protection, movement because of a rigid skeleton

• polysaccharide + some protein

• hardened by:

  • Calcium Carbonate

  • Sclerotization (protein cross linking)

• Secreted by epidermal cells

• Ex: crustacean cuticle

• epicuticle ~3% of total exoskeleton thickness

• lipoprotein

• waterproof but limit gas exchange

• procuticle: bulk of exoskeleton (chitin + some protein)

Arthropoda: Jointed, Segmented Appendages

• procuticle is thin + flexible in many body regions

• joints where segments articulate

• pairs of antagonistic muscles

• coelom is not involved in locomotion

  • rather coelom is a small hemocoel (“circulatory system”)

• locomotion by contraction/relaxation of striated/skeletal muscles

Arthropoda: Striated muscle + Neural control

• most inverts have smooth muscles but arthros have striated muscle

  • quick contractions

  • poly-neuronal innervation

Arthropoda: Ecdysis

• shed exoskeleton to grow

• molting

• exuvium = old exoskeleton

• ecdysis = splitting old skeleton + crawling out + forming new skeleton

Arthropod Growth: 4 stages to molt cycle

1. Pre-ecdysis - preparatory feeding, Ca²⁺ release into bloof

2. Ecdysis - shedding of exoskeleton

3. Post-ecdysis - calcification/hardening of new exoskeleton

4. Intermolt - period between molts

Arthropod Growth: Epidermis

• Epidermis secretes proenzymes

  • digest base of old exoskeleton

• Epidermis secretes a new epicuticle + molting fluid, old exoskeleton about to be shed

Arthropod Growth: Forming New Skeleton

• Proenzymes digest endocuticle

• New procuticle added

• Exoskeleton splits @ weak points

  • ex: anterior margin in limulus abdomen of crab + animal crawls out

• “soft shelled”

• shell then hardens

What happens when an arthropod loses a limb?

• limb bud forms

• new limb forms on new exoskeleton @ pre-ecdysis

• new limb unfolds during ecdysis

Arthropod Growth: Control of Molt Cycle

• hormones control molt cycle

  • ex: crustacean = y-organ in head produces ecdysteroid hormones, which trigger ecdysis process

  • inhibition process - MIH - molt inhibiting protein

    • produced in x-organ + sinus glands of eye stalks

  • inhibits ecdysone production + function

  • other hormones control ecdysone

Main Arthropod Groups

• Subphylum Trilobitomorpha = Trilobites

• Subphylum Chelicerata (Classes Merostomata, Arachnida (not discussed), Pycnogonida)

Arthropod: Subphylum Trilobitomorpha (Trilobites)

• ~4000 sp., all extinct

• Cambrian, extinct 285 mya.

• Benthic

• Oval w/ 3 lobes

Arthropod: Subphylum Chelicerata

• 2 body regions

  • prosoma

  • opisthosoma

• lack antennae

• Chelicerae = 1st pair of feeding appendages

  • 2nd pair are pedipalps

  • 4 pairs of walking legs

Arthropod: Subphylum Chelicerata - Class Merostomata

• horseshoe crabs

• Cambrian

• 4 extant sp.

• shallow water, soft bottoms

• omnivores, feed on molluscs, annelids, algae

Class Merostomata: Characteristics

• one pair of lateral compound eyes

• 8 other “simple” eyes (dorsal + ventral surface)

• 6 pairs of prosome appendages

• Chelicerae = 5 walking legs (pedipalps + 4)

  • gnathobase (armed, spiny modification, helps eating)

• opisthosome is not segmented

• 6 pairs of appendages

  • 1st are reproduction

  • other 5 are book gills

    • respiration

    • locomotion

Arthropod: Subphylum Chelicerata - Class Pycnogonida

• ~1160 sp.

• lack distinct tagmata

• reduced opisthosome

• Ovigers - used to carry fertilized eggs

  • groom legs

  • carry eggs in males

Class Crustacea: Characteristics

1) 3 tagmata

• head, thorax, abdomen cephalothorax

2) Head w/ 5 appendage pairs

• 1st antennae, 2nd antennae, mandibles, 1st maxillae, 2nd maxillae

3) Thorax w/ 8 thoracopods

• maxillipeds (1-3) - handle + sort food

• pereopods (4-8) - locomotion

4) Abdomen w/ 6-7 segments

• appendages = pleopods

  • swimming, jumping, respiration, copulation, brooding eggs

5) Biramous appendages

• branched

Crab vs. Shrimp

Crab: shortened body, no uropods, pleopods modified for reproduction, enlarged cheliped

Shrimp: elongated body, uropods, pleopods (swimming, respiration, reproduction), reduced cheliped

Class Crustacea: Reproduction

• Gonochoristic

  • internal fertilization w/ sperm storage

  • spermatophore in some sp.

  • maintenance of eggs or not

• Many marine taxa have larvae that hatch from egg + develop w/in plankton (meroplankton)

  • ex: callinectes

Development in Crustaceans: Copepods

1st - Nauplius

2nd - After 4-5 molts, Copepodite

3rd - After 5 molts, Adult

Development in Crustaceans: Barnacles

1st - Nauplius

2nd - After 3-5 molts, Cyprid

3rd - After smelling SIPC (settlement inducing protein complex, pheromone), Adult

Development in Crustaceans: Arthropods/Isopods

• Direct development

• Adults use thoracic marsupium (carry their fertilized eggs in a water-filled ventral brood pouch)

Class Crustacea: Respiration & Excretion

• Open Circulatory System

• Most w/ hemocyanin

• Gas exchange @ gills (bronchial chamber or @ base of pleopods)

• Ammonia waste (through gills)

• Antennal/green glands for salt/water balance

Class Crustacea: Digestion

• extracellular digestion

  • cardiac + pyloric stomachs w/ gastric mill

  • hepatopancreas

Class Crustacea: Nervous System

• highly developed

  • sensory (eyes + vision)

  • motor adaptations

  • conserved

Why is the mantle important in molluscs?

It covers the visceral mass, secretes the shell, and typically forms the mantle cavity containing the gills and anus.

How does counter-current exchange improve gas exchange?

Blood meets increasingly oxygenated water, maintaining an oxygen diffusion gradient and allowing more oxygen to enter the blood.

Why don’t bivalves have a radula?

They commonly filter-feed, using their gills to collect food and labial palps to move it toward the mouth.

How does torsion benefit gastropods?

It moves the mantle cavity toward the front, allowing the vulnerable head to withdraw into the shell more quickly.

How do cephalopods use their funnel?

They force water through the funnel for jet propulsion and water movement through the mantle cavity.

Why is a hydrostatic skeleton useful to annelids?

Coelomic fluid provides support against which muscles work, allowing segments to change shape for movement and burrowing.

What is the difference between setae and parapodia?

Setae are bristles used mainly for locomotion; parapodia are fleshy, paired appendages that often bear setae.

How do metanephridia help annelids maintain homeostasis?

They remove wastes and selectively reabsorb substances, helping regulate water and salt balance.

What is the advantage of segmentation in annelids?

Segments can move somewhat independently, allowing more efficient movement and burrowing.

Why must arthropods molt to grow?

Their rigid exoskeleton limits expansion, so they shed it and form a larger new exoskeleton.

What would happen if MIH stopped inhibiting molting hormones?

Inhibition of ecdysteroid production would decrease, potentially allowing the molting process to begin.

How do crustacean appendages differ from typical chelicerate appendages?

Crustaceans typically have branched (biramous) appendages, while chelicerates have specialized appendages including chelicerae and pedipalps.

How do horseshoe crab book gills function?

They primarily perform gas exchange and can also assist with locomotion.