Invertebrate Zoology Exam 3

Phylum Mollusca

• Soft-bodied

• Over 100,000 species, body plan is diverse

Features

1. Dorsal epithelium (mantle) - secretes shell

2. Radula - used for feeding

3. Ventral epithelium - forms “foot”

Mollusc Features

Mantle - specialized epithelium for shell

Nacreous layer - base layer, thin and smooth, pearl formation 

Central prismatic layer - thick and tough 

Periostracum - outer layer, organic, textured 

Mantle cavity - between mantle and viscera, contains ctenidia (gills): respiratory function/digestion, reproduction

Osphradium - chemoreceptor

Coelom (body cavity) - small, reduced, surrounds heart/gonads

Mollusc Circulatory System

Circulatory system - variation, some closed, most open

• Series of sinuses from hemocoel (blood-filled body cavity) 

• Heart pumps fluid - no pigment, some hemoglobin (iron) 

• Some hemocyanin in blood (copper) 

• Gas exchanged a gills: counter current exchange 

Mollusc Digestive System 

• Radula in mouth (ribbon of teeth), digestion is complete (mouth to anus) 

• Teeth are chitinous

• Odontophore - under radula, cartilage like, attached to musculature = forms buccal mass/odontophore complex 

• Rasp food from substrate or boring 

• Digestion is aided by digestive gland enzymes and storage  

Mollusc Excretory System

• Removes metabolic wastes from cells

• Waste is dissolved in fluid

• Osmoregulation - water + salt homeostasis

• Excretory system rids excess water, ions, and toxins

• Excretion in different species is done via ammonia (choice of most molluscs), urea, or uric acid

Ammonia - requires more water to excrete, but less energy

Urea and Uric acid - save water, less toxic, require more water

Types of excretion:

1. Osmoconformers - balance with environment, diffusion + osmosis, used in the ocean

2. Osmoregulators - homeostatic mechanisms, consistent concentrations despite outer environment

Nephridial systems:

Protonephridia - flame cells, one one opening

Molluscs - metanephridia: internal/external opening, funnel collects, nephridiopore releases

Mollusc Reproduction

• Typically dioecious

• Hermaphrodism - simultaneous protandric (born as males, but later turn into female)

• Ovotestis + reciprocal copulation

• Fertilization can be internal or external

• Reproduction can be sexual or asexual: Asexual - lacks diversity, more energy efficient Sexual - energy needed to find mate, only use half of each parent’s DNA, creates diversity

• Molluscs display sophisticated reproduction

• Hectocotylus arm - the modified, specialized arm of a male octopus used to transfer sperm to the female

• Love darts - calcareous or chitinous darts that transfer a mucus containing hormones

Mollusc Development

• Larval stages can be seen sometimes

• Trochophore larvae - disperse (free-swimming, ciliated larvae)

• Can develop to veliger - feeding

• Bivalves modified veliger - glochidia

Mollusc Nervous System

• Brain

• Model system for nerve impulses - giant fibers 

Class Aplacophora

• “Not plate bearing”

• Might be ancestoral

• Cylindrical and vermiform (worm-like)

• Food formed as keel

• Marine, in sediment

• No shell (have calcareous “spicules), foot is reduced, no fossil record

Class Polyplacophora

• “Many plate bearing,” chitons

• 800 species

• Marine, intertidal areas

• Shell: 7-8 interlocking plates

• Thick and flattened mantle (girdle) - used for suction

• Calcareous spicules in skin

• Water flows through body on both sides, over gills

• Linear (straight) digestive tract, full digestive tract, odontophore complex

• Ladder-like nervous system

• Sensory (photo, chemo, mechano receptors)

Class Monoplacophora

• “One plate,” one valve

• Marine, 31 species

• Paired structures, 3-6 ctenidia (gills) pairs, 6-7 pairs of nephridia, 8 pairs of retractor muscles

• Linear digestive tract

• Flattened foot with one shell

• Ancestor of “advanced molluscs”

• Variation on single shell

Class Gastropoda

• “Stomach foot,” largest class of molluscs

• 60,000 species, found everywhere, do everything

• Visceral mass: twisted 90-180°, on top of foot, covered by “univalve”

• Operculum on foot (some), disc that allows the body to be retracted and the shell to seal

• Shell in whorls around columella (central axis), sinistral (opening on left) or dextral (opening on right)

• Twisting of body = torsion, gills and anus are anterior, no clear reason (potentially sensory)

Subclass Prosobronchia (Prosobranchs) 

• 35,000 species

• Marine 

• Cone snails, many threatened (mainly due to shell collectors) 

• Many venomous (potential applications in medical research)

Subclass Opisthobranchia (opisthobranchs)

• “Posterior gills”

• Linear digestive tract

• Lost features - shell, operculum, mantle cavity, reduced gills

• Sea slugs/nudibranchs

• Sea hares

• Sea butterflies - have parapodia/pteropods

Pulmonata

• “Lung snails,” terrestrial snails + slugs

• Vascular mantle cavity - “lung”

• Some ciliary movement, but largely through pedal waves

• Can only move forward, uses a mucus trail

Class Bivalvia (bi-valves)

• Includes clams, scallops, mussels 

• Features - two shells closed by adductor muscles 

• Laterally flattened 

• “No” head 

• Large mantle cavity 

• Sedentary 

• No radula, suspension feeders - gill cilia draw water into mantle and filter it 

Subclass Protobranch

• “First gill” - gills like gastropods 

• Food collected by palp proscides and labial palps 

• Gills for respiration

Class Lamellibranchia (Lamellibranchs)

• “Plate gills”

• Gills for gas exchange/feeding

• Byssal threads = used for attachment, encrusts them to surfaces, potential industrial use as glue

• Many freshwater species in this group, many reproduce through “vampire” larvae that attach to fish gills (glochidia larvae)

Superorder Anomalodesmata

• “Irregular ligament”

• Hinge lacks “teeth”

• Septibranch type gills

• Due to their carnivorous diet, have chitin in the stomach/gills that grinds prey

Class Scaphopoda

• “Spade foot,” tusk shells

• Tubular

• Anterior feeding tentacls

• No gills or heart, foot pumps fluids

Class Cephalopoda (Cephalopod)

• “Head foot,” includes octopi, squid, cuttlefish, and nautilus 

• Distinct head (cephalization) 

• Reduced/absent/chambered shell 

• Closed circulatory system 

• Foot is highly modified 

• True brain, enclosed by cartilage 

• Radula + ctenidia modified 

• Active predators/complex behaviors

Nautilus (genus)

• More ancestorial, shell is highly adapted

• Shell is chambered, used for buoyancy

• Septa, siphuncle runs through it (controls buoyancy)

• Locomotion by the ejection of water

Cephalopod Features

• Locomotion by arms/fins

• Shell is internal

• No shell protection - intelligence, toxins, camo, and tools used for defense

• Camo = chromatophores

• Single, systemic heart with brachial (gill) hearts

• No countercurrent exchange or cilia in gills

• Muscles of the mantle flood the cavity (used for gas exchange)

• Cephalopods are deaf, may be a defense against predators like whales

Phylum Annelida (Annelids)

• Derived group with spiral cleavage

• Segmented worms

• One or more setae (chitinous spines)

• Metamerism - body is segmented, organs are in repeating units, seen in chordates/arthropods also

• Septa separate segments, sheet of mesoderm lines interior (triploblastic), creates peritoneum that lines the interior body cavity (coelom) \

• Individual segments form a hydrostatic skeleton

• Flexible body wall - circular/longitudinal muscles

• Body wall is used in gas exchange, moisture is required

• Elaborate excretory system, metanephridia (open at both ends) front = nephrostome, pulls coelomic fluid in, runs it through tubules (resorption, osmoregulation)

• Waste released out nephridia pore (gametes also released from here, gonads are often reduced, gametes are derived from peritoneum)

• Larvae with protonephridia (excretory organ that filters waste and excess water)

Class Polychaeta 

• “Many setae” 

• 65% of annelid species 

• Marine 

• Parapodia - outfolding of body wall, increases surface area, used for gas exchange/locomotion 

• Parapodia are integrated with the skeleton, muscles attached 

• Internal chitinous rods (acicula), work with setae, prevent sliding 

• Pair of eyes, sensory projections on the head, elytra plates (protective plates) 

• Have a well-developed head, jaws, and muscular pharynx 

Polychaete Groups

• Two groups - errant and sedentary 

• Errant - move through sinusoidal waves 

• Sedentary - lost/reduced/modified parapodia 

Sedentary Polychaetes

• Live in burrows/tubes, secretions + substrates

• Parapodia for water movement - feeding, breathing, and waste disposal

• Some have feathery radioles (feathery, tentacle-like structures that are used for respiration and filter feeding)

• Larvae is used for dispersal

Reproduction - discrete sexes, gametes from peritoneum

• Epitoky - specialized, swimming form, epitokes bud off and the atoke is left behind

• Triggers are complex

Fertilization - usually external = trochophore larvae (free-swimming larvae)

• “Wheel bearer,” 3 rings of cilia

• Telotroch on pygidium (hind segment of body) - new segments bud off this

• Cilia and digestive tract - can survive in environment, allows dispersal, some polychaetes lack larvae (use eggs instead)

• Limited asexual reproduction, usually by fragmentation

Class Polychaeta - Family Siboglinidae

• 170 species, marine (deep ocean)

• Trophosome for symbionts - organ with beneficial bacteria

• Reduced segmentation, seen in opisthosoma only (posterior, used for digging and as an anchor)

• Anterior - beard of tentacles, used in gas exchange

• Chitinous tube homes

• Lack a digestive system, microvilli (cell projections) on tentacles

• Most live in areas with hydrogen sulfide and methane, used for chemosynthesis

• Trophosome has chemosynthetic bacteria, produce sugars for food

• No reliance on light

Reproduction - dioecious

• Trochophore larvae with digestive tract, used for dispersal

Class Echiura (Echiurans) 

• “Serpent-like” 

• Shall, marine, “sausage-like” 

• Extendable proboscis - ciliated, particles fall into the gutter and are then transported into the mouth 

• Complete digestive tract

Class Echiura - Genus Urechis

• Peristatic waves draw water into burrow

• Ecosystem engineers, creation of burrows

Class Clitellata

• “Pack saddle”

• 85% are oligochaetes (subclass)

• hermaphroditic

• Clitellum - secretes cocoons for embryos, mucus used for sperm transfer, albumen for embryos (food source)

• Permanent gonads

Subclass Oligochaeta (Oligochaetes)

• “Few setae”

• Freshwater/terrestrial

• Earthworms = ecosystem engineers

• No parapodia, few setae, reduced anterior sense organs, special respiratory structures

• Set body plan

• Segmented heart/circulatory system, use hemoglobin, segmented ganglia

• Unsegmented digestive tract - crop, gizzard, intestine (typhlosole)

Locomotion - peristaltic waves, sequential contraction, circular/longitudinal muscle

Circular - forward

Longitudinal - anchor

Reproduction - testes/ovaries, mutual sperm transfer, received in spermathecae (all assisted by clitellum)

Subclass Hirudinea

• Leeches

• Freshwater, some terrestrial

• Shared features with oligochaetes

• Lack setae and septa - movement is different, suckers are used for an “inchworm” motion

• Suckers adapted to attachment, reduced coelom - filled mesenchyme

• Feeding/sucking proboscis

Phylum Nematoda

• “Thread”

• Diverse, simple body plan

• Most abundant multi-cellular animals

• Found everywhere

• No circulatory system, respiratory, or excretory (use diffusion)

• Ecdysozoa: “molting animals” - undecided classification

Nematode Features

• Sensory amphids derived from cilia

Coverings and cavities: 

• No segmentation, tapered worms 

• Collagen forms outer cuticle, secreted by epidermis 

• Syncytial epidermis - has multiple nuclei 

• Crossways/trellis-like cuticle, inelastic fibers (rigid) 

• Ecdysis - molting 

• Molt cuticle, 4x over life 

• Each molt is unique

• Growth between molts, grow by increasing cell size (eutelli) 

Cavity - pseudocoelom 

• Mesoderm lines ectoderm, but not the endoderm 

• Pseudocoel - fluid performs circulation 

Nematode Features 2

Locomotion - no circular muscles, no waves 

• Pressure in pseudocoel is high, stiff cuticle - roundworm 

• No locomotor cilia 

• Sinusoidal waves by longitudinal muscles - inefficient, requires a thick medium 

Nervous - anterior nerve ring, nerve cords run down the body 

• Muscles connect to nervous cords, noncontractile segments 

• Ocelli 

• Anterior amphids, posterior phasmids = both lined with sensory hairs, chemoreceptors (detect pheromones) 

Digestive tract - tube 

• Pharynx is highly pressurized, pumps fluid/food into the digestive tract, anus is sealed 

Reproduction: 

• Dioecious, internal fertilization 

• Males have copulatory spicules, sperm enters the gonopore, sperm = amoeboid, crawls 

Phylum Arthropoda

• “Jointed foot”

• 85% of all animals

• Segmented, chitinous exoskeleton

• Loss of mobile cilia

• Segmentation - fusion to tagmata (distinct body segments), due to specialization

• 8 classes, over 80 orders, 2400 families

Arthropod Features

Exoskeleton - hard, external covering

• Protects organs, aids in locomotion

• Secreted by epidermis

• Waxy outer layer = epicuticle

• Inner layer = procuticle, chitinous

• Exoskeleton hardens by “tanning” - sclerotization

• Proteins are cross-linked

• Procuticle = thick/thin sections for movement, forms joint-pairs of antagonizing muscles

• Often includes resilin = spring-like protein, releases muscles

Hemocoel - used for circulation

• Coelom doesn’t aid movement, is highly reduced

Molting - ecdysis

• Growth occurs in spurts

• Biomass - continuous

• Old cuticle splits due to pressure, caused by fluid or gas

• New cuticle forms before the split, not yet hardened

Circulatory:

• Open system, hemolymph travels through vessels - hemocoel - heart with ostia

Vision:

• Ocelli and/or compound eyes

• Eye focuses light, nervous system monitors variation, processes image 

• Ommatidia, wide field of view 

Reproduction - sexual/dioecious 

• Some parthenogenesis 

• Primarily internal fertilization