BIOL 201: Invertebrate Zoology

Chapter 9: Intro to Bilateria

Rob SwatskiAsst. Prof. Biology, HACC-York

Bilateria

Most (99%) eumetazoans display bilateral symmetry

(lumped into Bilateria taxon)

Led to enhancement of neuromuscular

systems: cephalization!

Allowed colonization of more

physiologically challenging habitats

– LAND!

Also allowed growth of larger body sizes

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Bilateral Symmetry

Has only 1 plane of bisection: midsagittal

plane

Produces left & right mirror-image halves

Body polarized along 2 axes: anterior-

posterior axis & dorsal-ventral axis

Most likely evolved from organisms that

spent more time at air-water interface 3

Bilateria: Symmetry Planes & Body Axes

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Functional Origin of Bilateral

SymmetryCommon in animals

that move horizontally through

habitat

Useful because food & mates are often

randomly distributed

Radial symmetry is more common in

sessile filter-feeders

Radial symmetry works well: plankton

& particulates are more uniformly

distributed 5

Cephalization

Development of head

Anterior concentration of central nervous system & sensory

structures

Brain is usually 1 or more ganglia

Longitudinal nerve cords: often paired

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Cephalization

Allows motile animals to detect & pursue food,

mates, shelter, etc.

Longitudinal nerve cords: often paired

Giant axons: rapid conductance & low resistance – escape

response

In contrast, most sessilebilateria are not

cephalized & usually have radially symmetric

structures (crown of tentacles w/cilia)

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Musculature

Often have obliquely / cross-striated muscle for rapid contraction

Outer circular muscle usually encloses an inner longitudinal

muscle layer (esp. in worms)

Contraction of circular muscles produces

elongation

Due to hydrostatic skeleton

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Elongation

Shortening

Circular & Longitudinal Musculature10

Musculature, cont.

Longitudinal muscle contractions allow for: bending, undulating, shortening, retraction

Peristalsis: coordinated / alternating contractions of circular & longitudinal

muscles

Used for forward propulsion & burrowing

Other types of musculature:

dorsoventral (flattening) & helical (twisting)11

Peristaltic Burrowing

Longitudinal & circular muscles12

Dorsoventral musculature 13

Helical musculature14

Compart-mentalization

Important for more specialized physiological

regulation

Cnidarians 1st evolved a cavity for extracellular digestion – but their

GVC is not specialized

Bilaterian gut isspecialized for digestion

& absorption

Other systems evolved that increase physiological

compartmentalization: coeloms & hemal

systems 15

BilaterianGut

Many have a true gut (open-ended hollow tube) consisting of specialized regions

Mouth & foregut: ingestion of food,

enzyme secretion, & physical digestion

Midgut: chemical digestion (hydrolysis)

& absorption

Hindgut & anus: formation, storage, & elimination of wastes; water reclamation &

ion regulation 16

Gut Regions & Specializations

Foregut

Buccalcavity

Pharynx Esophagus

Midgut

Stomach Intestine Cecum

Hindgut

Rectum Cloaca

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Foregut

Buccal cavity

Receives food

May have teeth

Pharynx

Muscular tube

May be protrusible;

used in feeding, digging

Esophagus

Links foregut &

midgut

Muscular & may be

ciliated

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Midgut

Stomach

Enlarged cavity

Extracellular digestion

Intestine

Forms feces

Joins hindgut

Cecum

Outpocketsof stomach or intestine

More SA: intracellular digestion,

absorption, & storage

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Hindgut

Rectum

Enlargement before anus

Receives indigestible

wastes

Cloaca

A variation of the rectum in some animals

Also receives wastes from

excretory ducts

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4 Types of BilaterianCirculatory Systems

Gastrovascular Cavity

Gut lined with gastrodermis; gut

tube & ceca

Hemal

Connective tissue compartment lined with basal lamina;

tubular vessels and/or hemocoelic

sinuses

Coelomic

True body cavity lined with mesothelium; small or large cavities, vessel-

like canals

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Coelom

Fluid-filled cavity or canal: unique to Bilateria

Lined with epithelium derived from mesoderm

(mesothelium)

Filled with coelomicfluid

Fluid circulated by mesothelial cilia or

muscular contractions of body-wall 23

Functions of Coeloms

Houses organs

Hydrostatic skeleton

Circulation & internal transport

Excretion Reproduction

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Segmentation

Segments: bilateral pairs of fluid-filled

coelomic cavities along length of body

Compartments separated by mesenteries

(longitudinal) & septa(transverse)

Benefit: individual regulation of segments

for specialized functions

Plays role in locomotion

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HemalSystem

A simple circulatory system consisting of: blood (hemolymph),

vessels, & sinuses

Blood circulated via circular musculature

and/or 1 or more hearts

Hemal systems are usually absent in

small animals (acoelomates)

Hemocoel: large sinus acting as the main body cavity

(pseudocoelomates)27

Parallel Blood Circuit

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Series Blood Circuit

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Excretion

Larger animals have ciliated tubules called

nephridia (little kidneys)

Remove nitrogenous wastes (urine) &

osmoregulate

Wastes are flushed out with water through a

nephridiopore

Different types of nephridia: filtration, secretion, & storage

30Nephridium animation

Metanephridia

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Protonephridia

flame cell

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Cleavage Patterns

Radial Cleavage

Cleavage planes are parallel or

perpendicular to polar axis of zygote

In cnidarians, lophophorates, &

deuterostomes

Spiral Cleavage

Cleavage planes are oblique to polar axis of

zygote

In flatworms, molluscs,

segmented worms, etc.

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Spiral Cleavage

Gastrulation(Triploblastic)

Ectoderm

Epidermis Covers body, secretes cuticle &

exoskeleton, contains sensory,

nervous, & glandular cells

Mesoderm

MesotheliumLines coelom,

forms muscles,

mesenteries, gonads, & CT

Endoderm

Gastro-dermis

Lines midgut, contains ciliated,

secretory, absorptive, & storage

cells

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Coelom Formation

Enterocoely

Outfoldings of archenteron pinch off In deuterostomes

Schizocoely

Coelomiccavities form

early in development

In protostomes

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Division of Bilateria

Protostomes

Molluscs, annelids,

& arthropods

Spiral cleavage & schizocoely

Blastoporebecomes

mouth

Deuterostomes

Echinoderms & chordates

Radial cleavage & enterocoely

Blastoporebecomes anus & mouth forms

elsewhere

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