Mollusks, Echinoderms,
and Arthropods
Biology 11
Phylum Mollusca pgs 428-432
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Examples: snails, clams, oysters, squid
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Habitat: mostly aquatic and terrestrial
7 classes: Gastropods, Pelecypod, Cephalopods,
Polyplacophora, Scaphopod, Aplacophora,
Monoplacophora
Body plan Development
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Bilateral Symmetry
Three germ layers
True coelom
Protosomes pg 428
Basic body plan includes:
Foot – for locomotion
 Mantle – hanging tissue
 Shell – internal or external, calcium carbonate
 Gills – respiratory structures (aquatic)
 Visceral mass – internal organs
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Mollusks Systems
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Sexual Reproduction – Hermaphroditic with cross
fertilization, or separate sexes.
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Circulatory System - Closed Circulatory System
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Digestion – Radula (a rough tongue) and filter feeding.
A complete digestive tract.
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Nervous System – Primitive brain and light receptors or
eyes.
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Respiration – Gills are covered by the mantle.
Phylum Echinodermata pgs 432-434
Echinos =spiny or bristly
 Derm= skin
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Examples: Starfish, sea urchin, sand dollar, sea
cucumber
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Habitat – aquatic actually means fresh water so
they actually live in MARINE environments.
Water Vascular system
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The major unifying characteristic of
echinoderms is their water vascular system.
Locomotion and feeding are accomplished by
this system, which operates much like a hydralic
pump.
Echinoderm
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Echinoderms are the closest relatives to
Chordates. It may not seem so at first, but
looking closely at embryological evidence, it is
believed that chordates and echinoderms shared
a common relative. This relative is believed to
have been something along the lines of sea
urchin - an orgasnism a bit larger than a golf
ball with spines extending radially. Interesting
little creatures, they aren't especially intelligent.
In fact, they don't do much of anything.
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Keep in mind that "close" is a relative term.
New York and Sydney are close compared to the
distance between the earth and Jupiter. It is all a
matter of context. In evolution, the context is a
matter of many millions of years.
Body plan Development
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Bilateral Symmetry in larvae,
Radial Symmetry in adults (Secondarily derived
pentaradial symmetry. What does that mean? It means
that echinoderms tend to be pentaradially symmetric.
The secondarily derived portion of that means that
they switch from being bilaterally symmetric to radially
symmetric as they mature. This is of no consequence to
taxonomists, they use the symmetry at the larval stage.
Three germ layers
True coelom
Deuterosomes pg. 432
Echinoderm Systems
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Water vascular system – unifying systems between all echinoderms. Allows for
some locomotion.
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Circulatory System – open circulatory system
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Digestion – one way digestive tube. Filter feeding and ingestion through the
mouth.
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Nervous System – oral and radial nerve cords but NO Brain.
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Respiration – diffusion
Reproduction- Echinoderms are monoecious, that means that the sexes are
separate. Males are males, females are females. There is no ambiguity.
However, with echinoderms, the sexes are not readily apparent using the
unaided eye.
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Phylum Arthropoda Pgs 435-441
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Arthropoda = jointed appendages
Examples: insects, crayfish, spiders
Habitat: ubiquitous (almost everywhere!)
Largest animal phyla
4 subphyla:
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Trilobitomorpha, Chelicerata, Uniramia, Crustacea,
Body Plan development
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Bilateral Symmetry
Three germ layers
True coelom
Various stages of metamorphosis
Segemented bodies, all include a head, thorax,
abdomen and a chitin exoskeleton.
Developed nervous systems, compound eyes or
photoreceptors etc.
Arthropod Systems
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Sexual reproduction – separate sexes
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Circulatory System – Open circulatory system
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Digestion – one way digestive tube, consisting or a
foregut, midgut and hind gut. Pg 438
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Nervous System – true brain with two ventral cords
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Respiration – tracheal system and gills
Application Questions
(to be completed for homework)
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Read Pgs 428-434 and do practice questions on
Pg 434, Questions #2, 3, 4, 7, 8
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Read Pgs 435-442 and do practice questions on
Pg 440, Questions #1, 2, 3, 4 and Pg 442
Question #5