Chapter 9 93
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HIGHER INVERTEBRATES
Chapter Outline
MOLLUSCS
Molluscan Body
Molluscan Shell
Bivalves
Bivalve Anatomy
Bivalve Adaptations to Different
Habitats
Reproduction in Bivalves
Cephalopods
Ecological Role of Molluscs
ANNELIDS: THE SEGMENTED WORMS
Polychaetes
Feeding and Digestion
Reproduction in Polychaetes
Symbiotic Relationships
ARTHROPODS: ANIMALS WITH
JOINTED APPENDAGES
Chelicerates
Horseshoe Crabs
Sea Spiders
Amphipods
Copepods
Barnacles
Ecological Roles of Arthropods
Arthropods as Food
Arthropods as Symbionts
SKIN
Echinoderm Structure
Sea Stars
Feeding in Sea Stars
Reproduction and Regeneration
Feeding in Sea Cucumbers
Defensive Behavior
Crinoids
Ecological Roles of Echinoderms
HEMICHORDATES
INVERTEBRATE CHORDATES
Tunicates
Ecological Roles of Hemichordates and
Invertebrate Chordates
KEY CONCEPTS
ARE YOU STILL WONDERING?
Chapter Objectives
• Describe the anatomy and physiology of molluscs, annelids, and nematodes.
• Recognize the ecological roles of molluscs and marine worms.
Key Terms
molluscs
head-foot
visceral mass
mantle
mantle cavity
radula
periostracum
conchiolin
prismatic layer
pheromone
bivalves
valve
umbo
adductor muscles
inhalant opening
exhalant opening
palps
siphons
setae
polychaetes
errant polychaetes
sedentary polychaetes
nonselective deposit
feeders
fecal casts (castings)
selective deposit feeders
epitoky
abdomen
telson
palps
mandibulates
mandibles
swimmerets
chelipeds
decapods
cryptid larva
arrowworms
grasping spines
endoskeleton
ossicles
pedicellariae
water-vascular system
madreporite
oral tentacles
Cuvierian tubules
eviscerate
crinoids
cirri
roe
holothurin
hemichordates
Chapter Summary
1. The phylum Mollusca includes chitons, tusk shells, gastropods, bivalves, and
cephalopods. The generalized molluscan body plan consists of two parts: a head-foot
and a visceral mass.
3. Echinoderms exhibit radial symmetry as adults, although their larvae exhibit
bilateral symmetry, suggesting that they evolved from bilateral ancestors.
4. Tunicates have bodies that are covered with a tunic composed of molecules similar
to cellulose.
5. Cephalochordates, also known as lancelets, are small animals that resemble eels.
96 Higher Invertebrates
Chapter Outline
I. Molluscs
A. Characterized by a soft body with a mantle that secretes the shell.
B. Marine, freshwater, and terrestrial habitats.
C. Body plan.
1. Head-foot.
D. Shell structure.
1. Periostracum: outer proteinaceous layer.
2. Prismatic layer: middle layer, majority of mass, composed of calcium carbonate and
protein.
3. Nacreous layer: inner layer; thin calcium carbonate sheets.
E. Chitons.
F. Scaphopods.
1. Tube-like shell open at both ends.
G. Gastropods; refer to Figure 9–7.
1. Most diverse group of molluscs; some members have secondarily lost their shell.
2. Univalve molluscs; coiled shell.
3. Operculum.
4. Nutrition.
a. Herbivores.
5. Naked gastropods (nudibranchs).
6. Reproduction and development.
a. Majority dioecious; some hermaphroditic species.
d. Impact bubble—queen conch.
H. Bivalves.
1. Bivalve anatomy.
a. Two shells hinged at the top.
2. Bivalve adaptations to different habitats.
a. Soft-bottom burrowers.
3. Reproduction in bivalves.
a. Dioecious.
I. Cephalopods.
1. Tentacles project from anterior portion of head.
a. Used to capture prey.
b. Used for defense.
c. Used for locomotion in some species.
2. Shell modifications.
3. Nautiloids.
4. Coleoids.
a. Bulky body, fins, and 10 appendages (eight short arms, two long tentacles).
b. Small internal shells; shell absent in octopus.
c. Squid.
J. Color and shape in cephalopods.
1. Chromatophores.
K. Feeding and nutrition.
1. Carnivorous.
2. Visual detection of prey—well-developed eye.
L. Reproduction.
1. Dioecious.
2. Some species have complex courting rituals.
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5. Most species lay fertilized eggs.
6. Some octopods incubate eggs until they hatch.
7. One reproductive event per generation.
M. Ecology.
1. Major source of food.
II. Annelida
A. Phylum Annelida.
B. Bodies divided into internal segments.
C. Hydrostatic skeleton.
D. Setae.
E. Class Polychaeta.
1. Entirely marine.
4. Feeding and digestion.
a. Predaceous errant polychaetes have chitinous jaws used to capture prey.
b. Tube worms: many are filter or suspension feeders.
5. Reproduction.
a. Asexual reproduction occurs in some polychaete species: budding and
fragmentation.
b. Fertilization is external; external embryonic development.
F. Sipunculids.
1. Phylum Sipuncula: peanut worms.
2. Shallow water; benthic; burrow in mud, sand, empty shells, coral, and rock crevices.
G. Class Echiura: spoonworms.
1. Benthic; burrow in substrate, or live in rock or coral crevice.
2. Mostly shallow water; some deep-water species.
H. Class Pogonophora: beardworms.
1. Benthic tube dwellers.
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3. Riftia, inhabits hydrothermal vent areas; endosymbiotic chemosynthetic bacteria.
III. Phylum Nematoda
A. Round worms; cylindrical body tapered at both ends.
B. Most numerous organism on Earth in terms of biomass.
IV. Ecological Roles of Marine Worms
A. Nutrient cycling.
B. Predator-prey relationships.
C. Symbiotic relationships.
V. Arthropods: Animals with Jointed Appendages
A. Phylum Arthropoda; crustaceans, insects, arachnids, etc.
E. Chelicerates: includes horseshoe crabs and sea spiders.
1. Cephalothorax.
2. Telson.
F. Mandibulates: includes crustaceans, barnacles, copepods, amphipods, krill, etc.
1. Three body regions: head, thorax, and abdomen.
2. Each body segment usually has a pair of appendages.
6. Decapods: crabs, lobsters, and true shrimp.
a. Five pairs of walking legs.
b. First pair modified for defense and feeding: chelipeds.
G. External embryonic development.
1. Zoea: larval crab, some larval shrimp.
2. Nauplius: larval form in shrimp, copepods, and barnacles.
3. Mantis shrimp.
a. Largely tropical species.
5. Amphipods.
a. Resemble shrimp and isopods.
b. Benthic, burrowing-tube building; some free-swimming.
c. Most are detritivores or scavengers, some herbivorous grazing on algae.
6. Copepods.
a. Microscopic planktonic organism.
7. Barnacles.
a. Sessile adults, part of fouling communities and commensal on whales, large fish,
and plants.
H. Ecological significance.
1. Important food source for fish, birds, marine mammals, and humans.
2. Symbionts for many organisms.
4. Fouling community constituent.
VI. Phylum Chaetognatha: Arrowworms
A. Common tropical planktonic organism.
VII. Echinoderms: Animals with Spiny Skins
A. Phylum Echinodermata.
B. Endoskeleton (internal) of calcium carbonate.
C. Ossicles.
H. Sea stars.
1. Aboral surface.
2. Mouth located on underside (oral surface); tube feet on this side of body.
3. Benthic, slow moving; five arms.
c. External fertilization and embryonic development, planktonic larval stage.
I. Class Ophiuroidea: basket starts, brittle stars, serpent stars.
1. Five arms, benthic, and widely distributed.
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3. Nocturnal feeders; avoid light.
4. Brittle stars will detach an arm to distract predators—arm continues to move and
distract predator from animal; regenerates missing arm.
J. Class Echinoidea: sea urchins, heart urchins, sand dollars.
1. Body enclosed in calcium carbonate endoskeleton (test).
2. Fused arms that form five ambulacral areas.
7. Irregular urchins are selective deposit feeders.
K. Class Holothuroidea: sea cucumbers.
1. Elongated body with thick body wall.
2. Respiratory tree.
7. Some species eviscerate; regenerates internal organs.
L. Class Crinoidea: sea lilies and feather stars.
1. Primitive echinoderms, well represented in the fossil record.
2. Sea lilies: stalked echinoderms living in deeper waters (excess of 100 m).
3. Feather stars are benthic and widely distributed.
M. Ecology.
1. Few echinoderm predators: sea otters, some molluscs, spider crabs.
2. Sea cucumbers are a delicacy in Asian cultures.
VIII. Hemichordates
A. Phylum Hemichordata.
B. Sessile benthic organisms that burrow into sediments.
IX. Chordates
A. Phylum Chordata; characterized by:
1. Dorsal notochord, pharyngeal gill slits, and post-anal tail at some point in life cycle.
2. Dorsal, hollow nerve tube.
B. Tunicates: sea squirts, salps, and larvaceans.
1. Sea squirts.
a. Incurrent and excurrent siphons bring water and food through body cavity.
2. Salps.
a. Free-swimming tunicate with excurrent and incurrent siphons on opposite
3. Larvaceans.
a. Free-swimming tunicates.
C. Cephalochordates: lancelets.
1. Long, slender, laterally compressed organisms that look like eels.
2. Benthic organisms, burrow in sediments.
D. Ecology.
1. Benthic deposit and suspension feeders.
2. Release nutrients from sediments through burrowing activities.
Suggestions on Presenting the Material
1. The key to getting students interested in this material is to help them establish a
2. Interesting predator deterrent behavior and/or reproductive behaviors and strategies
are great intellectual hooks to capture student interest. Ask students to work in
3. Another strategy is to assign groups of students a marine environment and have
them “design” an invertebrate that is specifically adapted to be successful in that
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Classroom Discussion Ideas
1. Explain the difference between higher and lower invertebrates.
2. Explain how an animal can be an invertebrate and a chordate at the same time.
Videos, Animations, and Websites
Videos
Attack of the Sea Slugs. (short documentary, 6 min)
Kings of Camouflage. (PBS, Nova episode)
A look at the cuttlefish and its adaptive capabilities.
The Shape of Life. (PBS, 2002)
A look at various types of life.
Cuttlefish—The Brainy Bunch. (Australia, Gisela Kaufmann, 52 min)
Websites
Arkive.
A collection of videos and images on 50 different marine invertebrates.
The Shape of Life.
Sea and Sky.
Links to information about marine invertebrates.
San Diego Natural History Museum.
The Marine Invertebrates department holds approximately five million specimens.
104 Higher Invertebrates
Suggested Answers to End of Chapter Questions
Multiple Choice
1. e. chitons
2. b. shipworm
Matching
1. c.
6. d.
Short Answer
1. What adaptations allow squids to be successful predators?
The squid’s large eyes, streamlined body, and highly maneuverable swimming
2. Name four commercially important crustaceans.
3. Explain how the radula is modified in gastropods for different types of feeding.
The radula of carnivorous molluscs is narrow and better adapted for boring holes
4. Describe how a sea star uses its water vascular system to move.
To move, a sea star pumps water into its water vascular system through the
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5. Explain how slow-moving animals such as sea cucumbers avoid predation.
Most types of slow-moving animals utilize chemical defenses to deter predators.
6. Describe how sea squirts feed.
Sea squirts filter-feed on plankton and particulate organic matter. Water is drawn
7. Why are arthropods such a successful group of animals?
The phylum Arthropoda is the largest phylum in terms of the number of species it
contains, and representatives are found in every type of habitat on Earth,
8. Distinguish between chelicerates and mandibulates.
Chelicerates are a group of primitive arthropods with a pair of pincher-like
9. How are regular and irregular echinoids particularly adapted to their individual
lifestyles?
Regular echinoids (the globose forms known as sea urchins) are epifaunal feeders,
106 Higher Invertebrates
10. Why do bivalves that burrow in soft sediments need siphons?
Siphons are tubular extensions of the mantle, and they allow a bivalve buried in
11. What important ecological contribution do burrowing organisms make to the
environment?
Burrowing organisms prevent surface sediments from compacting and also
12. Distinguish between selective and nonselective deposit feeders.
Nonselective deposit feeders ingest both organic and mineral particles and then
digest the organic material, especially the bacteria that grow on the surface of the
13. Why are nematodes important marine organisms?
14. Why were acorn worms at one time considered members of the phylum Chordata?
Acorn worms possess several chordate characteristics: pharyngeal gill slits; a
Thinking Critically
1. What is the advantage of being able to alter the sex ratio in populations of the slipper
limpet, Crepidula fornicata?
The ability to alter the sex ratio of a population is an adaptive trait because it
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2. On a field trip to the ocean you discover a gastropod that you have never seen before.
Because you are very interested in gastropod feeding, you want to know whether this
animal is an herbivore or a carnivore. How could you determine with a fair amount
of certainty which type of feeder this animal is?
If you find a gastropod and are uncertain of its feeding strategy, examine the shell.
3. What type of shell characteristics would you expect to observe in a gastropod that
spends most of its life burrowing through soft sediments?
A burrowing gastropod that spends a large portion of its life moving through soft
4. A sample of a deep-sea dredging contains an animal that is small, flat, and round. It
has a ventrally located mouth, a stomach, but no intestines. To which of the groups of
invertebrates introduced in this chapter does this animal probably belong? What
other characteristics would the animal need to have to confirm your identification?
Sea stars are radially symmetrical benthic animals with a ventral mouth and a
5. Some of the invertebrates covered in this chapter brood their eggs and others do not.
What are the advantages to each of these reproductive strategies?
Brooding eggs increases the chances that the eggs will survive to hatch. It does
Suggested InfoTrac® Articles
“Secrets of the Shell.” Ballarini, R. and A.H. Heuer. American Scientist, (2007).