CHAPTER OUTLINE
31.1 Evolutionary Trends Among Animals
Animals are extremely diverse, but in general they are heterotrophic, digest food internally,
typically have the power of movement, are multicellular, adults are typically diploid, and they
undergo sexual reproduction. Invertebrates lack an internal skeleton, or endoskeleton, of bone or
cartilage, they evolved first and far outnumber the vertebrates (animals with an endoskeleton).
Anatomical Data
We will focus on symmetry and embryonic development to classify animals.
Type of Symmetry
Animals can be asymmetrical, have no symmetry, radially symmetrical,
organized circularly, or bilaterally symmetrical, have definite right and left
halves.
Embryonic Development
Sponges have the cellular level of organization; they are multicellular, but lack
true tissues and are therefore in a separate group called the parazoans. True
tissues appear in eumatazoans as they undergo embryological development.
Some animals have two germ layers (ectoderm and endoderm), while some have
three (ectoderm, endoderm, and mesoderm). Animals with three germ layers are
either protostomes or deuterostomes, which differ in terms of three major
developmental events.
31.2 The Simplest Invertebrates
Sponges
Sponges are placed in phylum Porifera because their saclike bodies are perforated by
many pores, they are aquatic and largely marine. They vary greatly in size, shape, and
color and are multicellular. They are sessile filter feeders and can reproduce both
asexually and sexually. They are classified on the basis of their skeleton.
Comb Jellies and Cnidarians
Comb jellies and cnidarians have true tissues and as embryos have ectoderm and
endoderm germ layers. They are radially symmetrical as adults.
Comb Jellies
Comb jellies (phylum Ctenophora) are solitary, free-swimming marine
invertebrates found primarily in warm waters. They propel themselves with cilia
and are made of a transparent jellylife substance called mesoglea.
Cnidarians
Cnidarians (phylum Cnidaria) are tubular or bell-shaped animals that reside
mainly in shallow coastal waters. They have specialized stinging cells called
cnidocytes. The outer epidermis is separated from the inner tissue by mesoglea
and they have an internal gastrovascular cavity for digestion. They have two
body forms: a polyp and a medusa.
Cnidarian Diversity
Examples of cnidarians include the sea anemone, coral, hydrozoans, and true
jellyfishes.
Hydra
A hydra is a small, tubular polyp. It has two tissue layers: epidermis and
gastrodermis, separated by mesoglea. They have a nerve net that carries impulses
and begin digestion in the central cavity. They can reproduce either sexually or
asexually.
31.3 The Lophotrochozoans
The lophotrochozoa are bilaterally symmetrical at some stage of their development. As embryos,
they have three germ layers, and as adults, they have the organ level of organization.
Lophotrochozoans are protostomes and include the lophophorans and the trochozoans.
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Lophophorans are aquatic and have a feeding apparatus called the lophophore. The
trochophores either have a trochophore larva today, or an ancestor had one in the past.
Flatworms
Flatworms have an extremely flat body, an incomplete digestive tract, and no body
cavity. Some are free-living and others are parasitic, they lack respiratory and circulatory
systems.
Free-Living Flatworms
Planarians have an excretory and a nervous system. They capture food and digest
it in the gastrovascular cavity. They can reproduce both sexually and asexually.
Planarians typically cross-fertilize, even though they are hermaphrodites.
Parasitic Flatworms
The parasitic flatworms include the tapeworms and the flukes.
Tapeworms
Tapeworms are endoparasites of various vertebrates, including humans.
They attach to the intestinal wall of the host and feed.
Flukes
Flukes are endoparasites of various vertebrates. They are usually named
for the type of vertebrate organ they inhabit.
Rotifers
Rotifers are trochozoans related to the flatworms. Rotifers have a crown of cilia, known
as the corona, on their heads. Most live in freshwater, but some are marine and terrestrial.
Molluscs
The mollusks are the second most numerous group of animals and inhabit a variety of
environments. They have a true coelom, bilateral symmetry, three germ layers, the organ
level of organization, and a complete digestive tract.
The Unique Characteristics of Molluscs
All molluscs have a body composed of at least three parts: the visceral mass, the
foot, and the mantle. They also often have a radula, a tonguelike organ that is
used to obtain food.
Gastropods
Gastropods include nudibranchs, conchs, and snails. The foot is ventrally
flattened and the animal moves by muscle contractions that pass along it.
Cephalopods
Cephalopods include octopuses, squid, and nautiluses. The foot has evolved into
a funnel or siphon about the head. They move by jet propulsion of water.
Bivalves
Bivalves include clams, mussels, oysters, and scallops. Their shells have two
parts.
The Visceral Mass
The clam has an open circulatory system and a heart. It also has a
nervous system and a digestive system.
Annelids
Annelids are segmented, have a hydrostatic skeleton, and specialization of the digestive
tract. They have an extensive closed circulatory system.
Polychaetes
Marine annelids are the Polychaeta, which refers to the presence of many setae.
Setae are bristles that anchor the worm or help it move.
Oligochaetes
The oligochaetes, which include earthworms, have few setae per segment.
Segmentation
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Segmentation is evidenced by body rings, coelom divided by septa, setae
on most segments, ganglia and lateral nerves in each segment, nephridia
in most segments, and branch blood vessels in each segment.
Reproduction
Earthworms are hermaphroditic. When mating, two worms lie parallel to
each other facing in opposite directions.
Comparison with Clam Worm
The comparison of the marine clam worm with the terrestrial earthworm
highlights the manner in which earthworms are adapted to life on land.
Leeches
Leeches have no setae and each body ring has several transverse grooves.
Among their modifications are two suckers that attach themselves to open
wounds if they are fluid feeders.
31.4 The Ecdysozoa
The ecdysozoans and trochozoa are protostomes. The term ecdysis means molting, and both
roundworms and arthropods periodically shed their outer covering.
Roundworms
Roundworms are nonsegmented worms that are prevalent in almost any environment.
They possess a pseudocoelom, a body cavity incompletely lined by mesoderm
Ascaris
Ascaris are most commonly parasites of humans and pigs and females are very
reproductively prolific.
Other Roundworms
Trichinosis is a fairly serious infection caused by eating pork that is not fully
cooked that contains encysted larvae of the round worm Trichinella spiralis.
Elephantiasis is caused by a roundworm called the filarial worm, causing the
limbs of an infected human to swell to an enormous size due to the impediment
of fluid return. Pinworm and hookworm infections are more common in the
United States.
Arthropods
Arthropods are extremely diverse. They have jointed appendages and an exoskeleton
made of chitin. Their success is dependent on: a rigid, but jointed exoskeleton,
segmentation, a well-developed nervous system, a variety of respiratory organs and
reduced competition through metamorphosis.
Crustaceans
Crustaceans are a group of largely marine arthropods that include barnacles,
shrimps, lobsters, and crabs. They are named for their hard shells.
Internal Organs
The digestive system includes a stomach. The coelom is reduced to a
space around the reproductive system. They have an open circulatory
system. In crayfish, the nervous system is very similar to that of an
earthworm.
Insects
Insects are very numerous and diverse. Insects have a body that is divided into a
head, a thorax, and an abdomen. The head bears sensory antennae, a pair of
compound eyes, and several simple eyes. The mouthparts are adapted to the way
of life. Wings may be present.
Internal Organs
The digestive system of a grasshopper is suitable for an herbivorous diet;
there is an excretory system, a respiratory system, and a circulatory
system.
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Reproduction and Development
Grasshopper reproduction is adapted to life on land. Metamorphosis is a
change in form and physiology that occurs as an immature stage, called a
larva, becomes an adult.
Comparison with Crayfish
The grasshopper and crayfish share a common ancestor, but they have
diverged in their morphology to adapt to aquatic versus terrestrial
environments.
Arachnids
The arachnids include scorpions, spiders, ticks, and mites. They have six pairs
of appendages.
31.5 Invertebrate Deuterostomes
Chordates are most closely related to the echinoderms as witnessed by their similar
embryological development.
Characteristics of Echinoderms
Echinoderms are a diverse group of marine animals. They have an endoskeleton and are
often radially symmetrical.
Echinoderm Diversity
Echinoderms are quite diverse and include sea lilies, motile feather stars, brittle
stars, and sea cucumbers.
Sea Stars
Sea stars have a five-rayed body with an oral and an aboral side. Locomotion
depends on their water vascular system. They do not have a respiratory,
excretory, or circulatory system. Sea stars reproduce both asexually and sexually.
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