Figure 11.9
The Portuguese man-o-war can deliver debilitating stings with its tentacles.
Coral reefs are built from colonial cnidarians called corals (Figure below). The corals are sessile polyps that can extend their tentacles to feed on ocean creatures that pass by. Their skeletons are made up of calcium carbonate, which is also known as limestone. Over long periods of time, their skeletons can accumulate to produce massive structures known as coral reefs. Coral reefs are important habitats for diverse types of ocean life.
Figure 11.10
Corals are colonial cnidarians.
Lesson Summary
Sponges are sessile filter feeders without true tissues.
The cnidarians, such as jellyfish, are radially symmetrical with true tissues.
Colonial cnidarians include the Portuguese man-of-war and corals.
Review Questions
What is the only animal to lack true tissues?
In what phylum are the sponges?
How do sponges gain nutrition?
Cnidarians are radially symmetrical. What does this mean?
What are some examples of cnidarians?
How do cnidarians sting their prey?
Describe the nervous system of the cnidarians.
How is a jellyfish different from a Portuguese man-o-war?
How are coral reefs built?
Where are most cnidarians found?
Further Reading / Supplemental Links
http://www.ucmp.berkeley.edu/porifera/porifera.html
http://animaldiversity.ummz.umich.edu
http://www.pbs.org/kcet/shapeoflife/animals/cnidaria.html
http://www.ucmp.berkeley.edu/cnidaria/cnidaria.html http://tolweb.org/tree?group=Cnidaria&contgroup=Animals http://www.ucmp.berkeley.edu/cnidaria/cnidaria.html
http://animaldiversity.ummz.umich.edu/site/accounts/information/Porifera.html
http://en.wikipedia.org/wiki/Cnidaria
Vocabulary
corals
Cnidarians that live on ocean reefs in colonies.
cnidarians
Invertebrates that have radial symmetry and include the jellyfish.
filter feeders
An organism that feeds by filtering organic matter out of water.
gastrovascular cavity
A large cavity having both digestive and circulatory functions.
medusa
Cnidarian with a bell-shaped body directed downward.
nematocysts
Specialized cells in cnidarians that can release a small thread-like structure and toxins to capture prey.
porifera
Filter-feeders with sac-like bodies; known as the sponges.
polyp
Cnidarian with a cup-shaped body directed upward.
sessile
Permanently attached and not freely moving.
Points to Consider
How do you think that worms are different from sponges and cnidarians?
How do you think that worms might be similar to sponges and cnidarians?
Lesson 11.3: Worms
Lesson Objectives
Describe the major features of the flatworms.
Describe the major features of the roundworms.
Describe the major features of the segmented worms.
Check Your Understanding
In terms of body structure, what does segmentation refer to?
What is a body cavity?
Introduction
Calling an animal a worm is an informal, non-scientific classification for animals that have long bodies with no appendages. Worms are bilaterally symmetrical, meaning that the right side of their bodies is a mirror of the left. Worms live in a variety of environments, including in the ocean, in fresh water, on land, and as parasites of plants and animals.
In this chapter we will discuss three types of worms: the flatworms, the roundworms, and the segmented worms. These worms are distinguished from each other by their body plan. The flatworms have flat ribbon-like bodies with no body cavity. The roundworms have a body cavity but no segments. The segmented worms have both a body cavity and segmented bodies.
Flatworms
Worms in the phylum Platyhelminthes are called flatworms because they have flattened bodies. Some species of flatworms are free-living organisms that feed on small organisms and decaying matter. These types of flatworms include marine flatworms and fresh-water flatworms such as Dugesia (Figures below and below). Other types of flatworms are parasitic and rely on a host organism for energy. For example, tapeworms have a modified head region with tiny hooks that help the worm attach to the intestines of an animal host (Figures below and below).
Figure 11.11
is a type of flatworm with a head region and eyespots.
Figure 11.12
Marine flatworms can be brightly colored.
Figure 11.13
Tapeworms are parasitic flatworms that live in the intestines of their hosts.
Figure 11.14
Tapeworms attach to the intestinal wall with a head region that has hooks and suckers.
Flatworms have no true body cavity and an incomplete digestive system, meaning that the digestive tract has only one opening. Flatworms do not have a respiratory system, so gas exchange occurs at surface of their bodies. Furthermore, there are no blood vessels or true circulatory system in the flatworms. Their gastrovascular cavity serves for both digestion and to distribute nutrients. The flatworms do have a ladder-like nervous system with a distinct head region with a concentration of nerve cells and sensory organs such as eyespots (Figure below). The development of a head region, called cephalization, arose with the development of bilateral symmetry in animals.
Roundworms
The phylum Nematoda includes non-segmented worms known as nematodes or roundworms (Figure below). Unlike the flatworms, the roundworms have a body cavity with internal organs. A roundworm’s complete digestive tract, meaning the digestive tract includes both a mouth and an anus, includes a large digestive organ known as the gut. Roundworms also have a simple nervous system with a primitive brain. Both their anterior and posterior ends have specialized sensory nerves. These nerves are connected with ventral and dorsal nerve cords that run the length of the body.
Figure 11.15
Nematodes can be parasites of plants and animals.
Roundworms can be free-living organisms, but they are probably best known for their role as significant plant and animal parasites. The heartworms, which cause serious disease in dogs while living in the heart and blood vessels, are a type of roundworm. Round worms can also cause disease in humans. Elephantiasis, a disease characterized by the extreme swelling of the limbs, is caused by infection with a type of roundworm (Figure below).
Figure 11.16
One roundworm parasite causes elephantiasis, a disease characterized by swelling of the limbs.
Segmented Worms
The phylum Annelida includes the segmented worms such as the common earthworm, some marine worms, and leeches (Figures below and below). These worms are known as the segmented worms because their bodies are segmented, or separated into repeating units. Most segmented worms feed on dead organic matter, while leeches can live in freshwater and suck blood from host organisms. Leeches can also be used medicinally to remove excess blood.
Figure 11.17
Earthworms are segmented worms.
Figure 11.18
Leeches are parasitic segmented worms.
Segmented worms have a well-developed body cavity filled with fluid, which serves as a hydroskeleton, a supportive structure that aids in muscle contraction. Segmented worms also tend to have organ systems that are more developed than the roundworms or flatworms. Earthworms, for example, have a complete digestive tract including an esophagus and intestines. The circulatory system consists of paired hearts and blood vessels, while the nervous system consists of the brain and a ventral nerve cord.
Type of Worm Body Cavity Segmented Digestiv
e System Example
Flatworm No No Incomplete Tapeworm
Roundworm Yes No Complete Heartworm
Segmented Yes Yes Complete Earthworm
(Source: Jessica Harwood, License: CC-BY-SA)
Lesson Summary
The flatworms have no true body cavity and include free-living Dugesia and parasitic tapeworms.
The roundworms, which can also be parasitic or free-living, are non-segmented worms with a complete digestive tract and a primitive brain.
The segmented worms include the common earthworm and leeches.
Review Questions
Are all worms classified into a single phylum?
Describe the respiratory system of the flatworms.
What is cephalization?
Name a parasitic flatworm.
How does the body plan of the roundworms differ from that of the flatworms?
Describe the digestive system of roundworms.
What features distinguish Phylum Annelida from the other worms?
Describe the skeletal system of a segmented worm.
Name a parasitic segmented worm.
Earthworms are in what phylum?
Further Reading / Supplemental Links
http://animaldiversity.ummz.umich.edu/site/accounts/information/Annelida.html
http://animaldiversity.ummz.umich.edu/site/accounts/information/Nematoda.html
http://animaldiversity.ummz.umich.edu/site/accounts/information/Platyhelminthes.html
http://www.ucmp.berkeley.edu/platyhelminthes/platyhelminthes.html
http://www.ucmp.berkeley.edu/phyla/ecdysozoa/nematoda.html
http://www.ucmp.berkeley.edu/annelida/annelida.html
http://animaldiversity.ummz.umich.edu
http://en.wikipedia.org/wiki/Annelida
Vocabulary
annelida
Invertebrate worms that have segmented bodies, such as earthworms.
cephalization
Having a head region with a concentration of sensory organs and central nervous system.
complete digestive tract
A digestive tract with two openings, a mouth and anus.
gastrovascular cavity
A large cavity having both digestive and circulatory functions.
hydroskeleton
Fluid-filled body cavity that provides support for muscle contraction.
incomplete digestive system
A digestive tract with only one opening.
nematoda
Invertebrate worms that include the roundworms.
platyhelminthes
Invertebrate worms that include the flatworms and tapeworms.
segmentation
A body plan that has repeated units or segments.
tapeworms
Intestinal parasites in the phylum Platyhelminthes.
Points to Consider
How might the vertebrates be different from the invertebrates?
Can you think of some examples of animals with a backbone?
Lab
Survey of Some Invertebrates
In this lab you will observe some examples of the invertebrates, those animals that do not have a backbone. The hydras are in the phylum Cnidaria. The Dugesia are in the phylum Platyhelminthes, the flatworms. The earthworm is in the phylum Annelida.
Materials:
compound and dissecting microscopes
slides and cover slips
pipettes
watch glass
culture of living hydra
Dugesia
construction paper
preserved earthworms
dissection kits
Procedure:
Hydra With a pipette, pull up some of the material from the bottom of the culture dish. Then squeeze a couple of drops onto a clean slide and cover with a cover slip. Observe your hydra under the microscope and sketch one below.
Dugesia With a pipette, place a couple Dugesia on a clean watch glass. Observe under the dissecting microscope. Sketch below, labeling the eyespots, auricles, and gastrovascular cavity.
With a dark piece of paper, cover half the watch glass. Do the Dugesia seem to prefer the shade or the light? Movement in response to light is called phytotaxis.
Earthworm Find the clitellum. What is its function?
Touch the ventral side of the worm to feel the setae. What are their function?
Lay the worm on the dissecting tray with the dorsal side up. Using the forceps and the scissors, carefully cut open the worm along a straight line from the clitellum to the mouth. Make sure to just cut the skin so you do not damage the internal organs. Sketch your worm below and label the following: aortic arches, crop, gizzard, pharynx, dorsal blood vessel, intestine, ventral nerve cord, and seminal vesicles.
Chapter 12: Other Invertebrates
Lesson 12.1: Mollusks
Lesson Objectives
Discuss what characteristics define mollusks.
Describe the different types of mollusks.
Explain why mollusks are important.
Check Your Understanding
What is an invertebrate?
How are animals classified?
Introduction
Perhaps the best example of a wide variety of attainable mollusks is along a walk on the beach (Figure below). There you can find the calcified shells of many different types of mollusks, most typically clams, mussels, scallops, oysters, and snails. Another reminder of the treasures that mollusks yield up may be as close as a jewelry collection (Figure below). There glossy pearls, mother of pearl (Figure below), and abalone shells reveal some of the unique features of mollusks (Figure below).
Figure 12.1
The beach yields a wide variety of mollusks.
Figure 12.2
Pearls being removed from oysters.
Figure 12.3
The inside of a bivalve, one of the mollusk classes described in Types of Mollusks, showing mother of pearl.
Figure 12.4
Shells of marine mollusks, including abalone.
As you learn about the different types of mollusks and their characteristics, consider how these features help the mollusks adapt to their living conditions. Then also admire their features and see how people’s ingenuity has used the mollusk’s design and beauty for practical and decorative purposes.
What are Mollusks?
Mollusks belong to the phylum Mollusca. The mollusk body is often divided into a head with eyes or tentacles, a muscular foot, and a mass housing the organs. In most species, the muscular foot is used for locomotion. Mollusks also have a mantle, a fold of the outer skin lining the shell, which in most mollusks secretes a calcium carbonate external shell, just like the ones you find on the beach.
The majority of marine mollusks have a gill or gills to absorb oxygen from the water. All species have a complete digestive tract that begins at the mouth and runs to the anus. Many have a feeding structure, the radula, found only in mollusks. The radula is composed mostly of chitin, a tough, semitransparent substance that is the main component of the shells of crustaceans and the outer coverings of insects. Radulae range from structures used to scrape algae off rocks to the beaks of squid and octopuses.
Larval development suggests a close relationship between the mollusks and other groups, notably the annelids, any of various worms or worm-like animals, including the earthworm and leech, characterized by a cylindrical, elongated, and segmented body. Unlike the annelids, however, mollusks lack body segmentation and their body shape is usually quite different, as well.
The giant squid (Figure below), which until recently had not been observed alive in its adult form, is one of the largest invertebrates. However, the colossal squid is even larger and can grow up to 46 ft. (14 m) long. The smallest mollusks are snails that are microscopic in size.
Figure 12.5
The colossal squid, one of the largest invertebrates, here measuring 30 ft (9 m) in length.
Types of Mollusks
Within the phylum Mollusca
, there are approximately 160,000 living species and an estimated 70,000 extinct species. Mollusks are typically divided into ten classes, of which two are extinct. Which classes are you most familiar with?
Living Molluscan Classes Molluscan Class Number of Species Habitat Features of Class/Examples
Caudofoveata 70 Deep ocean Worm-like organisms
Aplacophora 250 Deep ocean Worm-like organisms
Polyplacophora 600 Rocky marine shorelines Chitons (Figure below)
Monoplacophora 11 Deep ocean Limpet-like organisms
Gastropoda 150,000 (80% of living molluscan diversity) Marine (some limpets live in deep ocean around hot hydrothermal vents), freshwater, and terrestrial Abalone, limpets, conch, nudibranchs, sea hares, sea butterfly, snails, and slugs (Figure below.)
Cephalopoda 786 Marine Most neurologically advanced of all invertebrates; include squid, octopus, cuttlefish, and nautilus (Figure below).
CK-12 Life Science Page 26
