February 22, 2012

35 Notes: Mollusks and Annelids

Section 1 MolluscaSection 2 Annelida

Describe the key characteristics of mollusks.

Describe the body plan of mollusks.

Name the characteristics of three major classes of mollusks.

Compare the body plans of gastropods, bivalves, and cephalopods.

35-1 Objectives

Phylum Mollusca is a diverse group of invertebrates such as clams, snails, slugs, squids, and octopuses.They are called mollusks, from the Latin molluscus, which means “soft.” Many mollusks have soft bodies and some have a hard shell.Some mollusks are sedentary filter feeders, while others are fast-moving predators.Mollusks are coelomates.Most aquatic mollusks and annelids have a larval stage called a trochophore.

Characteristics of Mollusks

· Phylum Mollusca is a diverse group of invertebrates such as clams, snails, slugs, squids, and octopuses.· They are called mollusks, from the Latin molluscus, which means “soft.” Many mollusks have soft bodies and some have a hard shell.· Some mollusks are sedentary filter feeders, while others are fast-moving predators.· Mollusks are coelomates.· Most aquatic mollusks and annelids have a larval stage called a trochophore.

Characteristics of Mollusks

112,000 species

February 22, 2012

Body Plan of MollusksThe body of a mollusk is generally divided into:

1. The visceral mass, which contains the heart and the organs of digestion, excretion, and reproduction

2. the head-foot, which consists of:the head, which contains the mouth and sensory structuresthe foot, a large, muscular organ for locomotion

The coelom is limited to a space around the heart.

3. A layer of epidermis called the mantle covers the visceral mass.

Body plan of Mollusks continued

· In most mollusks, the mantle secretes one or more hard shells containing calcium carbonate. CaCO3· This disadvantage of a shell is offset by having gills.· The gills are protected within the mantle cavity.· Most mollusks are bilaterally symmetrical. · The nervous system consists of paired clusters of nerve cells called ganglia. · The main feeding adaptation of many mollusks is the radula.

Types of Mollusks

February 22, 2012

Class Gastropoda· The largest and most diverse class of mollusks is Gastropoda, whose members are called gastropods. · Most gastropods, including snails, abalones, and conchs, have a single shell. Others, such as slugs and nudibranchs, have no shell.· Gastropods undergo the process of torsion during larval development.· Gastropods have an open circulatory system in which a heart pumps hemolymph from gills or lungs into the hemocoel.

Torsion in Gastropods

40,000 species

Snails· Snails live in moist and aquatic environments.· Most snails locate food using eyes at the end of tentacles.· Snails survive dry periods by retreating into their shells and sealing the opening with a mucous plug.

Other Gastropods· Slugs are terrestrial and lack shells. · Nudibranchs are marine and lack shells. · Pteropods, or “sea butterflies,” have a foot that is modified into a winglike flap and used for swimming rather than crawling.

Class Gastropoda continued

February 22, 2012

Class Bivalvia

The class Bivalvia includes aquatic mollusks such as clams, oysters, and scallops.

They are called bivalves because their shell is divided into two valves connected by a hinge.

Each valve consists of three layers.

Most bivalves are sessile filter feeders.

Bivalves lack a distinct head and have three pairs of nerve ganglia.

10,000 species

Class Bivalvia ContinuedClams· Clams are bivalves that live buried in mud or sand.· The mantle cavity of a clam is sealed except for a pair of hollow, fleshy tubes called siphons.· Water enters through the incurrent siphon. · Water leaves through the excurrent siphon.

Other Bivalves· Oysters become attached to a hard surface early in their development. · Scallops can move through the water by repeatedly opening their valves and snapping them shut.

Anatomy of a Clam

The Clam GunClam digging video

Successful Clam digging is determined by the tides.

February 22, 2012

Class Cephalopoda· The class Cephalopoda includes octopuses, squids, cuttlefishes, and chambered nautiluses.· They are called cephalopods, which means “head-foot.” · They are specialized as free-swimming predators. · They have tentacles and beaklike jaws on the head. · Their nervous system is very advanced.· They have a closed circulatory system. · Many use pigments to hide and disguise themselves.

Class Cephalopoda Continued

Squids· Squids are cephalopods with ten tentacles.· They propels themselves by pumping jets of water with the mantle through an excurrent siphon.

Octopuses· Octopuses have eight tentacles and are similar to squids.· They often crawl along the ocean bottom or lie in wait for prey.

Chambered Nautiluses· The chambered nautilus is the only existing cephalopod that has retained its external shell.· The shell is coiled and divided into a series of chambers. The body is confined to the outermost chamber.

February 22, 2012

35-2 AnnelidaObjectives:

Identify the structures that provide the basis for dividing annelids into three classes.

List the advantages of body segmentation.

Describe the structural adaptations of earthworms.

Compare the three classes of annelids.

Characteristics of Annelids

· The phylum Annelida is made up of bilaterally symmetrical, segmented worms.· This phylum includes common earthworms, feather-duster worms, and bloodsucking leeches. · They are called annelids, which means “little rings” and refers to the many body segments.· Annelids have a true coelom that is divided into separate compartments by partitions.· Most annelids have external bristles called setae (singular, seta), and some have fleshy protrusions called parapodia (singular, parapodium).

Characteristics of Annelids

15,000 species

Class Oligochaeta

· Annelids of the class Oligochaeta generally live in the soil or in fresh water and have no parapodia.· Oligochaeta means “few bristles”; these annelids have a few setae on each segment. · The most familiar member is the earthworm.

Structure and Movement· An earthworm’s body has over 100 nearly-identical segments.· Circular and longitudinal muscles line the interior body wall.· Locomotion is made possible by segmentation.

Types of Annelids

February 22, 2012

Class Oligochaeta continued

Feeding and DigestionEarthworms ingest soil as they burrow through it.

Soil is moved through these structures:· mouth· pharynx· esophagus· crop· gizzard · intestine - includes the typhlosole· anus

Earthworms play an important role in the condition of soil.

Class Oligochaeta continued

Circulation· Contractions of the aortic arches and the dorsal blood vessel force blood through the closed circulatory system.

Respiration and Excretion· Oxygen and carbon dioxide diffuse through the skin, which contains many small blood vessels and must be kept moist.· Cellular wastes and excess water are excreted through nephridia.

Neural Control· The nervous system of an earthworm consists of a chain of ganglia connected by a ventral nerve cord. · Sensory structures are found in all segments but are concentrated at the anterior end.

Cutaneous

Class Oligochaeta Continued

Reproduction

· Earthworms are hermaphrodites, but an individual worm cannot fertilize its own eggs.· During mating, earthworms press their ventral surfaces together.· They are held together by their setae and by a film of mucus secreted by each worm’s clitellum. · The sperm from each worm move through the mucus to the seminal receptacle of the other. · The clitellum secretes a tube of mucus and chitin.· Fertilization occurs inside the tube, which forms a protective case for the young worms.

February 22, 2012

Anatomy of the Earthworm

Class Polychaeta and Hirudinea

Polychaeta· Most annelids are members of the class Polychaeta, which means “many bristles.” · Polychaetes differ from other annelids in that they have antennae and specialized mouthparts.· They are the only annelids that have a trochophore stage.

~ 9,900 species

Hirudinea· Hirudinea is the smallest class of annelids, consisting of about 500 species of leeches. · Leeches have no setae or parapodia. · Many leeches are carnivores but some are parasites that suck blood from other animals.

February 22, 2012

35 Review - Mollusks and Annelids

Section 35-1

1. Describe the characteristics of mollusks

2. Describe a true coelom and list organisms we have studied that have one. 3. Describe why the evolution of a coelom was significant

4. Describe the relationship between mollusks and a mantle

5. Describe the function of the cilia of a trochophore

6. Describe the function and occurrence of Trochophores

7. Describe the function of a characteristic structure found in many mollusks, the radula

8. List members of the following phyla: Platyhelminthes, Nematoda, Mollusca, Annelida

9. Describe the body cavity of all of the animal phyla that evolved after the mollusks

10. List all classes of the phylum Mollusca

11. List all mollusks that have a closed circulatory system

12. List organisms have mantles and be able to recognize a basic drawing of them

13. List an organism where jet propulsion is the usual means of locomotion in water

14. Recognize a drawing of a bivalve, cephalopod, trochophore, and gastropod

15. Recognize drawings of locomotive structures of Mollusks

February 22, 2012

16. Describe shells of mollusks

17. Describe the function of adductor muscles

18. Discuss the function of an open circulatory system

19. Name the twisting of the visceral mass of gastropods

20. Discuss respiration in the various species of gastropods

21. Discuss characteristics common to all of the cephalopods

22. Discuss the mechanism of jet propulsion in a squid

23. List the only cephalopod that has retained its external shell

Section 35-2

24. Name the phylum of Segmented worms

25. Discuss the structure of each segment of an annelid

26. List an example of segmentation in humans

27. List the most significant evolutionary advancement of annelids over mollusks

28. Discuss any similarity between annelids and arthropods

29. Recognize drawings of organisms having segmented body plans

30. Describe the relationship between a true coelom and mollusks and annelids

February 22, 2012

31. Describe how coordinated movements of an earthworm’s body segments are possible

32. Describe the relationship between earthworm movement and circular muscles and setae

33. Name the small tubules that collect wastes from the coelom of annelids and discharge the wastes from the body

34. List and describe the function of the three regions of the digestive tube of the earthworm

35. Discuss the advantage(s) of a closed circulatory system over an open circulatory system

36. List structures of earthworm respiration, circulation, digestion, and excretion

37. Discuss the function of the clitellum of an earthworm

38. Discuss the benefits to the environment Earthworms provide

39. Describe the relationship between squid motion and siphon

40. Discuss the structure of the body of the water leech

41. Discuss general characteristics of Leeches

42. Describe the relationship between leeches and blood