Chapter 27: Mollusks and Annelids

1. Chapter 27:Mollusks and Annelids Section 1: Mollusks

2. Mollusks • Phylum Mollusca • Evolved in the sea more than 600 million years ago • More than 100,000 mollusk species • Divided into 7 classes • Live everywhere • Range in size from snails as small as a grain of sand to giant squids that may grow more than 20 meters long • Wide range of forms and colors

3. Examples of Mollusks

4. What is a Mollusk? • Very different but all share similar developmental patterns • Their different forms are the results of variations on the same basic body plan • Mollusks are defined as soft-bodied animals that have an internal or external shell • Although some present day mollusks lack shells, they are thought to have evolved from shelled ancestors • Most mollusks have a special kind of larva called a trochophore • Swim in open water and feed on tiny floating plants • Also seen in segmented worms • Evolved from a common ancestor

5. Form and Function in Mollusks

6. Form and Function in Mollusks • The body plan of almost all mollusks consists of four basic parts • Foot • Usually contains the mouth and other structures associated with feeding • Many different shapes • Mantle • Thin, delicate tissue layer that covers most of the body • Shell • Made by glands in the mantle that secrete calcium carbonate • Visceral mass • Contains the internal organs

7. Form and Function in Mollusks • Basic body parts have taken on different forms as mollusks evolved adaptations to different habitats • Type of foot and shell mollusks have are used to group them into classes

8. Feeding • Every mode of feeding is seen in this phylum • Many mollusks feed with a tongue-shaped structure called a radula • Layer of flexible skin that carries hundreds of tiny teeth • Inside is a rod of cartilage • When the mollusk feeds, it places the tip of the radula on the food and pulls the skin back and forth over the cartilage

9. Feeding

10. Feeding • Although they may have a radula, carnivorous mollusks such as octopi and certain sea slugs typically use sharp jaws to eat their prey • Produce poisons • Mollusks such as clams, oysters, and scallops are filter feeders • Use feathery gills to sift food from the water

11. Respiration • Gills serve as organs of respiration as well as filters for food • Aquatic mollusks breathe by using gills located inside their mantle cavities • Land snails and slugs breathe by using a specially adapted mantle cavity that is lined with many blood vessels • The surface is constantly kept moist so that oxygen can enter the cells • Because the mantle loses water in dry air, most land snails and slugs must live in moist places • Prefer to move around at night, during rainstorms, and times when humidity is high

12. Respiration

13. Internal Transport • Oxygen that is taken in by the respiratory system and nutrients that are the products of digestion are carried by the blood to all parts of a mollusk’s body • The blood is pumped by a simple heart through an open circulatory system • Blood does not always travel inside blood vessels • Instead, blood works its way through body tissues in open spaces called sinuses • The flow of blood through sinuses is not efficient enough for fast-moving octopi and squids • Closed circulatory system • Blood always moves inside vessels

14. Excretion • Mollusks must eliminate waste products • Undigested food becomes solid waste that leaves through the anus in the form of feces • Cellular metabolism produces nitrogen-containing waste in the form of ammonia • Must be removed from body fluids • Simple tube-shaped organ called nephridia • Remove ammonia from the blood and release it to the outside

15. Response • Vary greatly in the complexity of their nervous systems • Clams and other two-shelled mollusks • Simple nervous system • Several small ganglia near mouth, a few nerve cords, simple sense organs • Octopi and other tentacled mollusks • Highly developed nervous systems • Well-developed brain • Complex sense organs • Can be trained to perform different tasks in order to obtain a reward or avoid punishment • Often studied by psychologists interested in the way animals learn

16. Reproduction • Sexes are separate and fertilization is external • Most mollusks release eggs and sperm into water • Find each other by chance • Free-swimming larvae develop • Tentacled mollusks fertilization takes place inside the body of the female • Some snails are hermaphrodites

17. Snails, Slugs, and Their Relatives • Class Gastropoda • Gastropods “stomach-foot” • Many have a one piece shell that protects their soft bodies • Some gastropods, such as slugs, have no shell • Protected by their behavior • Some also contain toxins or chemicals that are secreted to warn predators • Bright colors warn predators to stay away

18. Snails, Slugs, and Their Relatives

19. Two-Shelled Mollusks • Class Bivalvia • Shells with a hinge • Common bivalves include clams, oysters, and scallops • Although larvae are free-swimming, they soon settle down to the relatively quiet life on the bottom of a body of water • Most are sessile • Mantle glands make the shells • Mantle glands also keep the shells inside surfaces smooth and comfortable by secreting layers of mother-of-pearl • If a grain of sand or small pebble gets caught between the mantle and the shell it forms a pearl

20. Two-Shelled Mollusks

21. Tentacled Mollusks • Cephalopods – members of the class Cephalopoda – are among the most active and interesting mollusks • Includes octopi, squids, cuttlefish, and nautiluses • “head-foot” • Most have 8 flexible tentacles equipped with a number of round sucking disks that are used to grab their prey • Move by a siphon and jet propulsion • Can secrete large amounts of dark colored, foul tasting ink • Can change color to to match their surroundings • Most modern cephalopods have an internal shell or no shell at all • Contains gases that allow them to float

22. Tentacled Mollusks

23. How Mollusks Fit into the World • Many different roles in living systems • Important source of food • Environmental monitors • Biological research • Can cause harm to crops • Cause sickness on occasion

24. Chapter 27:Mollusks and Annelids Section 2: Annelids

25. Annelids • The soft-bodied earthworm is the most common terrestrial, or land-dwelling, segmented worm • There are approximately 9000 species of segmented worms that live in moist soil, in fresh water, and in the sea • Segmented worms, or annelids, live just about everywhere in the world

26. What Is an Annelid? • Phylum Annelida • An annelid is a round, wormlike animal that has a long, segmented body • Annelids range in size from tiny aquatic worms less than half a millimeter long to giant earthworms more than 3 meters long • Annelids also vary greatly in color, patterning, number of bristles, and other superficial features

27. Form and Function in Annelids • The many segments of an annelid’s body are separated by internal walls called septa • Most of the body segments are virtually identical to one another • However, some segments are modified to perform special functions • For example, the first few segments may carry one or more pairs of eyes, several pairs of antennae, and other sense organs

28. Feeding • The digestive tract extends from the mouth to the anus • Food enters through the mouth and travels through the gut, where it is digested • Like mollusks, annelids have evolved structures and behaviors that allow them to use a wide variety of foods • One feeding organ that has evolved many different forms in different groups of annelids is the pharynx, or the muscular front end of the digestive tube

29. Feeding • Many annelids can extend the pharynx through the mouth • In carnivorous annelids, this type of pharynx usually has two or more sharp jaws attached to it • When a suitable animal approaches, the worm lunges forward, rapidly extends the pharynx, and grabs the prey with its jaws • When the pharynx returns to its normal position, it carries the food back to the gut

30. Many polychaete annelids, such as the sandworm Nereis, use hook like jaws to capture prey or nibble on algae

31. The digestive system of an earthworm is shown here. The pharynx pumps a mixture of food and soil into a tube called the esophagus. The food then moves through the crop, where it can be stored, and through the gizzard, where it is ground into smaller pieces. The food is digested in the intestine. Undigested materials pass through the intestine and are eliminated through the anus.

32. Respiration • Aquatic annelids often breathe through gills • Terrestrial annelids take in oxygen and give off carbon dioxide through their skin • Because the skin must stay moist to make gas exchange possible, the worms die if the skin dries out • To help guard against this, terrestrial annelids, such as earthworms, secrete a thin protective coating called a cuticle to hold moisture around them

33. The spaghetti worm uses its long tentacles to pluck bits of detritus from the ocean floor In plume worms, a brush-shaped structure on the head is used in filter feeding and in respiration.

34. Internal Transport • Annelids typically have closed circulatory systems organized around two blood vessels that run the length of their bodies • In each body segment is a pair of smaller vessels called ring vessels that connect the two main blood vessels and supply blood to the internal organs • In annelids such as earthworms, several of the ring vessels near the anterior end of the worm are larger than the other ring vessels and have muscle tissue in their walls • These vessels are often called hearts because they contract rhythmically and help pump blood through the system

35. Excretion • Annelids produce two kinds of wastes • Solid wastes pass out through the anus at the end of the gut • Wastes resulting from cellular metabolism are eliminated by nephridia • A pair of nephridia in each body segment removes waste products from the body fluids and carries them to the outside

36. Response • Many annelids are active animals with well-developed nervous systems • The brain sits on top of the gut at the front end of the body • Two large nerves pass around the gut and connect the brain with a pair of ganglia below • From these ganglia, a ventral nerve cord runs the entire length of the worm • Nerves from each segment of the worm enter and leave the nerve cord at a pair of small ganglia • These nerves help carry messages from sense organs and coordinate the movements of muscles

37. Response • Sense organs are best developed in the free-living marine species of annelids • Have sensory tentacles, statocysts, chemical receptors, and two or more pairs of eyes • Many other annelids have much simpler sensory systems • Earthworms have no specialized sense organs • They rely on simple sensory cells in the skin

38. Sense organs are best developed in free-swimming annelids such as the paddleworm, which has a pair of beady eyes and a number of sensory tentacles on its head.

39. Movement • Annelids have two major groups of muscles in their body walls • Longitudinal muscles • Runs from the front of the worm to the rear • When contracted, they make the worm shorter • Circular muscles • Runs in circles around the body of the worm • When contracted, they make the worm skinnier

40. Reproduction • Most annelids reproduce sexually • In some annelids, the sexes are separate • However, annelids such as earthworms and leeches are hermaphrodites that undergo internal fertilization • Although an individual worm produces both sperm and eggs, it rarely fertilizes its own eggs

41. Reproduction • Instead, worms pair up, attach themselves to each other, and exchange sperm • Each worm stores the sperm it has received in special sacs • When eggs are ready for fertilization, a band of thickened, specialized segments called the clitellum secretes a mucus ring into which eggs and sperm are released • The ring then slips off the worm’s body and forms a cocoon that shelters the eggs

42. Sandworms, Bloodworms, and Their Relatives • Class Polychaeta • Common and important marine worms • Polychaetes are characterized by paired paddle like appendages on their body segments • These appendages are tipped with bristles • Polychaetes live in cracks and crevices in coral reefs, in sand, mud, and poles of rocks, and even out in the open water

43. Although they look very different from each other, both the fanworm and the fireworm are polychaetes. The fanworm is a filter feeder that retreats into its tube when threatened. The fireworm defends itself with poisonous bristles that break off and penetrate skin at the slightest touch. The pain caused by these bristles gives the fireworm its name.

44. Earthworms and Their Relatives • Class Oligochaeta • Contains earthworms and related species • Oligochaetes are annelid worms that live in soil and open water • Most oligochaetes live in soil or freshwater • Oligochaetes have fewer bristles than polychaetes

46. Leeches • Class Hirudinea • Contains the leeches, most of which live in tropical countries • Freshwater organisms that exist as external parasites, drinking blood and body fluids from their host • All leeches have powerful suckers at both ends of their bodies • These suckers are used to attach a leech to its host

47. Leeches • Leeches penetrate the skin of their host in one of two ways • Use a muscular proboscis • Tubular organ that they force into the tissue of their host • Use razor sharp jaws • Once the wound has been made, the leech uses its muscular pharynx to suck blood from the area

48. Leeches • Both types of leeches release a special secretion from their salivary glands to prevent the blood from clotting as they drink it • Some leeches also produce a substance that anesthetizes the wound – thus keeping the host from knowing it has been bitten • During feeding, a leech can swallow as much as 10 times its weight in blood • Can take up to 200 days to digest • A leech can live for a year before it must feed again

50. How Annelids Fit into the World • Important in many habitats • Aquatic species are food for many fish, crab, and lobster • Earthworms perform an essential task in conditioning soil • By constantly burrowing through the ground, they help aerate the soil • Without the efforts of these annelids, the structure and fertility of farm soils would degenerate quickly, lowering crop yields