Biology Chapter 27 Mollusks and Annelids

1
Biology - Chapter 27Mollusks and Annelids

• Charles Page High School
• Stephen L. Cotton

2
Section 27-1Mollusks

• OBJECTIVES
• Explain how mollusks perform their essential life
  functions.

3
Section 27-1Mollusks

• OBJECTIVES
• Describe and give examples of the three major
  classes of mollusks.

4
Section 27-1Mollusks

• OBJECTIVES
• Discuss how mollusks affect humans and other
  living things.

5
Section 27-1Mollusks

• Phylum Mollusca- soft-bodied with an internal or
  external shell
• evolved in the sea over 600 million years ago
• have had a long and successful adaptive radiation
• more than 100,000 species today

6
Section 27-1Mollusks

• live everywhere, from deep ocean trenches to tree
  tops
• range in size from snails as small as grains of
  sand to giant squids more than 20 meters
• differently looking but grouped together because
  of similar development patterns

7
Section 27-1Mollusks

• Most have a special kind of larvae called a
  trochophore- swim in open water, and feed on tiny
  floating plants
• Figure 27-2, page 586
• trochophore larvae also present in the annelids,
  which are also in this chapter

8
Section 27-1Mollusks

• Mollusks are soft-bodies animals with either an
  internal or external shell
• name comes from the Latin word molluscus, meaning
  soft
• some today lack shells (like slugs), but thought
  to have evolved from shelled ancestors

9
Section 27-1Mollusks

• Body plan consists of four basic parts (Figure
  27-3, page 586)
• 1. Foot
• 2. Mantle
• 3. Shell
• 4. Visceral mass

10
Section 27-1Mollusks

• 1. Foot- the soft muscular foot usually contains
  the mouth and other structures associated with
  feeding
• can take many different shapes- flat for
  crawling spade-shaped for burrowing tentacles
  for capturing prey

11
Section 27-1Mollusks

• 2. Mantle- a thin, delicate tissue layer that
  covers most of a mollusks body- much like a
  cloak
• 3. Shell- found in almost all mollusks made by
  glands in the mantle that secrete calcium
  carbonate (CaCO3)

12
Section 27-1Mollusks

• 4. Visceral mass- located just below the mantle
  in most mollusks- this area contains the internal
  organs
• These basic body parts have taken on different
  forms as mollusks adapted to different habitats

13
Section 27-1Mollusks

• The type of foot, and the kind of shell that
  mollusks have are used to group them into the
  classes we will study
• Feeding many types of feeding have developed,
  and they feed on many different kinds of food

14
Section 27-1Mollusks

• Most mollusks are herbivores, carnivores, or
  filter feeders- but a few are detritus feeders
  and others are parasites
• some, such as the snails and slugs, feed with a
  tongue-shaped structure called a radula, which is
  rough like sandpaper
• Figure 27-4, page 587

15
Section 27-1Mollusks

• Mollusk herbivores use the radula to scrape algae
  off rocks in the water, or to eat the buds,
  roots, and flowers of plants
• Mollusk carnivores use the radula to drill
  through the shells of other animals, then swallow
  the soft tissues inside

16
Section 27-1Mollusks

• In a carnivorous snail called cone shells, the
  tiny rasping teeth of the radula have evolved
  into long, hollow darts that are attached to a
  poison gland- can stab prey such as small fish
• octopus and sea slugs typically use sharp jaws to
  eat their prey

17
Section 27-1Mollusks

• Mollusks such as clams, oysters, and scallops are
  filter feeders, using their feathery gills to
  sift food from the water
• cilia on the gills move the mixture of mucus and
  food into the mouth

18
Section 27-1Mollusks

• Respiration gills serve as the organs of
  respiration, as well as filters for food
• most only use gills for breathing
• aquatic mollusks include snails, clams, octopus-
  use gills inside the mantle cavity

19
Section 27-1Mollusks

• Land mollusks such as land snails and slugs
  breathe by using a specially adapted mantle
  cavity that is lined with many blood vessels
• this surface is kept moist so that oxygen can
  enter the cells
• snails and slugs live in moist areas for this
  reason

20
Section 27-1Mollusks

• Internal Transport oxygen and nutrients are
  transported by the blood to all parts of the body
• pumped by a simple heart through an open
  circulatory system
• the blood is not always in blood vessels, but in
  sinuses

21
Section 27-1Mollusks

• Open circulatory systems work well for
  slow-moving or sessile mollusks like snails or
  clams
• but faster-moving mollusks such as the octopi and
  squids have a closed circulatory system, in which
  blood always moves inside blood vessels

22
Section 27-1Mollusks

• Excretion like other animals, mollusks must
  eliminate wastes
• undigested food becomes solid waste that leaves
  through the anus in the form of feces
• cellular metabolism produces nitrogen-containing
  waste as ammonia

23
Section 27-1Mollusks

• Ammonia is quite poisonous, thus must be removed
  from body fluids
• this is done by tube-shaped organs called
  nephridia
• the nephridia remove ammonia from the blood, and
  release it to the outside

24
Section 27-1Mollusks

• Response mollusks vary greatly in complexities
  of their nervous systems, and ability to respond
  to environmental conditions
• clams and other two-shelled mollusks are
  basically inactive, burrowing in mud or sand
• have simple nervous systems

25
Section 27-1Mollusks

• These inactive forms may have several small
  ganglia near the mouth
• a few nerve cords
• simple sense organs such as chemical and touch
  receptors statocysts (for balance) and ocelli
  (eyespots)

26
Section 27-1Mollusks

• Octopi and other tentacled mollusks are active
  and intelligent predators
• have the most highly developed nervous systems of
  all members of their phylum
• may be more intelligent than even some
  vertebrates!

27
Section 27-1Mollusks

• They have numerous sense organs that help them
  distinguish shapes by sight, and texture by touch
• Octopi can be trained to perform different tasks
  in order to obtain a reward, or avoid punishment

28
Section 27-1Mollusks

• Reproduction in most mollusks, the sexes are
  separate, and fertilization is external
• in snails and almost all two-shelled mollusks,
  they release eggs and sperm into open water in
  enormous numbers

29
Section 27-1Mollusks

• In tentacled mollusks and certain snails,
  fertilization takes place inside the body of the
  female
• some other hermaphroditic mollusks, such as
  certain oysters, switch from one sex to the
  other- sometimes producing eggs, and sometimes
  sperm

30
Section 27-1Mollusks

• 1. Class Gastropoda- are called gastropods, the
  name meaning stomach foot
• appropriate name because they move by means of a
  broad, muscular foot located on the ventral
  (stomach) side

31
Section 27-1Mollusks

• Gastropods include familiar pond snails and land
  slugs, as well as more exotic mollusks such as
  abalones, sea butterflies, sea hares, and
  nudibranchs
• Figure 27-8, page 590

32
Section 27-1Mollusks

• Many gastropods have a one-piece shell that
  protects their soft bodies
• when threatened, many snails can pull up
  completely into their coiled shells
• some have small shells, or like slugs- lack
  shells completely

33
Section 27-1Mollusks

• Most of these without shells are not completely
  helpless
• they are protected by behavior, spending daylight
  hours under rocks or logs
• some sea hares have a special ink they squirt
  into the surrounding water

34
Section 27-1Mollusks

• Others, such as the sea butterflies, escape
  predators by swimming rapidly
• many nudibranchs (or sea slugs), have chemicals
  in their bodies that taste bad or are poisonous
  some even have nematocysts to sting with

35
Section 27-1Mollusks

• The bad-tasting, poisonous, stinging, or
  otherwise booby-trapped nudibranchs are usually
  bright colored- this warns the predators to stay
  away
• thus, the shell-less gastropods do have means of
  protection

36
Section 27-1Mollusks

• 2. Class Bivalvia- have two shells that are
  hinged together at the back and held together by
  one or two powerful muscles
• common bivalves include clams, oysters, and
  scallops
• may be tiny, or like the giant clam 1.9 meters in
  length

37
Section 27-1Mollusks

• Although bivalve larvae are free-swimming, they
  soon establish life at the bottom of a body of
  water
• some secrete sticky threads to attach themselves
  to rocks
• most sessile however scallops can flap their
  shells to move

38
Section 27-1Mollusks

• Mantles of bivalves, like other mollusks, contain
  glands that manufacture the shells, and also keep
  the shells inside surface smooth and comfortable
  by secreting layers of mother-of-pearl

39
Section 27-1Mollusks

• If a foreign object (sand or a pebble) gets
  caught between mantle and shell- the mantle
  covers it with this secretion
• after time, these objects become completely
  coated, and are called pearl
• often very valuable

40
Section 27-1Mollusks

• 3. Class Cephalopoda- called cephalopods, are
  among the most active and interesting
• includes octopi, squids, cuttlefish, and
  nautiluses
• cephalopoda means head-foot their head is
  attached to the foot, divided into tentacles

41
Section 27-1Mollusks

• Cephalopods range in size from tiny cuttlefish
  less than 2 cm long, to giant squids which are
  thought to grow greater than 20 meters long
• they have flexible tentacles equipped with round
  sucking disks to grab fish and prey

42
Section 27-1Mollusks

• Most modern cephalopods have small internal
  shells, or no shells at all
• the only present-day cephalopod with a shell are
  a few species of nautiluses
• Figure 27-11, page 592

43
Section 27-1Mollusks

• Cuttlefish have small shells that are found
  inside their bodies- thin and coiled
• this is the cuttlebone on which pet birds
  condition their beaks
• this helps the nautilus and cuttlefish to remain
  upright and allow it to float in the water -
  Figure 27-12, page 593

44
Section 27-1Mollusks

• However, there are other means of protection for
  cephalopods
• can move quickly, either by swimming or crawling,
  or a form of jet propulsion of drawing water in
  and then out
• changing their color releasing dark,
  foul-tasting ink

45
Section 27-1Mollusks

• How Mollusks fit into the World
• feed on plants prey on animals clean up their
  surroundings by eating detritus
• some are hosts to symbiotic algae or to
  parasites others are parasites themselves
• food for many, even humans

46
Section 27-1Mollusks

• Can be a check on pollution levels, since some
  are filter-feeding bivalves
• snails and other mollusks never seem to develop
  cancer why is that? Scientists are interested
  in finding out!

47
Section 27-1Mollusks

• Although beneficial in many ways, there are also
  some negative relationships with humans
• land slugs and snails are plant eaters that do
  damage to crops
• shipworms will damage wood ships- termites of
  the sea

48
Section 27-1Mollusks

• Since many are filter-feeders, if we eat them, we
  are likely to get high concentrations of
  pathogens (things that cause disease), toxins, or
  pollutants that can result in sickness or even
  death

49
Section 27-2Annelids

• OBJECTIVES
• Describe how annelids perform their essential
  life functions.

50
Section 27-2Annelids

• OBJECTIVES
• List and give examples of three classes of
  annelids.

51
Section 27-2Annelids

• Phylum Annelida - segmented worms, or annelids
• approximately 9,000 species
• can live everywhere in the world, except
  Antarctica and Madagascar
• many are aquatic, thus only a few species are
  familiar to us

52
Section 27-2Annelids

• They are round, wormlike animals with a long,
  segmented body
• from Latin annellus- which means little ring, and
  refers to the ring-like appearance of the body
  segments

53
Section 27-2Annelids

• Range in size from tiny aquatic worms less than
  half a millimeter long to giant earthworms more
  than 3 meters
• vary greatly in color, patterning, number of
  bristles, and other superficial features
• most appear very wormlike

54
Section 27-2Annelids

• The many segments of an annelids body are
  separated by internal walls called septa
• most of the body segments are identical to each
  other, but some are modified to perform special
  functions- first few may have eyes, antennae, etc.

55
Section 27-2Annelids

• Feeding the digestive tract (gut) is a long tube
  within the body cavity that extends from the
  mouth to the anus
• Figure 27-14, page 595
• pharynx is one structure that has evolved many
  different forms in different annelids

56
Section 27-2Annelids

• Pharynx is a muscular front end of the digestive
  tube
• many can extend it through the mouth- in many
  carnivorous annelids it usually has two or more
  sharp jaws attached to help grab prey

57
Section 27-2Annelids

• Others use the pharynx to tear off bits of algae
• some detritus feeders have the pharynx covered
  with sticky mucus to grab food particles other
  detritus feeders use it as a pump
• leech sucks blood from a host

58
Section 27-2Annelids

• Respiration aquatic annelids often breathe
  through gills
• the feather-duster worms have brightly colored
  gills that protrude from their burrow
• Figure 27-15, page 595
• others may have delicate gills located on the
  side of the body

59
Section 27-2Annelids

• Many annelids can exchange gases through their
  skin but the skin must remain moist to make gas
  exchange possible
• terrestrial annelids (such as the earthworm)
  secrete a thin protective coating called a
  cuticle to hold moisture

60
Section 27-2Annelids

• Internal Transport annelids typically have
  closed circulatory systems organized around two
  blood vessels that run the length of their bodies
  (Fig. 27-16, p.596)
• blood moves toward the head in the dorsal vessel
  toward the posterior in the ventral vessel

61
Section 27-2Annelids

• Each segment has a pair of smaller vessels called
  ring vessels that connect the dorsal and ventral
  blood supply
• in annelids such as earthworms, several ring
  vessels are larger, more muscular- often called
  hearts

62
Section 27-2Annelids

• Excretion like other animals, there are two
  types of waste
• solid waste passes out through the anus at the
  end of the gut
• waste from cellular metabolism are eliminated by
  nephridia- a pair of which are in each body
  segment

63
Section 27-2Annelids

• Response many annelids are active animals with
  well-developed nervous systems
• brain sits on top of the gut at the front end of
  the body
• two large nerves pass around the gut, and connect
  brain with a pair of ganglia below

64
Section 27-2Annelids

• From these ganglia, a ventral nerve cord runs the
  entire length of the worm
• Sense organs are well-developed in the
  free-living marine forms
• sensory tentacles statocysts chemical
  receptors two or more pairs of eyes (light
  detectors)

65
Section 27-2Annelids

• Many other annelids have much simpler sensory
  systems
• earthworms have no specialized sense organs, but
  rely on simple sensory cells in the skin
• little protection from predators burrow or swim
  away

66
Section 27-2Annelids

• But, some annelids do fight back
• several carnivorous annelids use their sharp jaws
• the marine fireworms have tufts of poisonous
  bristles that easily break off and penetrate
  skin, causing painful sores and a burning
  sensation

67
Section 27-2Annelids

• Movement two major muscle groups in their body
  walls
• longitudinal muscles run from the front to the
  rear when they contract, the worm gets shorter
• a group of circular muscles when they contract
  they make the worm skinnier

68
Section 27-2Annelids

• Reproduction some are able to reproduce
  asexually by budding
• most reproduce sexually
• some species have separate sexes and external
  fertilization happens in open water
• proper timing is essential

69
Section 27-2Annelids

• In the South Pacific, islanders eagerly await the
  autumn spawning season of the annelids called
  palolo worms
• they are considered a great delicacy to humans,
  as well as sea birds and fishes that gather to
  feed

70
Section 27-2Annelids

• Some such as leeches and earthworms are
  hermaphroditic
• undergo internal fertilization
• however, they rarely fertilize their own eggs -
  page 599
• when eggs are ready, and band called the
  clitellum secretes a mucus ring

71
Section 27-2Annelids

• The eggs and sperms are secreted into the
  clitellum
• and then the ring slips off the worms body and
  forms a cocoon that shelters the eggs

72
Section 27-2Annelids

• 1. Class Polychaeta - many common and important
  marine worms many ocean plankton are small
  polychaetes
• polychaetes have paired paddle-like appendages on
  their body segments tipped with bristles that
  give their name

73
Section 27-2Annelids

• Note Figure 27-19, page 599 of the sea mouse-
  bristles are so long they look like hair or fur
• the polychaetes live in cracks and crevices in
  coral reefs in sand, mud, or piles of rocks or
  even in open water
• some colorful others dull

74
Section 27-2Annelids

• 2. Class Oligochaeta - contains earthworms and
  related species
• examples are earthworms and tubifex worms- red
  threadlike aquatic worms sold as tropical fish
  food in pet stores
• oligo means few, so these have fewer bristles

75
Section 27-2Annelids

• The bristles (called setae) are felt as
  roughness on the ventral side of the earthworm
  help anchor it in its burrow
• mostly found underground
• squiggles of mud are sometimes seen above ground,
  and called castings

76
Section 27-2Annelids

• Eating materials in the soil, this passes
  through some will be digested- other
  indigestible materials pass out the anus in large
  quantities- the castings
• some tropical varieties castings are enormous- 18
  cm long, and 2 cm in diameter!

77
Section 27-2Annelids

• 3. Class Hirudinea - contains the leeches, most
  of which live in tropical countries
• typically no more than 6 cm long some tropical
  species as long as 30 cm
• most are freshwater organisms

78
Section 27-2Annelids

• Usually exist as external parasites, sucking
  blood and body fluids from the host
• roughly 1/4 of all leeches are carnivores rather
  than parasites feeding on snails, worms, and
  insect larvae either swallow whole or suck soft
  parts from the body

79
Section 27-2Annelids

• Leeches have powerful suckers at each end of the
  body
• penetrate either by a) muscular proboscis, or b)
  slice into the skin with razor sharp pair of jaws
• uses its muscular pharynx to suck blood from the
  area

80
Section 27-2Annelids

• A special secretion from the salivary gland keeps
  the blood from clotting, and it may also
  anesthetize the wound- thus the host does not
  even know it has been bitten!
• Can swallow as much as 10 times its weight in
  blood

81
Section 27-2Annelids

• Such a huge meal may take up to 200 days to
  digest with the help of symbiotic bacteria in the
  gut
• may live up to a year before needing to feed again

82
Section 27-1Mollusks

• How Annelids Fit Into the World
• Annelids are important in many habitats
• some are members of the ocean plankton, where
  they are food for many fishes, crab, and lobster

83
Section 27-1Mollusks

• How Annelids Fit Into the World
• earthworms condition soil by burrowing helps
  aerate soil
• speed the return of nitrogen and other important
  nutrients from dead organisms
• thus, spraying insects is not always good