Biology 320 Invertebrate Zoology Fall 2005

1
Biology 320Invertebrate ZoologyFall 2005

• Chapter 22
• Phylum Gastrotricha and Phylum Nematoda

2
Phylum Gastrotricha

• 500 species
• Found in all three habitat types
• Marine and FW - interstitially or on plants
• Terrestrially - in water film covering soil
  particles
• Microscopic to 4mm long
• Eutelic
• Genetically predetermined and constant number of
  cells
• Cell size increases, but not cell number

3
Body Form

• Bowling-pin-shaped
• Name means hairy belly
• Locomotory cilia on ventral surface
• Adhesive tubes located near head and/or on
  lateral body
• Adhesive organs at posterior
• All are similar to duo-glands of turbellarians

4
Body Wall

• Cuticle
• Not chitinous
• Not molted
• Epidermis
• Musculature
• Circular
• Longitudinal

5
Organ Systems

• No hemal system
• Acoelomate, so dont even possess a hemocoel
• No respiratory system
• Nervous system
• Brain
• Pair of ventrolateral nerve cords
• Sensory organs
• Cerebral organs possess all three types of
  receptors
• Sensory bristles found on body surface
• Excretory system
• One to several pairs of protonephridia
• Nephridiopores open ventrolaterally
• Osmoregulatory in function
• Ammonotelic, so ammonia diffuses across body
  surfaces

6
(No Transcript)
7
Movement

• Ciliary gliding for forward movement
• Rapid rearward withdrawal as an escape response
• Can use a combination of adhesion and muscular
  contractions to
• Inchworm
• Somersault

8
Nutrition

• Gut lacks stomach
• Large muscular pharynx
• Y-shaped in some
• Possess pharyngeal pores that open to surface and
  release excess ingested water
• Feed on small organic particles (live or dead)
• Bacteria and protozoans

9
Reproduction

• Hermaphroditic with indirect sperm transfer
• Posterior copulatory organ that is loaded with
  sperm from anterior male gonopore
• Spermatophore transferred to seminal receptacle
  via posterior female gonopore
• Internal fertilization

10

• Fertilized eggs are released by rupturing body
  wall
• Some species are parthenogenic
• Can produce two types of eggs
• Summer
• Winter (resting)
• Direct development
• Sexual maturity is reached in about three days
• Life span is approximately 40 days

11
Phylum Nematoda

• Roundworms
• 20,000 described species
• May be the largest animal phylum
• Found worldwide and live in all habitat types
• Interstitially
• Inside of animals and plants
• 53C hot springs
• Epiphytic bromeliads
• Very abundant
• One cubic meter of mud off of the coast of
  Holland contains over 4,000,000 nematodes

12

• Ecologically important
• Link between decomposers and higher trophic
  levels
• Some are excellent decomposers and nutrient
  cyclers
• 90,000 nematodes in one rotting apple
• Caenorhabditis elegans is an important model
  organism for genetics / development studies
• Eutelic
• C. elegans has 1031 cells

13
Body Form

• Vermiform
• Tapered at both ends
• Important adaptation for living interstitially
• Microscopic to 50 cm
• One mm is typical
• Mouth at anterior end, with three to six lips
  that bear sensilla
• Caudal gland on posterior that is similar to a
  duo-gland
• Lack cilia

14
Body Wall

• Cuticle
• Not chitinous
• Often annulated
• Must be molted
• Molted four times while growing
• Molting ceases upon reaching adulthood
• Ecdysone controls molting
• Epidermis
• Secretes cuticle
• Stores nutrients
• Endoparasites may absorb nutrients with
• Has four longitudinal extensions called epidermal
  cords
• Longitudinal nerve cords housed here
• Musculature
• Four bands of longitudinal fibers separated by
  epidermal cords
• No circular muscles

15
(No Transcript)
16
Locomotion

• Use sinusoidal undulations of dorsoventral plane
  to move forward
• Alternate contractions of dorsal and ventral
  longitudinal muscles
• Efficient movement requires a substratum to act
  against
• Removing worms from natural substratum hinders
  locomotion
• Many swim or are capable of swimming
• Some crawl like earthworms or inchworms

17
Organ Systems

• Hemal system
• Large nematodes have a hemocoel (pseudocoel)
• Fluid filled and fluid may contain hemoglobin
• Functions as hydrostat
• Small varieties are acoelomate
• No respiratory system
• Nervous system
• Collar-like brain surrounds pharynx
• Dorsal, ventral, and lateral longitudinal nerve
  cords in epidermal cords
• Sensory organs
• Papillae low projections of cuticle on lips and
  head
• Tactile setae
• Ocelli one on each lateral aspect, near pharynx

18
(No Transcript)
19
Nutrition

• Will eat just about anything
• Carnivorous, herbivorous, or omnivorous
• Will eat bacteria, protists, fungi, other
  nematodes, and plant cells
• Some are deposit or detritus feeders
• Actually feed on bacteria associated with these
  types of organic matter
• Some fungi actually prey on nematodes
• Trap them with hyphal threads

20
Digestive System

• Similar to that of gastrotrichs
• Cuticle of foregut contains teeth, ridges, rods,
  or plates
• Varies according to feeding habits
• Example Mononchus uses teeth to latch onto
  other nematodes
• Body contents are pumped out
• Will eat over 1000 nematodes over its 14 week
  life span
• Diagnostic tool for taxonomists
• Herbivores have a stylet for piercing plant cells
• Some parasites use body surface to absorb
  nutrients

21
Excretory System

• Ammonotelic
• All have one or more excretory glands
• Some also have an excretory canal system
• Interestingly, is formed from one huge H-shaped
  cell
• Pore opens midventrally near pharynx
• The canal system is osmoregulatory in C. elegans,
  but other nematodes secrete different substances
• Gelatinous matrix for eggs
• Glycoprotein coat for cuticle
• Digestive enzymes to erode host tissue in animal
  parasites
• Molting fluid

22
Reproduction

• Most gonochoric
• Some hermaphroditic
• Some parthenogenic
• Internal fertilization with copulation
• Sexually dimorphic
• Males smaller than female
• Male has curled posterior that resembles a hook
• Male has a cloaca, as where vagina is located
  midventrally on the middle of the females body
• Male has copulatory spicules (resemble curved
  blades) that are used to hold female gonopore open

23

• Both male and female have C-shaped gonads
• Females produce pheromones to attract males
• Sperm are aflagellate and amoeboid
• Eggs have characteristic shapes, and experts can
  diagnose infections
• Some free-living types, such as Turbatrix, are
  viviparous

24

• Hermaphroditic varieties typically self-fertilize
  and rarely cross-fertilize
• Little genetic recombination
• Egg numbers vary greatly between species
• 50 in some marine species
• 200,000 per day in the parasite Ascaris
• Eutelic
• Around 1000 cells
• Number of cells in individual organs is also
  constant
• Cells grow in size, not in number
• Direct development
• Egg, three juvenile instars, and adult
• Four molts (first two of which may occur in egg
  before hatching)
• Molting ceases upon reaching adulthood

25
Parasitism

• Can be ectoparasites or endoparasites
• Can have one host or multiple hosts
• Can infect plants, animals, or both
• Juveniles, adults or both can be infective

26

• Ascaroid nematodes
• Ascaris, hookworms, pinworms, and Trichinella
• Infect one host humans, cats, dogs, pigs, cows,
  etc.
• Infection occurs when eggs or juveniles are
  ingested
• Eggs hatch in intestine
• Juveniles penetrate intestinal wall
• Juveniles enter hemal system
• Juveniles break through alveoli in lungs
• They migrate up respiratory tree
• Are coughed up in sputum
• If sputum is swallowed, end up in intestine

27
(No Transcript)
28

• Ascaris
• Human intestinal worm Ascaris lumbricoides
• Can reach 50 cm in length
• Widely distributed, but is found in SW US
• Eggs found in soil and are resistant to harsh
  environmental conditions
• Children are susceptible to infection because
  they put things in their mouths
• Feeds on fluid gut contents of host
• Can cause malnutrition and death in the event of
  intestinal blockage
• Secrete substances which inhibit host digestive
  enzymes from harming them

29

• Hookworms
• Necator americanus
• Juveniles enter through skin of foot
• Follow typical ascaroid path
• Attach to intestinal lining with hooks
• Feed on hosts blood
• Infections of more than 25 worms can lead to
  serious blood loss and tissue damage
• Common in tropics
• 380 million infected worldwide

30

• Pinworms
• Enterobius vermicularis
• Typically infect humans
• Worldwide distribution
• Adults live in intestine
• At night, female crawls out anus and deposits
  eggs
• Scratching traps eggs under fingernails

31

• Trichinella spiralis
• Infects mammals with a condition known as
  trichinosis
• Juveniles are carried to skeletal muscles by
  blood
• Form calcified cysts in muscle
• Transmitted when undercooked flesh is eaten
• Example humans eating undercooked pork
• Severe infection causes pain and stiffness

32

• The following groups of parasitic nematodes have
  two hosts (intermediate and definitive)
• Filarioids
• Dracunculoids
• Filarioids
• Elephantiasis, heart, and eye worms
• Thin, threadlike worms that inhabit the lymphatic
  system or other tissues
• Intermediate host is typically a blood sucking
  insect
• Definitive host is usually a bird or mammal

33

• Wuchereria bancrofti
• Found mainly in Africa and Asia
• Intermediate host is a mosquito, definitive host
  is a human
• Threadlike adults live in lymph glands
• Blockage of lymph vessels causes edema
• Long term blockage causes a condition known as
  elephantiasis
• Enlargement of appendages, breasts, scrotum, etc
• Life cycle
• Eggs hatch into microfilariae that migrate to
  surface blood vessels at night (when mosquitoes
  are biting)
• Microfilariae migrate from mosquito gut lumen to
  proboscis
• Injected into definitive host

34

• Dirofilaria immitis
• Canine heartworm
• Loa Loa
• African eye worm
• Sometimes crosses cornea

35

• Dracunculus medinensis
• A dracunculoid
• Guinea worm
• Intermediate host is a FW copepod and definitive
  host is human
• Adult female lives below skin of human and
  produces an ulcer
• Juveniles released in water
• Ingested by a copepod
• Humans ingest copepod by drinking contaminated
  water
• Worms removed surgically or by winding on a stick
• Caduceus (symbol of medical profession) is really
  a nematode curled around a stick