Biology 625
Fall semester lecture note outline

Updated: 24 September 1999

The text below simply represents a crude lecture outline of one of the topics covered in class. It is not meant to substitute for attending lectures or ignoring the textbook. Additional material, including line drawings, kodachromes, and more extensive information on life-cycles and basic biology, will be supplied in the lectures.

TOPIC #4. Basic characteristics of the Class: Digena

  1. 2 suckers usually
    1. oral (usually anterior) sucker. Surrounds mouth.
    2. acetabulum (ventral sucker).
  2. 2-more hosts in life cycle
  3. All but a few monecious (self or cross fertilize)
  4. Types of adult flukes
    1. distome [most common; oral sucker & ventral acetabulum]
    2. amphistome [oral sucker; posterior sucker, usually posterior to testes]
    3. monostome [oral sucker only]
    4. gasterostome [mouth in center of oral, ventral sucker]
    5. holostome [forebody & hindbody; tribocytic organ posterior to acetabulum]
    6. echinostome [collar of spines around anterior sucker]
  5. Tegument
    1. Two Zones
      1. outer cytoplasmic syncytium with microvilli [mitochondria, ER, vacuoles, lipids, etc.] [pinocytosis; spines]
      2. inner area of nucleated cell bodies - cytons
      3. zones separated by basal lamina (C.T.)
      4. also, circular & longitudinal muscle under basal lamina
    2. numerous chemicals in tegument
      1. mucopolysaccharides to inhibit host digestive enzymes
      2. acid & alkaline phosphatases, esterases, and aminopeptidases for digestion
  6. Alimentary tract
    1. incomplete gut
    2. pharynx, when present, masticates food
    3. esophagus leads to 2 blind caecae
    4. entire gut secretion by cells along gut; proteases, lipases, etc.
    5. in cecae, absorption too
    6. different species feed on different things:
      1. blood
      2. mucus
      3. epithelium
  7. Male reproductive system
    1. usually 2 testes; taxonomic importance
    2. vas efferens - vas deferens - cirrus pouch
    3. cirrus pouch encloses seminal vesicle, prostate glands, and cirrus; some with external seminal vesicle outside of pouch
    4. sperm stored in seminal vesicle
    5. prostate secretes fluid to keep sperm alive
  8. Female reproductive system
    1. usually single ovary
    2. 2ndry oocytes released
    3. through short oviduct, into ootype
    4. 3 organs enter into ootype:
      1. Mehlis gland
        1. cluster of unicellular glands, enhance egg tanning by maintaining correct pH. Different cell types in gland
        2. secretions cause release of shell globules from vitelline glands
        3. secretes first membrane around egg
        4. lubricates uterus
        5. activates sperm, which are passed down ootype
      2. Common vitelline duct [passage of vitelline gland secretions for eggshell formation]
      3. Duct from seminal receptacle [absent in some species]
      4. Occasionally a fourth duct, vitelline reservoir, as diverticulum of vitelline duct
  9. Excretory system
    1. protonephridia (flame cells)
    2. excretory bladder in posterior, with excretory pore
  10. Egg
    1. usually operculate
    2. lid pops off during hatching (in schistosomes, no lid; shell splits)
    3. miricidium produces lytic enzymes in response to proper stimulus (i.e. light). Membrane between operculum and cushion dissolves; water enters. Cushion expands, and cement of operculum dissolved
  11. Miricidium
    1. hatches from egg - penetrates mollusc (rarely annelids)
    2. asexual stages from miricidium
    3. morphologic characteristics:
      1. apical stylet [in some species]
      2. apical papilla [where ducts from glands open; also nerve endings for chemoreception]
      3. apical gland [histolytic enzymes]
      4. cephalic glands [lytic enzymes]
      5. photoreceptors [eyespots]
      6. germinal mass [initiate asexual stages]
      7. cilia [locomotion]
      8. excretory pore
    4. actively swim
    5. chemoreception to snail mucus [1980, Acta Tropica 37: 303-318]
    6. attaches to snail with apical papillae; lytic enzymes dissolve tissues. About 30 min for penetration.
  12. Sporocyst
    1. asexual stage, various shapes
    2. no mouth or digestive system
    3. absorbs nutrients through tegument
    4. may produce rediae, daughter sporocysts, or cercariae
  13. Redia
    1. develop directly from miricidium, or from embryos generated from sporocysts
    2. elongate; crawl actively
    3. muscular pharynx, mouth, blind-ended sac caecum
    4. 1-more ambulatory buds for movement often
    5. because ingestion, many species cause extensive damage to host
    6. will form daughter rediae or cercariae
  14. Cercariae
    1. from redia or sporocyst
    2. free-swimming, leaves snail and seeks host
    3. miniature immature fluke with tail
    4. senarios
      1. penetrate skin of definitive host
      2. encyst on vegetation as metacercariae
      3. encyst as metacercariae in intermediate host
      4. eaten by intermediate or definitive host
      5. mesocercariae (unencysted juvenile) in tissues
    5. Types, some systematics based on sucker placement of cercariae
      1. monostome
        1. 1 sucker only, anterior
        2. 2 eyespots
        3. long, simple tails
        4. develop from redia
        5. give rise to monostome adults
      2. amphistome
        1. posterior sucker, often anterior too
        2. eyespots
        3. develop from redia
        4. give rise to amphistome adults
        5. all in superfamily: Paramphistomoidea
      3. gasterostome
        1. mouth ventral
        2. develop into gasterostome adults
        3. all in family: Bucephalidae
      4. distome
        1. 2 suckers, one oral and one ventral
        2. most common type
    6. another classification, based on cercarial tail shape and other structures
      1. pleurolophocercous
        1. dorsoventral fold on tail
        2. all in superfamily: Opisthorchioidea
        3. develop from redia
      2. furcocercous
        1. forked tail
        2. some with eyespots
        3. with pharynx, holostomes and strigeids
        4. without pharynx, schistosomes
      3. echinostome
        1. collar of spines anteriorly
        2. echinostome adults
      4. xiphidiocercariae
        1. with anterior stylet
        2. develop from sporocysts
      5. ophthalmocercariae [with eyespots]

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