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 #16. Introduction to the Phylum: Nematoda

  1. Characteristics of the phylum
    1. elongate; bilaterally symmetrical
    2. eutely (consistent cell number)
    3. pseudocoelomate
    4. complete gut in most species
    5. body covered by non-living cuticle
    6. most are dioecious
    7. variable in size, from less than 1 mm in length to over 1 meter in length
    8. most are free-living; some parasitic

  2. Some basic fun facts
    1. 3-9 billion worms, representing dozens and sometimes over 30 species, can be found in good farm land
    2. Roundworms were discussed by Aristotle in his writings; the Book of Numbers talks about worms (serpents crawling out of peoples bodies) that probably represent Dracunculus; Egyptian papyrus talked about hookworms

  3. Body wall consists of 3 parts
    1. Outer cuticle (i.e Ascaris as a model)
      1. Epicuticle (trilaminate)
      2. Exocuticle (2 zones; outer zone unstriated; inner zone striated)
      3. Mesocuticle (obliquely striated; allows some movement)
      4. Endocuticle (unorganized fibers)
    2. Hypodermis
      1. secretes cuticle; living cells
      2. lies underneath a basal lamina, directly under cuticle
      3. usually syncytial (occasionally cellular)
      4. nuclei of syncytium lie in four (usually) hypodermal cords that project into pseudocoel and run longitudinally; one mid-dorsal; one mid-ventral; two lateral (dorsal and ventral with longitudinal nerves; laterals contain excretory canals)
    3. Wall musculature
      1. Spindle shaped cells that run longitudinally
      2. Each cell divided into two portions
        1. contractile part, with myofibrils
        2. non-contractile part, with cell body (myocyton).
      3. attach to endocuticle via small fibers that extend from the contractile portions, through basal lamina
      4. Cytoplasmic processes from myocytons run to nerves (i.e., in most animals it is the nerves that have processes that lead to the muscles)
      5. Also muscle-to-muscle connections in many species and neuro- transmission can occur directly between these muscle cells.

  4. Examples of some cuticular ornamentation
    1. Cuticular ridges form distinct patterns in nematodes, and this pattern is termed the synlophe.
    2. lips
      1. The primitive condition is for a nematode to have 6 lips
      2. In most parasitic nematodes, lips have usually fused to form 0, 2, or 3 lips
    3. alae (=wing)
      1. compressed, lateral, longitudinal, cuticular thickenings
      2. may be longitudinal alae along body, cervical alae at anterior end, or caudal alae at posterior end
    4. ballonets
      1. in gnathostomes only
      2. 4 inflated areas surrounding the lips; with small spines
      3. each connected to internal cervical sac of unknown function
    5. head bulbs (bulbed regions surrounding the lips of a few nematodes that are not ballonets)
    6. cordons (longitudinal, cord-like thickenings (ridges) at the anterior end of some worms)
    7. head shields
      1. cuticular collar, posterior to the lips
      2. dorsal and ventral portions recurve to shield lips
    8. cephalic vesicles (inflated regions of the cuticle at the anterior end of the worm)
    9. collarettes (cuticular collar surrounding the head region posterior to the lips; prepuce-like)
    10. bosses (irregular scutes, or bumps, at anterior end of some nematodes)

  5. Basic structure of the alimentary tract
    1. complete gut, although some with anal atropy
    2. stomodeum (buccal cavity plus esophagus); lined by cuticle
    3. proctodeum (rectum); lined with cuticle
    4. mouth [usually circular; 0-6 lips (all derived from 6-lipped ancestors)]
    5. buccal cavity
      1. area between mouth and esophagus, often forming a ridged structure termed a buccal capsule
      2. may be reduced or absent in some species
      3. armament common in parasitic and predatory forms; include teeth, stylets, spears, rods, cutting plates, ridges, etc.
    6. esophagus (pharynx)
      1. muscular, pumping organ that pulls food into gut and forces food into intestine
      2. may have 1-more enlargements, termed bulbs
      3. muscles run from tri-radiate esophagus to cuticular lining of esophagus
      4. generally 3 esophageal glands (5 or more in Enoplea), one in each inter-radial zone.
        1. Uninucleate or multinucleate, and open independently into lumen of esophagus.
        2. digestive enzymes (amylases, proteases, chitinases, cellulases, etc.; anti-coagulants in some hookworms)
        3. In the Enoplean taxa Trichurida and Mermithida, the anterior portion is a thin-walled muscular tube whereas the posterior portion is a thin tube surrounded by a column of singular, glandular cells termed stichocytes. The entire structure is termed a stichosome.
      5. rapid contraction of buccal muscles anteriorly opens mouth and anterior portion of esophagus; food pulled in; hydrostatic pressure closes mouth when muscles relax
      6. posterior bulb has one way valves
      7. few species with esophageal cecae (1-5) near intestinal junction
    7. intestine
      1. esophageal valve at junction
      2. may be intestinal cecae at junction
      3. straight tube; non-muscular; with microvilli
      4. single layer of cuboidal or columnar cells
      5. action of esophagus forces food back
      6. three regions (few enzymes produced)
        1. ventricular region (mainly secretory)
        2. mid-region (absorptive)
        3. pre-rectal region (absorptive)
      7. intestinal digestion relatively minor, due to rapid food movement through the gut (i.e., in Ascaris only about 3 min)
      8. excretion also occurs across gut wall
    8. rectum; lined by cuticle
      1. females (short, terminal, cuticle-lined rectum runs between anus and intestine)
      2. males (rectum receives products of reproductive system in its terminal portion, and thus is technically a cloaca)
      3. depressor ani is a powerful muscle bundle that opens anus when it contracts; hydrostatic pressure around gut forces waste out
      4. rectal glands often present

  6. Excretion (secretory-excretory system)
    1. most occurs through intestinal wall
    2. most parasitic forms have tubular system
      1. 2 long canals in lateral hypodermis interconnect anteriorly
      2. opens to exterior through a median ventral duct and pore
    3. major waste product ammonia (some urea)
    4. under osmotic stress, can excrete higher amounts of urea
    5. carbon dioxide and fatty acids (both end products of metabolism) are also excretory products

  7. Nervous system (Ascaris)
    1. circumesophageal nerve ring (4 nerve cells and 4 glial cells)
    2. 1 pair ventral ganglia, 1 pair lateral ganglia, and a dorsal ganglian
    3. series of nerves arise and travel posteriorly;
    4. 6 labial nerves arise and travel anteriorly
    5. 2 amphidal nerves arise and travel anteriorly
    6. ventral nerve trunk the largest nerve running posteriorly as chain of ganglia; the final ganglion (preanal ganglion) branches to form posterior nerve ring
    7. sense organs
      1. cephalic papillae
        1. tactile receptors
        2. 1-3 sensory nerve endings each
        3. sometimes labial papillae, caudal papillae, and usually cervical papillae (deirids) at level of nerve ring
      2. amphids
        1. pair of sensory pits on either side of head
        2. tend to be reduced in parasitic nematodes
        3. up to 23 sensory nerve endings per amphid
        4. in most species, chemoreceptors
        5. in some groups, i.e. hookworms, amphids also appear to secrete anti-coagulants
      3. phasmids (in Rhabditia only)
        1. many similar in structure to amphids (i.e., chemosensory)
        2. some with associated glandular cells
        3. cuticle lined ducts, open at apices of caudal papilla near tip of tail
    8. main neurotransmitter acetylcholine
    9. stimulatory and inhibitory nerves
    10. inhibitory muscle fibers release gamma-aminobutyric acid (GABA) to hyperpolarize muscle and decrease the rate of action potentials
    11. ivermectins increase GABA release, results in hyperpolarization (increased chloride ions into cell)

  8. Male reproductive system
    1. usually one testes; sometimes two
    2. hologonic - germs cells proliferate throughout length of gonad (rarer type; in Trichuroidea)
    3. telogonic - germ cells proliferate only at the inner end of the gonad (most common type)
      1. germinal zone at terminal end
      2. maturation or growth zone
      3. storage zone or seminal vesicle
      4. muscular ejaculatory duct, opens into cloaca
      5. few nematodes with cement gland to plug vulva after copulation
    4. copulatory spicules
      1. in most species
      2. most as pair of spicules that arise from cloacal wall
      3. fibrous sheath
      4. hold vulva open during copulation
      5. important taxonomic characteristic
    5. gubernaculum
      1. in many species
      2. dorsal sclerotization of cloacal wall
      3. guides exsertion of spicules during copulation
    6. telamon
      1. in some stronglids
      2. additional ventral sclerotization of cloacal wall
      3. similar function to gubernaculum
    7. sperm
      1. no flagella; no acrosome; move by pseudopodia when mature
      2. dividing spermatogonia attach by cytoplasmic bridges to central supporting structure while maturing in testes termed a rachis.
    8. copulatory bursa
      1. flap-like expansions of the cuticle (Strongyloidea) or whole body (Dioctophymidea) in males
      2. supported by thin extensions of the body termed rays in the Strongyloidea
      3. male grasps females with bursa during mating

  9. Female reproductive system
    1. most with two ovaries; few with other numbers
      1. monodelphic = 1 ovary
      2. didelphic = 2 ovaries
      3. polydelphic = 3-more ovaries
    2. most with two uteri that converge from opposite directions
      1. amphidelphic = 2 uteri converge from opposite ends at junction with vagina
      2. prodelphic = 2 uteri parallel and converge from anterior direction
      3. opisthodelphic = 2 uteri parallel and converge from posterior direction
    3. germinal zone in ovary, produces oogonia
    4. oogonia become oocytes; move posteriorly through growth zone attached to a rachis as well; move to spermatheca (sperm storage area) for fertilization; finish meiosis here; shell formation during meiosis
    5. uterus muscular; moves eggs containing embryos down to ovijector (highly muscular portion of uterus)
    6. vulva never opens posterior to anus, always anterior unless opens into rectal area to form cloaca.

  10. Fertilization
    1. Generally, excretory/secretory products of males attract the females
    2. Some females seek out coiled posterior end of male tail (thigmotactic)
    3. Caudal papillae in males tail detects vulval area
    4. Spicules probe for vulva; exsertion of spicules and sperm transfer

  11. Development of the embryo
    1. Penetration of oocyte by sperm initiates eggshell formation
    2. generally 3-4 layers of eggshell
      1. fertilization membrane secreted by embryo after fertilization; increases in thickness to form chitinous shell. Outer vitelline membrane may also be present
      2. lipid layer is then secreted by embryo directly between shell and embryo, composed of esterified glycosides
      3. in many species, an outer proteinaceous layer is added by the uterine wall
      4. in pinworms, 2 uterine layers and the lipid layer is very thin
      5. in some, proteinaceous plugs in ends
      6. lipid layer of ascarids composed of 75% ascarosides which are unique glycosides that render the eggshell impermeable to substances other than gases and lipid solvents

  12. Hatching
    1. Parasitic worms generally hatch only in response to various stimuli
    2. Ascarid eggs require body temperature, low oxygen, high carbon dioxide, and neutral pH
    3. A few larval nematodes have an oral stylet, but most change permeability of lipid layer. High concentrations of trehalose around juvenile result in high osmotic pressure. Hatching stimuli cause trehalose to leak from eggs allowing juvenile to hydrate and internal pressure may rupture eggshell

  13. Molting (ecdysis)
    1. Undergo 4 molts
    2. New cuticle produced under old (hypodermis detaches from old sheath and produces new one which is highly folded under old one)
    3. larva produces exsheathing fluid (leucine aminopeptidase), stimulated by carbon dioxide, which allows old cuticle to loosen and rupture

  14. Larval types
    1. rhabditiform
      1. L1 (and L2) of many parasitic species
      2. esophagus joined to terminal bulb by narrow isthmus
    2. filariform
      1. L3 of many species
      2. esophagus elongate without distinct posterior bulb; isthmus absent
    3. microfilaria
      1. immature L1 of filarids
      2. embryo, with body covered by flattened epidermal cells
      3. no gut

  15. Typical life histories
    1. non-parasitic life-cycles (egg - L1 - L2 - L3 - L4 - adult)
    2. parasitic life-cycle; egg ingested
      1. (egg - eaten - hatch - L1 - L2 - L3 - L4 - adult)
      2. (egg - eaten - L1 - L2 - L3 - hatch - L4 - adult)
    3. parasitic life-cycle; larval penetration of skin (eggs out with feces - hatch in soil - L1 - L2 - L3 - penetrate skin - L4 - adult)
    4. parasitic life-cycle; vector employed (L1 in environment or microfilariae in blood - ingested by arthropod - L2 - L3 - injected into definitive host - L4 - adults)

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