Phylum Nematoda Lecture Notes

Symmetry and Structure: Nematodes are characterized as being bilaterally symmetrical and possessing a pseudocoel.
Reproductive Status: Most species are dioecious, and they exhibit considerable sexual dimorphism.
Morphology:
- Body shape is elongated and tapered at both ends.
- Size ranges from less than $1\,\text{mm}$ to greater than $10\,\text{meters}$ .
Digestive Tract: Usually complete, featuring a mouth located at the extreme anterior and an anus located near the posterior tip.
Cuticle:
- Possess a noncellular cuticle secreted by an underlying hypodermis.
- The chitinous cuticle is shed exactly $4$ times during ontogeny, referred to as the $4$ molts.

Composition: The body wall consists of three primary layers: the cuticle, the hypodermis, and the body wall musculature.
Hypodermis Function and Structure:
- An important function of the hypodermis is the secretion of the cuticle.
- In adult worms, the hypodermis is syncitial.
- Nuclei of the hypodermis lie in $4$ epidermal cords.
Epidermal Cords:
- Run longitudinally and divide the body into $4$ quadrants.
- Dorsal and Ventral Cords: Contain the longitudinal nerve trunks.
- Lateral Cords: Contain the lateral canals of the excretory system in most species.

Muscle Arrangement: Muscles of the body wall are arranged entirely longitudinally; there is no circular muscle layer.
Muscle Anatomy: Nematode muscles consist of two parts:
- A contractile portion.
- A noncontractile cell body, known as the myocyton.
Contraction Mechanism: Muscle contraction occurs similarly to vertebrate striated muscle, utilizing the proteins myosin and actin.
Support System: The cuticle, hypodermis, somatic musculature, and the fluid-filled pseudocoel function together as a hydrostatic skeleton.

Definition of Pseudocoel: It differs from a true coelom because it has no peritoneal (mesodermal) lining.
Physical Properties:
- It is a fluid-filled cavity enclosed by the body wall.
- The pseudocoelomic fluid is known as hemolymph.
Hydrostatic Skeleton Requirements: Its function as a skeleton depends on three factors:
1. An enclosed volume of non-compressible fluid.
2. The ability of muscle contraction to apply pressure to that fluid.
3. The transmission of the fluid pressure in all directions.
Locomotion: The alternation of contraction and relaxation in the dorsal and ventral muscles produces the characteristic S-shaped motion observed in nematode locomotion.

Complexity: The nervous system is relatively simple.
Nerve Concentrations: There are $2$ main concentrations of nerve elements, which are connected by longitudinal nerve trunks:
- One concentration is located in the esophageal region.
- One concentration is located in the anal region.
Sense Organs: Nematodes have a variety of sensilla (small sense organs).
- Sensilla are most prominent in the cephalic and cauda (tail) regions.

Components: The digestive system is complete, consisting of a mouth, gut, and anus.
Ingestion:
- Buccal muscles and anterior esophageal muscles contract to open the mouth and suck in food.
- Hydrostatic pressure within the pseudocoel acts to close the mouth and the esophageal lumen.
Digestion Process:
- Food passes down the esophagus via muscle contractions until it reaches the intestine.
- A posterior bulb serves as a one-way valve.
- Hydrostatic pressure pushes food posteriorly; newly ingested food and locomotor activity assist this movement.
Intestine Structure: The intestine is constructed of a single layer of intestinal cells.
Defecation:
- The depressor ani (anal dilator muscles) contract to open the anus.
- Hydrostatic pressure causes defecation when the anus is opened.
Dietary Sources: Nematode food sources include blood, tissue cells, fluids, and intestinal contents.

Excretion Pathways: Most excretion occurs through the intestine.
Organs: There are no flame cells or nephridia present.
Canal Structure: Two long canals located in the lateral hypodermis connect near the anterior end.
Pores: The secretory-excretory canal opens to the excretory pore.
Osmoregulation: The ability to osmoregulate varies greatly and depends on the specific habitat of the nematode.

iClicker Question 1: Which of the following is not associated with the body wall of nematodes?
- A. Hypodermis
- B. Tegument
- C. Body wall musculature
- D. Cuticle
- E. None of the above
- Answer: B. Tegument (Note: Terminology used in other phyla like Platyhelminthes, but not Nematoda).
iClicker Question 2: How many molts are needed for nematodes to become reproductive adults?
- A. 1 molt
- B. 2 molt
- C. 3 molt
- D. 4 molt
- E. 5 molts
- Answer: D. 4 molt.

Sexual Status: Most are dioecious.
Dimorphism: Females are generally larger; the tail of the male is more curled.
Reproductive Strategy: Most nematodes are oviparous.
Gonad Structure: Gonads are solid cords of cells; germ cells move via the rachis.
Male Anatomy:
- Testes are generally paired, though some species have only one.
- Nematode spermatozoa are unique in that they use a pseudopodium (amoeboid movement).
- Males possess a cloaca; the ejaculatory duct opens into the rectum.
- Copulatory spicules of the male are used during copulation.
Female Anatomy: The female reproductive system opens through a central vulva.

Classification: A tissue parasite.
Size and Longevity: Measures between $3\,\text{cm}$ to $5\,\text{cm}$ in length; adults can live for several years.
Reproductive Output: Females produce between $3,000$ and $20,000$ eggs per day.
Development: Embryonation occurs over $21$ days in moist and shady soil.
Infection Cycle:
- Eggs are swallowed and hatch in the small intestine.
- Larvae then enter the intestinal crypts of the large intestine.
Attachment: The slender anterior end embeds in the gut mucosa, while the enlarged posterior portion remains in the lumen.