Chapter 22 - Nematodes: Form, Function, and Classification

1. Chapter 22 - Nematodes: Form, Function, and Classification

2. General Characteristics • Body elongate, cylindrical, and tapered at both ends • Body design is a tube within a tube, the outer tube being the body wall and underlying muscles and the inner tube being the digestive tract • Between the tubes is the fluid-filled pseudocoelom, in which the reproductive system and other structures are found; the pseudocoelom is filled with hemolymph

3. General Characteristics cont. • Although there are some structural differences between pseudocoeloms and coeloms, they confer many of the same advantages: • A space within the body cavity allows for the reproductive and digestive systems to evolve more complex shapes and functions • A fluid lined chamber offers protection to the gut and other organs; acts as a cushion • The fluid filled body cavity acts as a skeleton - hydrostatic skeleton, providing support and rigidity for a soft bodied animal • Sexual dimorphism is evident: at the curved posterior end of the male there is a copulatory organ as well as other specialized organs; males are usually smaller than females

4. Cuticle • An elastic cuticle covers the body surface of nematodes; it is periodically molted • The presence of enzymes in the cuticle indicates that it is metabolically active and not an inert covering • Specialized structures such as spines, bristles, warts, papillae, and ridges may be present on the cuticle; these structures may be sensory and some may aid in locomotion • The cuticle not only covers the entire external surface, but it also lines the buccal cavity, esophagus, rectum, cloaca, vagina, and excretory pore • Cuticle consists of 4 basic layers: epicuticle, exocuticle, mesocuticle, and endocuticle

5. Cuticle cont. • Epicuticle - thin; with a carbohydrate containing glycocalyx; acts as a protective barrier • Exocuticle • Mesocuticle - consists of obliquely oriented, collagenous, fibrous sublayers that vary in number and angular arrangement to each other; they sublayers can shift their angles of orientation, thus providing flexibility to the cuticle • Endocuticle - fibrous, but orientation of the fibers is not distinct • A basal lamina separates the cuticle from the underlying hypodermis

6. Hypodermis • Beneath the basal lamina lies the thin, cellular (or in some cases syncitial) hypodermis which secretes the cuticle • Protrude into the pseudocoelom along the middorsal, midventral and lateral lines to form the longitiudinal hypodermal cords, partially dividing the pseudocoel into quadrants • Hypodermal organelles such as nuclei and mitochondria are confined to the cords • The lateral cords are the largest and contain the primary excretory canal when these are present, while the dorsal and ventral cords contain longitudinal nerve trunks

7. Musculature • Within and closely associated with the hypodermis are one or more layers of longitudinally arranged muscle cells, the somatic musculature • Each muscle cell comprises a contractile portion containing myofibrils and a non-contractile portion in which are found the various organelles, such as the nucleus, mitochondria, etc. • Collectively, the cuticle, hypodermis, and somatic musculature make up the body wall

8. Musculature cont. • An arrangement of multiple longitiudinal rows of muscle cells in each quadrant is termed polymyarian, one with no more than2 rows of cells is called holomyarian, and one with 2 to 5 rows is meromyarian

9. Digestive Tract • Consists of an anterior mouth, a gut that comprises 3 major regions (foregut, midgut, and hindgut), a cloaca and a subterminal vent

10. Digestive Tract cont. • The cuticle lined foregut begins at the mouth, which in many species opens into a buccal capsule and continues as the esophagus • Action of the esophagusis is often enhanced by one or more muscular enlargements called bulbs • The glandular portion of the foregut ranges from a few unicellular glands to large prominent glands lying along the esophagus

11. Digestive Tract cont. • The esophagous empties into the midgut (intestine) through a junction called the esophago-intestinal valve • The midgut is a strait tube lined with a single layer of cells bearing microvilli and a prominent glycocalyx • The midgut is nonmuscular, the food being moved posteriorly by the muscular activity of the foregut and the overall body movements • Digestion can be intra- or extracellular or both • In females, the midgut empties into the cuticle lined hindgut or rectum - a short, flattened tube joining the midgut and the anus • In males, the posterior most portion of hindgut receives the products of the reproductive system via the vas deferens and is therefore called a cloaca

13. Nervous System • There are 2 major nerve centers in nematodes: • 1. The circumesophageal commissure, or nerve ring • 2. The rectalcommissure • Associated with the nerve ring are ganglia from which longitudinal nerves emanate • From the ventral longitudinal nerve is born the rectal commissure

14. Nervous System cont. • Parasitic nematodes possess both mechanoreceptors and chemoreceptors • Mechanoreceptors - located around the mouth are papillae of 2 types: labial papillae on the lips surrounding the mouth and cephalic papillae behind the lips • Other papillae may be found at different levels of the nematode body, e.g. caudal papillae, observed in many males; aids in copulation

15. Nervous System cont. • Chemoreceptors - Amphids are chemoreceptors located in shallow anterior depressions or pits • Phasmids are a set of chemoreceptors at the posterior end

16. Excretory System • When present, the basic component is comprised of 1 or 2 renettes, large unicellular glands that empty through an excretory pore • The renettes and the excretoy pore are usually located anteriorly • Note: • It has not been shown conclusively that this system has a function in the removal of wastes • It may be strictly osmoregulatory

17. Male Reproductive System • On or two testes • Vas deferens (sperm duct) extends distally to the cloaca • Two specializations of the vas deferens are evident before it enters the cloaca: the seminal vesicle (sperm storage) and the ejaculatory duct • Sperm has no flagella or acrosomes

18. Male Reproductive System cont. • Male nematodes are usually equipped with one or more copulatory spicules, cuticular structures are encased within spicule pouches located laterally in the cloacal wall • The spicules are important during copulation in that they keep the female vulva open, thus facilitating the entry of sperm into the female reproductive tract • Other accessory structure may be present, including a sclerotized spicular guide or gubernaculum; serves to guide the spicules when they are extended

19. Female Reproductive System • Usually didelphic - equipped with 2 cylindrical ovaries and uteri • The uteri unite to form a common vagina that opens through a gonopore or vulva, usually located near midbody • Oogonia are produced at the proximal end of the ovary, which is known as the germinal zone • As the oogonia develop into oocytes, they move distally along the rachis into the growth zone

20. Female Reproductive System cont. • Approaching the oviduct, the oocystes detach from the rachis and pass to a portion of the oviduct called the spermatheca, where sperm are stored • Once fertilized, the developing egg is moved down the tract by a combination of uterine peristalsis and hydrostattic pressure • The distal portion of the uterus, the ovijector, is usually muscular and acts in conjunction with the muscles of the vulva to expel ripe eggs

21. Female Reproductive System cont. • Eggs can hatch either within the host or in the external environment • Hatching of eggs in the external environment is , in part, controlled by such ambient factors as temperature, moisture, and oxygen tension • In some species, the eggs only hatch once they have been ingested by a host • In these cases the stimuli for hatching may be carbon dioxide tension, pH, salts and temperature

22. Molting • Nematodes undergo 4 molts each of which involves: formation of new cuticle, loosening of the old cuticle, rupturing of the old cuticle, and escape of the larva • This sequence of events is controlled by exsheathing fluid secreted by the larva • In some nematodes, there is a lag phase at some stage of development, during which a phase of the life cycle is temporarily arrested • This phenomenon is called hypobiosis (developmental arrest) - it is thought to be an adaptation that allows the larva to withstand adverse environmental conditions while awaiting the access of a new host

23. Larval Forms • Larval stages preceding each molt of the 4 molts in the life cycle of parasitic nematodes are generally referred to as first-, second-, third, and fourth-stage larvae (e.g., L1, L2 ,L3, L4) • The first stage larva being the stage prior to the first molt • However various other designations also are used for specific nematode larval forms

24. Larval Forms • Rhabditiform larva - The first stage larva of Strongyloides and hookworms; the esophagus of this small larva is joined to a terminal esophageal bulb by a narrow isthmus

25. Larval Forms cont. • Filariform larva - after molting twice, the rhabditiform larva of Strongyloides and hookworms usually retain the remnants of their last cuticle and becom ensheathed, 3rd stage or filariform larva • The esophagus is typically elongate and cylindrical and has no terminal bulb • This larva is usually the stage that is infective to the definitive host

26. Larval Forms cont. • Microfilaria - the prelarvae of filarial worms (e.g. Wuchereria bancrofti)are known as microfilariae • The larval body surface is covered by a thin layer of flattened epidermal cells • The primordia of various adults structures are visible within the pseudocoelom