Chapter 22 - Nematodes: Form, Function, and

Classification

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

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

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

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

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

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

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

Digestive Tract

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

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

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

Nervous System

• There are 2 major nerve centers in nematodes: 1. The circumesophageal commissure, or nerve ring 2. The rectal commissure

• Associated with the nerve ring are ganglia from which longitudinal nerves emanate• From the ventral longitudinal nerve is born the rectal commissure

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

Nervous System cont.

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

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

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

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

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

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

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

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

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

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

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

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