1 Chapter 22 - Nematodes: Form, Function, and Classification Structure of the Adult Nematodes are elongate, cylindrical, and tapered at both ends The basic 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 fluid in the pseudocoelom is called hemolymph Pseudocoeloms and coeloms, they confer many of the same advantages: 1. A space within the body cavity allows for the reproductive and digestive systems to evolve more complex shapes and functions 2. A fluid lined chamber offers protection to the gut and other organs; acts as a cushion 3. The fluid filled body cavity acts as a skelton - 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 Also, 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 Although the cuticle is generally smooth, various structures such as spines, bristles, warts, papillae, and ridges may be present on it Some of these specialized 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 It consists of 4 basic layers: epicuticle, exocuticle, mesocuticle, and endocuticle A basal lamina separates the cuticle from the underlying hypodermis which forms the cuticle
2 The hypodermis protrudes into the pseudocoelom along the middorsal, midventral and lateral lines to form the longitiudinal hypodermal cords These partially divide 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 Collectively, the cuticle, hypodermis, and somatic musculature make up the body wall A convenient classification system to describe the muscle arrangement has been devised based on the number of rows of muscle cells per quadrant An arrangement of multiple longitiudinal rows of muscle cells in each quadrant is termed polymyarian, one with nomore than2 rows of cells is called holomyarian, and one with 2 to 5 rows is meromyarian Each muscle cell comprises a contractile portion containing myofibrils and a noncontractile portion in which are found the various organelles, such as the nucleus, mitochondria, etc. Sensory processes usually extend from the non-contractile portion of each cell to the longitudinal nerve trunks Digestive Tract The digestive tract is complete; it consists of an anterior mouth, a gut that comprises 3 major regions (foregut, midgut, and hindgut), a cloaca and a subterminal vent Foregut The cuticle-lined foregut begins at the mouth, which in many species opens into a buccal capsule and continues as the esophagus When present, it may contain spears, stylets or teeth for attachment to or penetration of the host for acquiring food The buccal cavity (or mouth) leads into the esophagous that is lined with cuticle The esophagous may be completely muscular or completely glandular, or the anterior half may be glandular and the posterior half muscular Esophageal action is often enhanced by one or more muscular enlargements called bulbs
3 Midgut 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 Hindgut 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 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 Chemoreceptors - Amphids are chemoreceptors located in shallow anterior depressions or pits Phasmids are a set of chemoreceptors at the posterior end
4 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 Reproductive Systems Nematodes are usually dioecious Male System While there is usually a single testis, 2 are not uncommon The vas deferens (sperm duct) extends distally to the cloaca Two specializations of the vas deferens are evident before it enters the cloaca These are the seminal vesicle (sperm storage) and the ejaculatory duct Male nematodes are usually equipped with one or more (commonly 2) copulatory spicules These cuticular structures are encased within spicule pouches located laterally in the cloacal wall The spicules are imporatnt during copulation in that they keep the female vulva open, thus facilitating the entery of sperm into the female reproductive tract Other accessory structure may be present, including a sclerotized spicular guide or gubernaculum This structure is located along the dorsal wall of the spicule pouch and serves to guide the spicules when they are extended Interestingly, nematode sperm has no flagella or acrosomes Some move by means of pseudopodia others move by means of cytoplasmic extensions Female System Female nematodes are 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
5 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 hydrostatic 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 Eggs can hatch either within the host or in the external environment In the latter case, the first-stage larva usually emerges Hatching of eggs in the external environment is,in part, controlled by such ambient factors as temperature, moisture, and oxygen tension Eggs will hatch only when the external conditions are favorable, thus assuring that the emerging larva does not enter an unduly harsh environment 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 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., L 1, L 2, L 3, L 4 ) The first stage larva being the stage prior to the first molt
6 However various other designations also are used for specific nematode larval forms as follows: 1. 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 2. 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 3. 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 adult structures are visible within the pseudocoelom This larva is generally found in circulating blood and cutaneous tissues