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., L1, L2 ,L3,
L4)
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