Invertebrate Anatomy OnLine
Ascaris suum ©
Copyright 2001 by
is one of many exercises available from Invertebrate
Anatomy OnLine , an
Internet laboratory manual for courses in Invertebrate Zoology. Additional
exercises, a glossary, and chapters on supplies and laboratory techniques are
also available at this site. Terminology
and phylogeny used in these exercises correspond to usage in the Invertebrate
Zoology textbook by Ruppert, Fox, and Barnes (2004). Hyphenated
figure callouts refer to figures in the textbook. Callouts
that are not hyphenated refer to figures embedded in the exercise. The glossary
includes terms from this textbook as well as the laboratory exercises.
Introverta, Nematoida, Nematoda P,
Secernentea C, Rhabditida O,
Ascaridida, Ascaridina, Ascaridoidea SF,
Ascaridae F, (Fig 22-35,
includes Gastrotricha, Nematoda, Nematomorpha, Priapulida, Kinorhyncha, and
have a secreted cuticle and lack locomotory cilia so that locomotion is
accomplished with muscles. The brain is a circular band around the anterior gut
composed of forebrain, midbrain, and hindbrain (Fig 22-2A). The pharynx is
radially symmetrical (Fig 22-15). In most the body is compact without a body
cavity but in large species there may be a spacious hemocoel. Eutely, with about
1000 cells, is common.
20,000 known nematode species inhabit terrestrial, marine, and freshwater
environments and are found in almost all moist habitats. The
taxon includes numerous plant and animal parasites, many of which are of medical
or agricultural importance, but most are free-living (non-parasitic). Most
nematodes, or roundworms, are long, slender, almost featureless externally,
tapered at both ends, and round in cross section (Fig 22-7A). The
body cavity, if present, is a hemocoel derived from the blastocoel.
body is covered with a thick extracellular cuticle secreted by a cellular or
syncytial epidermis that is molted during juvenile development (Fig 22-11,
epidermal nuclei are sunken below the epithelial layer into four longitudinal
epidermal cords that extend the length of the animal. The
body wall has well-developed longitudinal but no circular muscles.
gut is complete with terminal anterior mouth and subterminal posterior anus. It
comprises ectodermal foregut and hindgut and an endodermal midgut. The body
cavity, or hemocoel, is not lined with mesothelium and there is no muscle,
connective tissue, or other mesodermal derivative associated with the midgut.
nervous system is a ganglionated circumenteric ring, hence the name
“cycloneuralia”, with several longitudinal nerve cords, the most important of
which is the ganglionated, double, ventral cord (Fig 22-11A). The
nerve cords are located in the longitudinal epidermal cords, along with the
epidermal nuclei. Cytoplasmic
innervation processes from the longitudinal muscles extend to the longitudinal
nerve cords and serve the function of motor neurons, which are absent. Sensory
equipment may include unique chemosensory amphids and sensory bristles around
nematodes lack cilia or flagella, even in the sperm. There
are, however, ciliary derivatives in the amphids (Fig 22-8B) and cilia are
present in the gut epithelium of some nematodes (Fig 22-16A). Roundworms
are ammonotelic and nitrogen excretion is mostly by diffusion across the body
is accomplished by an excretory canal system in some and perhaps by excretory
nephridia are present.
are typically gonochoric and fertilization is internal with copulation. Sexual
dimorphism is common. Nematode
sperm have no flagella and probably employ amoeboid locomotion. Nematode
development features a phenomenon known as chromosome diminution in which much
of the chromosome material of presumptive somatic cells degenerates and is lost
(Fig 22-20). Germ
cells, however, retain the full complement of genetic material. Development
is direct and includes four juvenile and one adult instar separated from each
other by molts. Most
nematodes are small (<3 mm) and free-living but some of the parasitic species,
such as Ascaris, may reach
Once known as Phasmidia, Secernentea includes
terrestrial nematodes and many important parasites. Free-living, non-parasitic
species, such as Cephalobus,
are usually soil dwellers. Phasmids are present and the amphids are porelike. Excretory
canals, and sometimes excretory glands, are present (Fig 22-17). Epidermal cells
can be mono- or multinucleated.
includes the largest nematodes and several members of the family are large
enough to be dissected in invertebrate zoology laboratories. Ascaris
suum, the pork roundworm, is a large, cosmopolitan species reaching lengths
up to 50 cm (Fig 22-21). It
is convenient for laboratory studies of nematode anatomy because of its large
size and availability. An
almost identical species, A.
lumbricoides, occurs in humans, sheep, cattle, apes, and squirrels. The
two were long thought to be the same species and until recently both went by the
name A. lumbricoides. Preserved
specimens are available from biological supply companies. Commercially supplied
specimens are usually labeled A.
lumbricoides but may be either
species. Cross section slides are also available commercially.
a preserved adult Ascaris in
a long narrow dissecting pan of tapwater. While
not essential, the dissection is best conducted with a dissecting microscope. Preserved
worms are delicate and should be handled carefully.
at the surface of the worm with the dissecting microscope and note that it is
firm and resists deformation. It
is covered with a thick proteinaceous cuticle which
plays an important role, in the absence of circular muscles, in containing the
high hydrostatic pressure of the hemocoel. Look
for the characteristic ornamentation of the cuticle, which in this species
consists of fine circumferential ridges. The
cuticle of preserved specimens is fragile and rough handling will cause it to
break or peel away.
Figure 1. View
of the left side of a male Ascaris.
the sex of your specimen. Females reach larger sizes than males. The
posterior end of males is curved ventrally and looks like a shepherd’s crook
(Fig 1). Two tinycopulatory spicules may
be visible protruding from the anus on the inside curve of the crook. The
posterior end of females is not noticeably curved.
Figure 2. Dorsal
dissection of a female Ascaris. The
right side of the reproductive system is omitted and the left side has been
moved to the left to expose the gut. The
reproductive system has been simplified and untangled for clarity.
between the anterior and posterior ends. The curled posterior end of males makes
this easy for that sex but females are straight with similar anterior and
posterior ends. In
both sexes, however, the mouth is
terminal at the anterior end but the posterior end has no terminal opening. Viewed
head-on with the help of a hand lens, the mouth can be seen to be surrounded by
three small lips. (It
is easier to view the mouth with a hand lens than with the dissecting microscope
because of the problem of orienting the long worm vertically on the stage.)
three lips of Ascaris are
formed by fusion of the six lips of the ancestral nematodes (Fig 22-8A). One
of the lips is dorsomedian in position whereas the other two are ventrolateral. The
arrangement of the mouth and its lips is radially symmetrical. The
subterminal anus of
both sexes is located slightly anterior to the posterior tip of the worm (Fig
is a transverse ventral slit and is the best landmark for recognizing the
ventral surface. Its
position just behind the tip on the ventral surface is referred to as
mouth, which is at the extreme tip of its end, is terminal.)
female gonopore, known as the vulva,
is located on the midventral line about 1/3 of the animal’s length posterior to
the mouth. It
is a small pore best found with magnification. The female reproductive system
opens to the exterior independently of the gut and there is no cloaca in this
sex. The male reproductive system does not have its own external gonopore and
sperm exit the animal via the anus (= vent). Two
protrusible copulatory spicules, which are part of the male copulatory
apparatus, may extend from the anus in some specimens.
Knowing ventral, anterior,
and posterior, you
can now find dorsal, right,
and left. Find
the plane of symmetry and
the dorsal midline.
four longitudinal cords in the body wall are visible from the exterior as thin,
pale stripes Fig 1, 2, 3, 4, 5, 22-10B,C, 22-11). These
are the dorsal, ventral,
and two lateral longitudinal
epidermal cords. They
are faint, but discernable with good light, and are concentrations of the
epidermal nuclei. The
two lateral cords are easiest to see. A tiny pore belonging to the excretory
canal system is located immediately posterior to the mouth on the ventral
midline but is usually not visible.
a worm in a dissecting pan of water with its ventral side down. Males
must be rotated a little to accommodate the curl of the tail.
the worm gently with thumb and forefinger of one hand and use a # 1 insect pin
to scrape a longitudinal middorsal incision through the cuticle and longitudinal
muscles of the body wall in the anterior third of the body. Do
your best to keep the incision on the dorsal midline even though there are not
many landmarks to guide you. The
two lateral epidermal cords, which are revealed by your incision are large and
conspicuous longitudinal ridges on the inside of the body wall that can be used
for orientation. You should make the dorsal incision so a lateral epidermal cord
is equidistant from either side of it. You
will not be able to see the cords at first and must wait until the incision is
long enough to spread the walls apart. In the anterior end of the body there are
few organs to damage but you should be careful nevertheless. You
can develop your skill with the insect pin in this region where the possibility
of damage is lessened.
cuticle tends to resist the cutting motion of the pin so that it separates
unevenly but several scrapes in the same place will penetrate it. The
longitudinal muscles inside the cuticle, on the other hand, help guide the pin
in the correct direction and there is no connective tissue or circular muscles
to impede the passage of the pin.
the incision anteriorly to the mouth. Deflect
the cut edges of the body wall and pin them to the wax using # 1 insect pins
inserted at 45 ° angles. Be
careful as you pin the body wall aside. The
internal organs, especially the gut, are very delicate and break easily. Further,
the gut often adheres to the body wall and is easily pulled past its breaking
point by movement of the body wall.
you have opened and pinned the anterior third of the worm, extend the incision
posteriorly to the end of the body, deflecting and pinning the walls. Opening
the middle region of the worm is a bit more difficult because it is packed with
the reproductive system (Fig 2).
wall will be studied in more
detail later using cross section slides (Fig 3, 4, 5, 22-11) but some of its
features are visible in gross dissection. The
heavy, transparent cuticle is
its outermost layer. Immediately
inside the cuticle is the inconspicuous, thin epidermis. Inside
the epidermis is a thick, white sheath of longitudinal
muscles composed of a single
layer of cells which protrude into the hemocoel (Fig 4).
or body cavity, is filled with fluid under exceptionally high pressure (higher
than that of any other animal) and is a hydrostatic skeleton. Virtually
all other organ systems are affected by this pressure and must be able to
function under its influence. The
pressure maintains the body shape and acts as a hydrostatic skeleton against
which the body wall muscles act to accomplish locomotion.
nuclei of the epidermal cells are concentrated in the four longitudinal epidermal
cords (Fig 1,2,3,4,5,
two lateral epidermal cords are
large and conspicuous and protrude into the hemocoel. The
dorsal and ventral cords are much less evident and the dorsal cord is usually
destroyed by the middorsal incision. You
must push the surrounding muscle cells aside to see the ventral and dorsal
epidermal cord includes at least one longitudinal nerve cord and an excretory
canal is present in each lateral cord. The
epidermal cords divide the somatic musculature into dorsal and ventral fields
and make convenient landmarks.
The gut is
a long, straight tube running from mouth to anus (Fig 2, 22-10). It
is composed of an anterior, ectodermal foregut, endodermal midgut, and
the terminal mouth.
The foregut, or stomodeum,
comprises the buccal cavity and pharynx, which, consistent with their ectodermal
origins, are lined with cuticle. The
mouth opens into the small, inconspicuous, thin-walled buccal cavity (Fig 2,
posterior to the buccal cavity is the longer, thicker-walled pharynx whose
heavily muscularized walls are used to suck food into the gut in opposition to
the high hydrostatic pressure of the hemocoel (Fig 2, 22-14B). The
posterior end of the pharynx is swollen slightly and the pharyngeal lumen is
triangular in cross section as is that of other cycloneuralians (Fig 22-15).
at a commercially prepared slide of a cross section made through the pharynx
(Fig 22-15). Note
the thick muscular pharyngeal walls and the triradiate lumen with its cuticular
lining. When filled
with food, the lumen expands and becomes circular.
midgut, or intestine,
begins immediately posterior to the pharynx (Fig 2, 22-10A). It
is a long dorsoventrally flattened tube that extends posteriorly almost to the
the ectodermal foregut, its walls consist solely of a simple columnar or
cuboidal epithelium and its basal lamina. There
is no associated muscle, connective tissue, or mesothelium.
intestine is the region of hydrolysis and absorption. In the middle of the body
your view of the intestine is probably obscured by the reproductive system but
you can find it again posterior to this region. Ascaris subsists
chiefly on monomers (sugars and amino acids) from the intestinal contents of its
host. These are absorbed by the microvilliated midgut epithelium.
intestine extends posteriorly to join the short ectodermal hindgut, or rectum (Fig
females, the rectum is difficult to differentiate from the intestine but in
males the rectum is acloaca which
receives the male gonoduct and the intestine before opening to the exterior via
the anus (Fig
ectodermal, the rectum is lined with cuticle.
metabolism is anaerobic and there are no special gas exchange structures.
Fluid Transport System
unpartitioned hemocoel (= pseudocoel) of nematodes obviates the need for a hemal
system and there is none. Transport
is thought to be accomplished by diffusion in small species and by movement of
the hemocoelic fluid (blood) in large species.
excretory system consists of an enormous H-shaped canal system contained within
a single cell (Fig 22-17A,B). The
uprights of the “H” are longitudinal canals located in the lateral epidermal
cords and extend over the entire length of the worm. The two longitudinal canals
connect with each other via a transverse canal near the anterior end of the
short excretory duct leads from the transverse canal to the excretory pore on
the anterior ventral midline. The
system is thought to be chiefly osmoregulatory. The excretory canal system is
difficult to observe in gross dissection of preserved whole specimens. The
excretory pore is located immediately posterior to the mouth on the ventral
midline but it is difficult to find.
of the nervous system of Ascaris requires
specially prepared material and will not be attempted. The
central nervous system consists of a characteristic cycloneuralian brain which
is a tripartite nerve ring around the pharynx (Fig 22-11A). Try
to find the brain. Dorsal, ventral and lateral longitudinal nerves arise from
the brain and extend posteriorly in the epidermal cords. Of
these the ventral nerve cord is most important and is a double ganglionated
cord. The dorsal cord is single and unganglionated. A
small lateral nerve cord is present in each lateral epidermal cord. Ascaris has
locomotory system comprises the pressurized hemocoel, which is a hydrostatic
skeleton, the antagonistic dorsal and ventral longitudinal muscle fields of the
body wall, and the elastic cuticle, which contains the hydrostatic pressure and
opposes the longitudinal muscles. When
one muscle field contracts, the opposite side of the body lengthens to relieve
the hydrostatic pressure and the cuticle on that side stretches (Fig 22-12A). Alternate
contractions of dorsal and ventral muscle fields result in sinusoidal waves in
the dorso-ventral plane passing along the length of the body. The arrangement of
the protein fibers in the cuticle allow changes in the length of the worm but
not its diameter. This
results in the characteristic dorso-ventral thrashing motion that can be
translated to efficient forward movement in a viscous medium or an environment,
such as wet sand or the walls of a host’s intestine, with solid or resistant
surfaces to push against. Nematodes
(except for the very small) are relatively helpless in pure water and are not
capable of directed motion but have efficient locomotion when there is a
substratum to push against.
living nematodes are available in the laboratory, place some in a Petri dish of
water and watch their motion with a dissecting microscope. Add
some sand grains to the dish and note the difference in the effectiveness of
reproductive system is a tube with the gonads continuous with the gonoducts so
gametes are not released into the hemocoel (Fig 22-18A, 22-19A). Both male and
female systems are long tapered tubes lying coiled in the hemocoel. The
upstream, solid, free ends of the tubes are small in diameter but expand and
become hollow as they extend downstream toward the gonopore. The solid upper
ends are the gonads, ovaries or testes. The hollow, larger regions are
specialized for various purposes, including transport and storage of gametes.
Mitotic divisions of primordial germ cells in the gonad produce diploid gonial
cells which move down the gonoduct undergoing gametogenesis on the way. Study
the reproductive system of your specimen and then look at a dissection of the
opposite sex. You should be familiar with both sexes.
ascarids have a Y-shaped reproductive system consisting of two tubes, each with
an ovary, oviduct, and uterus forming an arm of the Y (Fig 2, 22-10). The
two arms join to form a common (unpaired) vagina which is the stem of the “Y”. The
vagina empties to the exterior via the single gonopore. It
is convenient to trace the system backwards beginning at the gonopore but keep
in mind that female gametes travel in the opposite direction. The
upper, small diameter ends are referred to as “ upstream”.
the approximate position of the vulva on the midventral line and look on the
inside surface of the body wall to find a short tube attached to the body wall. This
tube is the vaginaand
the vulva opens into it (Fig 2, 22-10A). The
vagina is formed by the union of two large, convoluted, tubular uteri (Fig
2, 22-19A). Each
uterus extends posteriorly, decreasing in diameter, almost to the posterior end
of the hemocoel.
of each uterus is the small-diameter oviduct (Fig
small swelling, the seminal
receptacle, is situated at the junction of the oviduct and uterus (Fig
2, 22-19A). The
oviductsextends anteriorly, without much change in diameter, to about the level
of the vagina where it turns and runs posteriorly again. The
two oviducts are coiled around the uteri, the gut, and themselves.
the middle of the body each oviduct decreases in diameter to become an ovary. The
ovaries are solid, not tubular, and form a mass of small-diameter threads in the
middle of the worm.
are produced by mitotic divisions of primordial germ cells in the upper ovary. The
oogonia move downstream to the upper oviduct where additional mitotic divisions
produce primary oocytes. The
cell is fertilized in the seminal receptacle before meiotic divisions begin,
while it is still a diploid oocyte. Once
fertilized, it enters the uterus where it develops a chitinous eggshell and
undergoes oogenesis followed by embryonic development. The
uterus contains shelled "eggs" in all stages of oogenesis and embryonic
reproductive system of male ascarids resembles that of females but is only one
tube rather than two (Fig 22-18A). The solid free upstream end of the tube is
the testis. It
is a small, white, solid thread coiled in the posterior third of the hemocoel. As
it twists back and forth it gradually increases in diameter. Eventually
it turns anteriorly and becomes the vas
deferens, or sperm duct. This
region of the male duct is hollow and contains male sex cells undergoing
makes several loops in the middle of the hemocoel, some of which extend
posteriorly to the region of the testis.
last loop of the vas deferens turns posteriorly again and expands abruptly to
become the seminal vesicle,
a large-diameter tube that runs straight posteriorly in the posterior third of
the animal. Its
diameter increases as it extends posteriorly and spermatozoa are stored in it.
reaching the posterior end of the body, the male duct decreases in diameter once
again and becomes the ejaculatory
muscular duct extends posteriorly from the end of the seminal vesicle to join
the rectum, or cloaca, immediately anterior to the anus. A
region with secretory epithelium, the prostate gland, lies between the seminal
vesicle and the ejaculatory duct. Two
spicules are located beside
the cloaca. The
spicules are housed in deep pouches opening from the cloaca (Fig 22-18A,B). During
copulation these are extended out of the anus and into the vulva of the female
to hold the vulva open (Fig 22-18D).
stained cross sections of the region of the pharynx (Fig 3, 22-15) and more
posterior sections showing the male and female reproductive systems (Figs
4, 5). Orienting
these sections is sometimes difficult and you cannot rely on position on the
slide for clues. The
lateral epidermal cords are much larger than the dorsal and ventral cords and
can be used to distinguish lateral from dorsal/ventral (Fig 4, 5). Distinguishing
dorsal from ventral is difficult but the best landmark is the gut, which is
usually (but not always) in the dorsal half of the hemocoel. Fortunately
it doesn’t make much difference if you can’t make the distinction between dorsal
and ventral but it is important that you distinguish lateral from dorsal and
body wall can be studied on any of your cross sections but, if possible, it is
most convenient to begin with the pharyngeal cross section (Fig 3). The
outermost layer of the body
wallis the thick cuticle. It
is a nonliving extracellular secretion that stains pink in most preparations
(Fig 3, 4, 22-11).
inside the cuticle is the thinner epidermis which
secretes the cuticle (Fig 3). On
most slides the epidermis appears as a thin, pale, pink layer but it may be
separated from the cuticle by a white space. If
present, this space is an artifact resulting from the slide-making process and
in life there is no space between cuticle and epidermis. Together
the epidermis and cuticle make up the integument.
epidermis is a syncytium whose nuclei are located in the four epidermal
cords sunken into the body
cavity. The right and left
lateral epidermal cords are
large and easily located (Fig 3, 4). The
inconspicuous excretory ducts are in these cords.
The dorsal and ventral
epidermal cords are much
smaller but can be found by careful inspection. The dorsal and ventral
longitudinal nerve cords are
usually visible in the dorsal and ventral epidermal cords respectively.
thickest part of the body wall is the longitudinal
muscle layer (Fig 3, 4,
is a single layer of large cells which bulge far into the body cavity and occupy
much of it. Each muscle
cell comprises an obvious peripheral and a less evident central portion. The
peripheral, or fibrillar,
region sits on the inside of the basal lamina of the epidermis and contains the
contractile fibers of the cell. It
is easily recognized because the fibers stain dark pink and form a thick outline
around this portion of the cell.
nucleus and most of the cytoplasm (or sarcoplasm), however, are in a large, but
less conspicuous, bulging, centrally located sarcoplasmic
region that extends deep
into the body cavity. The
outlines of this part of the cell are not as apparent as are those of the
two lateral epidermal cords divide the longitudinal muscle sheath into
antagonistic dorsal and ventral muscle fields. In
nematodes the axons of motor neurons do not exit the central nervous system
(nerve cords) and do not extend to the muscles. Narrow
sarcoplasmic extensions, or innervation
processes, arise from the apical ends of the muscle cells and run to a
dorsal or ventral nerve cord to synapse with neurons confined to the cord. The
innervation processes carry motor commands from the central nervous system to
the muscles in the absence of the peripheral axons that perform this function in
white space in the interior of the worm is the hemocoel (=
pseudocoel), or body cavity, and in it are the digestive and reproductive
systems (Fig 3,4). The pharynx (=
esophagus) is round in cross section and has thick muscular walls (Fig 3). At
rest its lumen is collapsed and is triradiate (= Y-shaped). The
lumen is dilated by contraction of the radial muscles in the pharyngeal walls. As
part of the foregut, the pharynx is lined by epidermis, which is not visible,
and has a thick cuticular
lining, which is.
The intestine is
usually flattened (Fig 4, 5) but may be dilated and very large. At
high power you can see that the intestinal walls are composed of a monolayered
very tall cells. The
basal ends of the cells rest on a basal
lamina, which you can see around the outside of the gut tube. The
basal lamina separates the epithelium from the hemocoel and there are no muscles
or mesothelium associated with the gut. The
apical ends of the epithelial cells are microvilliated and form an absorptive brush
border which is visible as a
dark line around the midgut lumen. The
hemocoel is bounded on the outside by somatic musculature, which is mesodermal,
and on the inside by the midgut epithelium, which is endodermal.
Figure 3. Cross section through the pharynx of Ascaris.
prepared slides usually have one section each from male and female specimens. These
sections are made through posterior regions of the body and include several
sections through regions of the reproductive tube(s).
the cross section through a male Ascaris (Fig
cross sections are usually smaller in diameter than female and lack the large
egg-filled uteri (Fig 5). The male reproductive system is a long much-coiled
tube. The plane of section will have passed through this tube many times and
each specialized region of the tube will probably be represented many times. The
number of times depends on the level of the section.
Figure 4. Cross
section of a male Ascaris. Nematode56La.gif
proximal, upstream end of the tube is the testis which
is solid, small in diameter, and enclosed by an epithelium (Fig 4, 22-18A). It
is filled with small, spherical primordial
germ cells and has no lumen. The
primordial germ cells are associated with a branching core, or rachis. Mitotic
divisions of the germ cells produce spermatogonia which move downstream to
undergo spermatogenesis. Several sections through the testis may be present.
next region of the male tube is the vas
deferens . It is
slightly larger in diameter than the testis and is also enclosed by a thin
interior is filled with spermatogoniaand
their daughters undergoing spermatogenesis. The
organization of the contents of the vas deferens is looser than that of the
testis and its sex cells are larger and not attached to a rachis. A
lumen is present although it may not be apparent since it is filled with
developing germ cells. There should be several sections through the vas deferens
in the hemocoel of your specimen.
next region of the male duct is the seminal
spermatozoa are stored here (Fig 22-18C). Most
slides have a single section through the seminal vesicle but one made too far
anteriorly will have none. In
life, the seminal vesicle is larger in diameter than the other regions of the
male system but in preserved material it often contracts and may be smaller. The
epithelial walls of this region are much thicker than those of upper regions of
the reproductive tube and this is the best way to recognize it. The
ejaculatory duct, which is the downstream-most region of the male system, is too
far posterior to be present in the same cross section as the testis and vas
the cross section of a female Ascaris and
its reproductive system. It
should be recognizable by its two large egg-filled uteri. The
female reproductive system consists of two long much-coiled tubes (Fig 2,
plane of section passes through these tubes many times and each region of a tube
will probably be represented many times. The
number depends on the level of the section. Most sections should include cuts
through the ovaries, oviducts, and uteri. The
twisting, coiled nature of the reproductive system results in multiple sections
through the ovaries and oviducts, but probably not the uteri. Unlike
that of the male, the female reproductive system is double so there are two
ovaries, two oviducts and two uteri. There
is only one vagina and gonopore but these will not be represented on the slide.
smallest sections are of the ovary (Fig
5, 22-19A). These
are easily recognized because they are solid whereas the oviducts and uteri are
hollow (albeit so full of eggs they may seem to be solid). The
ovary has a central cellular core, known as the rachis,
from which radiate a single layer of long narrow primordial
germ cells. Their
nuclei are usually easy to see. Mitotic
divisions of the germ cells produce oogonia which move downstream as they
undergo oogenesis. The ovary and oviduct are surrounded by thin epithelia which
are often pulled away from the germinal cells leaving a white space between
space is an artifact. The rachis may appear to be a tiny lumen but it is not. If
you look closely, you will see pale, lightly staining cells in it. (If
there are no cells here and the center of the organ is white, you are looking at
an oviduct, not an ovary.)
The oviduct has
much the same appearance as the ovary but is a little larger in diameter and is
hollow (Fig 5). Its
epithelium is similar to that of the ovary and is thick but it has a smalllumen instead
of a rachis. Around
the lumen is a single thick layer of oogonia (upstream) or oocytes (farther
of which there should be two in most cross sections, are much larger than either
oviducts or ovaries. They
are filled with large, shelled “eggs” in various stages of oogenesis and
development and the uterine wall is thick and muscular.
ascarid life cycle involves only one host which becomes infected when it ingests Ascaris eggs
in its food or water. These
hatch in the intestine and juvenile worms migrate to the liver where they enter
the host’s hemal system. They
are carried in the blood to the lungs where they enter the lumen of the alveoli. From
here they crawl to the pharynx, then follow the gut lumen to return to the small
intestine where they mature into adult roundworms and feed on chyme.
Figure 5. Cross
section of a female Ascaris featuring
the reproductive system. The
details of the body wall have been omitted. Nematode57La.gif
numbers of worms may be present in a single host (Fig 22-21). After
maturation, copulation occurs and females produce and release shelled eggs which
leave the host in the feces. When
fully developed and infective, an ascarid “egg” contains a tiny juvenile worm
capable of infecting a new host. Ascarid
eggs are difficult to kill and remain viable in soil for as long as 20 years and
are widespread in the environment. Development
is direct with no distinct larva. The
juveniles undergo four molts to become adults (Fig 22-9).
of eggs and subsequent infection is most common in children due to their habit
of playing in grass and sand and indiscriminately ingesting a variety of
potentially contaminated materials.
CG. 1950. Ascaris
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Long, narrow dissecting pan (aluminum ice cube tray with wax
bottom is ideal)
# 1 stainless steel insect pins
10X hand lens
Cross section slides of male and female reproductive systems
Cross section slide through pharynx
Living small nematodes such as Cephalobus
section slides of the reproductive region of male and female Ascaris are
available from Carolina Biological and Ward’s Natural Science. Cross
section slides of the pharyngeal region are available from Ward’s