Invertebrate Anatomy OnLine
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,
Cephalobidae F, (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.
a small, free-living nematode that inhabits soil where it feeds on dead plant
material and bacteria. It
is viviparous and females gestate the embryos and release juveniles. Living
cultures are available at low cost from biological supply companies.
Place a small bit of culture in poikilotherm saline solution
(see Supplies chapter) in a small culture dish and observe the worms with the
dissecting microscope. Adults are 1 mm or less in length and exhibit the
quintessential vermiform shape typical of nematodes, being cylindrical in cross
section and tapered at both ends. The
anterior end, however is blunter than the acutely pointed posterior end and the
two are easily distinguished.
Figure 1. Male Cephalobus.
Note the characteristic thrashing motion that results from
the alternating contraction of dorsal and ventral longitudinal body wall muscles
in the absence of circular muscles (Fig 22-12A). This motion is ineffectual in a
smooth-bottom culture dish but in a complex substratum, with sand grains or
other particles to push against, the worms move rapidly and efficiently in the
a few sand grains to the dish and observe the efficacy of the longitudinal
muscles in a complex substratum. <
Make a wetmount of a few nematodes and cover it with a
not use Protoslo or wax feet on the coverslip. Examine the preparation with the
compound microscope and find relatively inactive worms to study. It
may be necessary to heat the slide slightly to kill the worms so you can observe
is accomplished with a small flame from a butane lighter or small gas laboratory
the flame briefly below
the slide. More
than one attempt may be required to achieve the correct amount of heat.
Search the slide for specimens and distinguish between males
and females. Choose the largest specimens you can find for observation. The male
posterior end tapers quickly to a point and is curved whereas that of females
tapers more gradually (Fig 1, 2). Males
also have a pair of copulatory spicules protruding from the cloaca near the
posterior end of the worm. Females,
being viviparous, usually contain juvenile worms of various sizes (Fig. 2). Study
representatives of both sexes.
In both sexes the reproductive system is a single tube
extending anteriorly from the gonopore but the location of the gonopore differs.
The upper, anterior end of the tube is reflexed, or bent posteriorly, and is the
gonad, where gametogenesis begins and gonial cells are produced (Fig 1, 2,
portions are modified for various functions in the two sexes.
The integument consists of a thick cuticle,
which is underlain by the epidermis, and a thick layer of longitudinal muscles
(Fig 22-10B). There
are no circular muscles and no mesothelium. The
body cavity is an unlined hemocoel filled
with the digestive and reproductive systems.
The gut and reproductive systems occupy all of the space in
the hemocoel and each is likely to obscure your view of the other. You
should work at 400X and use the fine focus to take optical sections in an
attempt to resolve the parts of the two systems. The
anterior end of the gut is easy to see and distinguish because there is no
reproductive system in this part of the worm.
Find a male and
study it first (Fig. 1). Distinguish
between the anterior and posterior
anterior end tapers to a blunt point whereas the posterior end tapers abruptly
to a sharp tip. The gut is
the most conspicuous feature. It
begins with a terminal mouth at
the extreme anterior end of the worm. The
mouth opens into a small buccal
cavity (Fig 1, 22-13A). The
buccal cavity is followed by the large pharynx that
is easy to see because of its thick muscular walls (Fig 1, 22-14B). The
pharynx is part of the foregut and as such is ectodermal and lined with cuticle. The
thickness of its walls is due to radial muscles which, upon contraction increase
the diameter of the lumen thus creating a suction that draws food into the mouth
(Fig 22-15). The
pharynx has three distinct parts. The
anterior region is the corpus and
is the longest (Fig 1). The
corpus is followed by a narrow isthmus which
opens into a spherical pharyngeal
brain is a band around the pharynx.
The midgut, or intestine,
which is endodermal and is not lined by cuticle, follows the pharyngeal bulb and
extends as a thick tube posteriorly almost to the posterior end of the worm (Fig
about the level at which the body begins to taper abruptly, the intestine
narrows and turns ventrally to become the cloaca (Fig
22-18A). The cloaca opens to the exterior via theanus on
the ventral midline.
The male reproductive system consists of a single long tube
extending over most of the length of the worm (Fig 1, 22-10D, 22-18A). It
is reflexed at the anterior end although this is not always apparent. The
reflexed anterior end of the tube is the testis,
where primordial germ cells undergo mitotic divisions to produce spermatogonia. The
remainder of the tube is the vas
deferens, or sperm duct, which stores sperm and joins the hindgut. The
vas deferens is regionally specialized into a seminal vesicle for sperm storage,
a secretory prostate, and a muscular ejaculatory duct. The chamber formed by the
union of the vas deferens and the hindgut is the cloaca. The cloaca opens to the
exterior by the anus. A
pair of copulatory spicules may
be visible beside, or protruding from, the cloaca.
Now find a female and
study it (Fig. 2). Distinguish
between the anterior and posterior
anterior end tapers to a blunt point whereas the posterior end tapers gently to
a sharp tip. The
easiest way to recognize females is by the presence of juvenile worms in the
reproductive system (Fig 2). The hemocoel of
females contains the gut and reproductive system, as does that of the male, but
appears more crowded because the reproductive system is larger.
The female gut is
similar to that of males except that there is no cloaca. In
female nematodes the gut and reproductive system are independent of each other
and they do not share ducts or openings as they do in the male. Consequently
there are two external openings rather than one (Fig 2, 22-10A).
You should be able to find the same regions of the gut, with
the exception of the cloaca, that you found in the male. The
anterior mouth opens
into a small buccal cavity which
leads to the large, conspicuous pharynx (Fig
pharynx is divided into corpus, isthmus and bulb. The
brain is a band around the pharynx. The intestine,
or midgut, extends posteriorly from the pharyngeal bulb as a large tube almost
the same diameter as the body. It
may be difficult to distinguish the intestine from the uterus. In
most commercial cultures the gut contains recognizable potato granules that can
be used to identify the gut. The
intestine narrows to become the short hindgut, which then opens to the exterior
via the anus on
the ventral midline very close to the posterior end of the body. Females
have no cloaca.
In Cephalobus the
female reproductive system is a single large tube that may fill most of the
hemocoel and obscure your view of the gut. The tube ends blindly at its anterior
end and opens to the exterior via female gonopore, on the midventral line about
2/3 of the distance from the tip of the head to the tip of the tail. The tube is
reflexed at the anterior end as is that of males. The upstream, anterior end is
the ovary where oogenesis begins with production of oogonia from primordial germ
cells. The ovary is difficult to discern in most specimens.
Figure 2. A female Cephalobus.
Posteriorly, the ovary widens to become the much larger and
more conspicuous uterus (Fig
2, 22-19). In
most mature females you should see coiled juvenile
worms of various ages in the
uterus narrows to become the short vagina which
opens to the exterior via the female gonopore. The
gonopore is on the midventral line about 2/3 of the distance from the anterior
to the posterior end of the worm. The seminal
receptacle is a large
diverticulum of the vagina on the posterior side of the gonopore (Fig 2). It
receives and stores sperm following copulation. In
many nematodes the reproductive system consists of two twin tubes, each with
ovary and uterus, which join to form the vagina (Fig 22-19).
During copulation the male inserts the copulatory spicules
into the vagina to hold it open while sperm are ejaculated from the cloaca (Fig
are stored in the seminal receptacle and make their way upstream to fertilize
the oogonia as they leave the ovary. Once
fertilized, the zygotes begin development and are gestated in the uterus until
they become small worms. They
are then released from the vagina through the gonopore.
J-S . 1978. Laboratory
Manual for Invertebrate Zoology Emphasizing Marine Forms. Hwong,
Los Alamitos, Calif. 152p.
LH. 1951. The
Invertebrates: Acanthocephala, Aschelminthes, and Entoprocta. The coelomate
Bilateria, vol. III. McGraw-Hill,
New York. 572pp.
Ruppert EE, Fox RS,
Barnes RB. 2004.
Invertebrate Zoology, A functional evolutionary approach, 7 th ed.
Brooks Cole Thomson, Belmont CA. 963 pp.
8-cm culture dish
poikilotherm saline solution (0.5-0.7%)