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 can be accessed by clicking on the links on the left. A
glossary and chapters on supplies and laboratory techniques are also available. 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.
Turbellaria C, Seriata,
Tricladida O, Dugesiidae F (Fig
or platyhelminths, are bilaterally symmetrical metazoans with three tissue
most triploblastic animals, they are compact and have no coelom (body cavity)
surrounding the viscera and no hemal system. The
gut, if present, has a single opening to the exterior. An
anterior brain with associated concentration of sense organs is present, as
expected of bilaterians. Flatworms
are complex animals with elaborate hermaphroditic reproductive systems. Fertilization
is internal with copulation. They
may be free-living or parasitic.
4500 described species of free-living flatworms are included in the
heterogeneous taxon Turbellaria (Fig 10-32*), whereas the parasitic flukes and
tapeworms belong to another taxon, Neodermata. Turbellarians are carnivores
varying in length from less than 1 mm to as many as 60 cm in length but the
great majority are small. Many are roughly the size of ciliate protozoans, with
which they are easily confused. The body is cylindrical in small species
(microturbellaria, Fig 10-2) and dorsoventrally flattened in large
(macroturbellaria, Fig 10-1). The
epidermis is ciliated. The
mouth is located somewhere on the ventral midline and opens into a blind gut, or
gastrovascular cavity, which lacks an anus and sometimes lacks a lumen. Most
are aquatic, inhabiting the oceans and fresh water, but a few are found in moist
terrestrial environments. Most
and fluid regulation areaccomplished with protonephridia (Fig 10-20). There
is no hemal system and transport is by diffusion, which is facilitated by small
body size and flattening. Nutriment
is delivered to tissues by diffusion from the gut, whereas oxygen diffuses
across the body surface.
end product of nitrogen metabolism is ammonia, which is lost by diffusion across
the body surface. The
nervous system consists of a bilobed brain, or cerebral ganglion, from which
longitudinal nerve cords arise and extend posteriorly for the length of the body
(Fig 10-11). Light-sensitive
pigment-cup ocelli occur in various positions on the body. Turbellarians
are hermaphroditic with internal fertilization.
of the great heterogeneity of Turbellaria there is no typical example although
the triclad, Girardia tigrina (= Dugesia
tigrina), is often used in teaching laboratories as if it were. Triclads
are macroturbellaria in which the gut comprises three branches, or rami (Fig
ramus is anterior and two are posterior to the plicate pharynx situated
ventrally, near midbody, at the junction of the three rami. Each ramus is itself
branched with many ceca ramifying to reach and deliver food throughout the body
tigrina is one of several
species of freshwater triclad flatworms collectively known as "planarians". This
exercise employs commercially available wholemounts, cross sections, and living
species supplied by commercial houses vary but are similar and all are suitable
for this exercise. The
exercise is written specifically for "brown planaria".
inhabit freshwater where they are most common under stones, leaves, or other
objects near the shore. They
are negatively phototactic. They
can be collected in local fresh waters, if present, by placing small pieces of
lean beef on the bottom for 10-15 minutes.
the dissecting microscope or low power of the compound microscope study a
composite wholemount with two fixed and stained planarians, one with the gut
injected with dye and the other plain, with uninjected gut (Fig 1, 10-31*). Note
the dorsoventral flattening and bilateral
the anterior-posterior axis,
which is the axis of symmetry. The plane
of symmetry includes this
axis and divides the worm into right and left sides. Specimens
are usually mounted on the slide with the back, or dorsal surface, up.
anterior end of the body is the head and
the remainder is the trunk. Girardia
tigrina has a pointed triangular
head but the head of some species is blunt and rounded. The posterior limit of
the head is marked by a pair of lateral, ciliated, chemosensory protrusions, the auricles. These
are variously developed in different species and are sometimes absent. The
two darkocelli, or eyespots, are easily seen at the level of
digestive system is best studied using a specimen that has been fed a colored
substance such as carmine powder or carbon black and thus has a gut filled with
pigment. A wholemount of such a specimen should be available, either on the
slide you are now using or mounted separately.
the cylindrical pharynx lying
on the midline at the center of the body (Fig 1, 10-31A). The pharynx is a
muscular, protrusible tube housed in a spacious pharyngeal
cavity which opens to the
exterior via the mouth. The
narrow pharyngeal lumen,
which may contain pigment, occupies the center of the pharynx. The pharyngeal
cavity is enclosed by the pharyngeal
Figure 1. The
planarian, Girardia tigrina,
in dorsal view. Flatworm27L.gif
mouth, which is rarely visible on wholemounts, is a small pore on the ventral
midline of the body immediately posterior to the pharynx. It
is the opening of the pharyngeal cavity to the exterior. During
feeding, the pharynx lengthens to many times its resting length and is extended
out of the mouth to reach the food.
proximal end of the pharynx opens into the intestine which,
in triclads, immediately divides into one anterior and
two posterior rami. The
name Tricladida (clad = branch) alludes to the presence of the three rami. Each
ramus ends blindly and bears abundant ceca,
or diverticula, so that no area of the body is beyond effective diffusion
distance from the food source.
protonephridia are present but are not visible in these preparations. Sometimes
some features of the reproductive system can be seen posterior to the pharynx
but it is rare that enough can be discerned to make sense of it (Fig 10-31E). Study
of the nervous system is also impractical (Fig 10-11C).
a prepared slide of planaria cross sections. Use
a commercial slides with three cross sections taken at different levels along
the worm. There
is usually one through the anterior end of the body, one through the pharynx
(Fig 3), and one through the posterior part of the worm (Fig 2). Slides
are not uniform however, and do not necessarily conform to this ideal, nor can
you rely on the order in which the sections are arranged on the slide.
and identify each of the three sections on your slide. The pharyngeal section is
usually easy to recognize (Fig 3). A
good pharyngeal section shows the large, unmistakable pharynx as a hollow, red
circle surrounded by a narrow white ring in the center of the section. The
anterior and posterior sections have one to many irregular circles distributed
through the parenchyma (Fig 2). These
circles are the intestinal rami and their ceca (Fig 2).
Anterior or Posterior
with either the anterior or posterior section (Fig 2), i.e. one that does not
pass through the pharynx. It
makes little difference which you choose and you may find it profitable to use
both but be certain you do not have the pharyngeal section. Be
sure you can tell dorsal from ventral. The
ventral surface is usually flatter than the arched dorsal surface (in most
slides it will be down).
Figure 2. Cross
section through the body of Girardia
tigrina posterior to the
body is covered by a monolayered, secretory epidermis (Fig
ventral, but not the dorsal, epidermis is ciliated,
a fact that can be verified by careful observation with high power (400X). The
cilia are used for locomotion and the epithelial cells are multiciliated. The
dorsal surface is not ciliated. Compare
the dorsal and ventral epithelia to be sure you can recognize cilia.
The epidermis is underlain by a distinct basal
lamina, which is visible as a thin, dark line just inside the
dorsal epidermis contains numerous secretory
vesicles and rod-shaped
membrane enclosed secretions, the rhabdites (rhabd
= rod). Rhabdites
are synthesized by epidermal gland cells insunk (submerged) below the basal
lamina into the parenchyma (Fig 10-5). When
expelled at the surface, rhabdites absorb water and expand to become sticky
mucus which may help trap small invertebrate prey.
the clusters of adhesive
gland cells situated at the
lateral edge of the ventral epidermis (Fig 2). These
are part of a cilia-free adhesive zone that encircles the worm. These cells
secrete an adhesive that helps the animal grip the substratum. The
ventral epidermis bears numerous gland cells that secrete mucus.
thick layer of body wall muscles,
consisting of outer circular and inner longitudinal fibers, lies just inside the
epidermis (Fig 2, 10-3C).
the muscle layer, the interior of the worm is filled with a mesenchymal
connective tissue, the parenchyma,
consisting of cells in a fibrous extracellular matrix and having a loose, open
appearance (Fig 10-13B). Many
large epidermal gland cells are submerged into it but they nevertheless open to
the surface via narrow necks passing through the basal lamina and epidermis.
muscles can be seen passing
vertically through the parenchyma connecting the muscle layers of the dorsal and
ventral body walls. These
muscles maintain the flat shape of the triclad body.
about in the interior are sections through the intestinal
rami and their ceca (Fig
2, 10-13B). These
are irregular circles of various sizes and unpredictable number. The
clear space in the interior of each is the gut
lumen and is surrounded by a
monolayered epithelium of large, vacuolated cells, which may be secretory,
absorptive, or phagocytic. This
epithelium is usually referred to as the gastrodermis,
or mucosal epithelium.
now to the pharyngeal cross section, which should resemble Figure 3. Review
the now familiar features you identified on the other sections and then study
the enormous pharynx in the center.
The pharynx occupies
almost the entire center of the section and the body wall is very thin above and
below it. Locate
and identify the two white, unstained spaces associated with the pharynx. The
one in the center of the pharynx is the pharyngeal
lumen whereas that
surrounding the pharynx is the pharyngeal
type of pharynx, known as a plicate pharynx and consisting of a muscular tube
retractable into a sheath, is characteristic of triclads and polyclads (Fig 1,
outward from the lumen in the center of the pharynx, the layers are, in order: pharyngeal
lumen, gastrodermis, inner
muscle layer, parenchyma (connective
tissue), outer muscle layer, epithelium, pharyngeal
cavity, and pharynx
sheath epithelium (Fig 3).
Figure 3. Cross
section through the pharyngeal region of Girardia
a living worm in a drop of water on a microscope slide without a coverslip. Living
worms can be picked up with a plastic pipet but they must be ejected quickly or
they will attach to its wall using their adhesive glands. They adhere
tenaciously and are difficult to remove from the pipet when this happens.
the worm with the dissecting microscope. Watch
it as it moves across the slide. The
major locomotory force is produced by the cilia of the ventral epidermis but
muscular activity also plays a role in locomotion, especially in making turning
the worm with a tiny needle to
encourage it to change directions. Try
to discover the contributions of the musculature to this maneuver and think
about which muscles would be involved in making a turn. Triclads do not swim.
the worm gently with the needle and look for evidence of adhesive ability. Where
do the adhesive cells seem to be located? _________________
the animal with transmitted light and look for the intestine and its ceca. These
may be obvious, especially if the worm has been fed recently.
> a. Position
a coverslip over the worm and place the slide on the compound microscope. The
weight of the coverslip will squeeze the worm enough to immobilize it and make
it thin enough to see some internal structure. Remove
some water from the preparation to further squeeze the animal. Do
not use 400X on these slides.
with 100-200X, on the edge of the head, reduce the light, and look for evidence
of beating cilia. Most
of the animal's cilia are ventral and thus difficult to see in a wholemount but
the head bears cilia associated with chemosensory receptors on the auricles and
their activity is obvious. Look
for them on the edge of the auricles. The
name "turbellaria" means "little disturbance" and is a reference to the movement
of water caused by the cilia of the auricles. Observe the cilia with phase
contrast if you have it.
the light so you can illuminate some of the interior. At
40X you may be able to see gut diverticula, especially if the animal has eaten
pharynx is the conspicuous, long, pale area in the center of the body. If
the animal is squeezed sufficiently, you may see the pharynx clearly and you may
even see the mouth opening at its posterior end. The
pharynx will probably move about in the pharyngeal chamber and may increase in
> b. Place
three or four worms in the center of a small (6 cm) culture dish. Place
a small piece (about 2-3 mm in diameter) of hard-boiled egg yolk, lean beef, or
beef liver in the dish with the worms. Turn
the microscope and room lights off. Girardia
tigrina is negatively
phototactic and may unwilling to feed in bright light although sometimes it
doesn’t seem to matter. Put the dish on the stage of the dissecting microscope
being careful that you do not disturb the worm. If
you are fortunate, you will see the worm protrude its pharynx and feed (Fig
sight is impressive and you may be startled by the length of the pharynx. Egg
yolk is especially good for this exercise because the bright yellow yolk
granules are easy to see as they move up the pharyngeal lumen to accumulate in
the intestine. After
several minutes of feeding look at the gut to see if the ceca and rami can now
be visualized. <
tigrina typically reproduces
asexually by architomy (Fig 10-22B), a type of fission in which the worm divides
into two fragments without prior differentiation of new parts. Transverse
cleavage just posterior to the pharynx divides the worm into an anterior, nearly
normal, worm with head, mouth, pharynx and most of the gut, and an incomplete,
headless posterior mass of tissues which must replace its missing parts.
division, the anterior end behaves normally but the posterior end remains
immobile until regeneration is complete and the missing parts replaced. You
are not likely to see fission occurring but, if your laboratory maintains
populations in aquaria, it is quite possible that you will see these headless
lumps stuck on the walls of the aquaria.
> c. Triclads
have remarkable regenerative abilities and are capable of repairing damage or
replacing missing body parts if experimentally fragmented. The
process mimics architomy. Add about 20 ml of pond water to the dish containing
your specimen(s). Remove the egg yolk and any worms in excess of one large worm.
a sharp scalpel to cut the remaining specimen in pieces. You
decide how you want to cut the worm. Some
obvious possibilities are to bisect it with transverse or longitudinal cuts. Or
you could trisect it. Or
you could split the head to see if you can produce a two-headed worm (if you
choose this option you will need to visit the lab daily to renew the incision
and keep the two halves of the head from growing back together.
a Sharpie to label the side of the dish with your name and the date. Cover the
dish with another dish and set it aside in a dark area of the laboratory. Make
careful sketches of the pieces of the worm. Observe
your specimens during each laboratory period for the remainder of the semester.
Make sketches each week and compare them with earlier sketches. Do
you see evidence of regeneration? <
call-outs, such as this one, refer to figures in Ruppert, Fox, and Barnes
without hyphenation refer to figures embedded in this exercise.
(ed) 1950. Selected
Invertebrate Types. Wiley,
New York. 597p.
LH. 1951. The
Invertebrates:Platyhelminthes and Rhynchocoela, vol. II. McGraw-Hill,
New York. 550p.
RW . 1989. Fresh-water
Invertebrates of the United States, 3 ed. Wiley,
New York. 628p.
A . 1916. Morphology
of Invertebrate Types. MacMillan,
New York. 263p.
SK, Maugel TK. 1987. Illustrated
invertebrate anatomy. Oxford
Univ. Press, Oxford.
RM, et al. 1991. Platyhelminthes:
Turbellaria, in Harrison, F. W. & B. J. Bogitsh (eds.). 1991. Microscopic
Anatomy of Invertebrates vol. 3 Platyhelminthes and Nemertinea . Wiley-Liss,
New York. 347p.
NW, Morse MP (eds). 1974. Biology
of the Turbellaria. McGraw-Hill,
New York. 530
Ruppert EE, Fox RS,
Barnes RB. 2004.
Invertebrate Zoology, A functional evolutionary approach, 7 th ed.
Brooks Cole Thomson, Belmont CA. 963 pp.
Slides and cover glasses
Plastic Pasteur pipet
6 cm Carolina culture dish
Hard-boiled egg yolk
Prepared slides: stained wholemount with
filled gut, stained cross sections of anterior, middle, and posterior regions,
Cultures: brown planaria
Composite wholemount slides (gut injected, gut plain)
Carolina, Triarch, Ward’s
Cross sections slides at three levels
Carolina, Triarch, Ward’s
Living Girardia tigrina (as