PRACTICAL 7 : Flatworms, Roundworms, and Rotifers

Objectives

At the end of this lab, students should be able to:

i. explain the basic body plan of some members of phyla

Platyhelminthes, Nematoda, and Rotifera.

ii. identify structures and functions of representative of these groups.

iii. state characteristics of members of these groups.

iv. explain the difference between acoelomate and pseudocoelomate,

and know which phyla associated with each.

Introduction

Flatworms of phylum Platyhelminthes, roundworm of phylum Nematode, and

rotifers of phylum Rotifera live in marine, freshwater, terrestrial, and parasitic

environment. Their morphology is more complex than that of sponges and

jellyfish. They have three germ layers: ectoderm, mesoderm, and endoderm

(described as triploblastic) and have organ made of interdependent tissues

and are the simplest animal having bilateral symmetry with distinct anterior

and posterior ends.

Flatworms have soft, wormlike bodies and are acoelomates (lack of

body cavity between the gut and body wall). They represent an important

evolutionary transition because they are the simplest forms to exhibit an

organ system level of organization and cephalization (a definite head with

sense organ). Although they are definite organs, the digestive system is still

of the gastrovascular type, incomplete with a mouth and no anus. Flatworms

are grouped in three classes: class Turbellaria: Planarians, class Trematoda:

Fluke, and class Cestoda: Tapeworms.

Roundworms are pseudocoelomates (have a false body cavity or

pseudocoel). They have a complete gut with both a mouth and an anus.

Together, the pseudocoel and the complete digestive system comprise a

tube-within-a-tube arrangement. Roundworms have slender, cylindrical

bodies that taper at both ends. A complex cuticle of protein covers them.

Free-living roundworms are Rhabditus and Turbatrix, Ascaris, and Trichinella.

Rotifers are pseudocoelomates, microscopic animals, easily found in

rain gutters and spouts, and in the slimy material around the bases of

building. Their body wall is very thin, usually transparent and covered by a

cuticle composed of protein. They are dioecious; however, many species

have no males, and the eggs develop parthenogenetically.

Materials

Preserved or living specimens of :

- Dugesia (planaria)

- Taenia pisiformis

Prepared slide of Dugesia

(planaria), Taenia pisiformis

Compound

microscope

Dissecting

microscope

Slide and coverslip

Complete dissecting

set

Distilled water

Procedure

Experiment 1 : Flatworms of phylum Platyhelminthes

Dugesia (planaria) (Figure 7.1) :

1. A living Dugesia was obtained and its morphology was examined with a

dissecting microscope. Features of planaria distinguish its head from its

tail was identified.

2. A prepared slaide of a stained planaria was examined. The ectoderm,

mesoderm, endoderm, gastrovascular cavity, and muscular pharynx

was identified.

Taenia pisiformis (Figure 7.2) :

1. A preserved Taenia pisiformis was obtained. The morphology of Taenia

pisiformis, internal parasites of dogs and cat was look for.

2. The specialized head or scolex, neck, and proglottis was identify at the

body of Taenia pisiformis.

3. Using a prepared slide, hooks and suckers was identified. Task that the

tape worm used these hooks and suckers was identified.

Figure 7.1 : Anatomy of a planarian. (A) Dorsal view of digestive and

nervous system. (B) Cross section and (C) longitudinal section of

planarian taken through the pharynx region. (D) Lateral view. (E)

Dorsal view. (F) Sagittal view of reproductive system.

Figure 7.2 : Structure and life cycle of the beef tapeworm, Taenia saginatus.

Discussion :

In the kingdom Animalia,the phylum Platyhelminthes represents all

flatworms.There are bilaterally symmetrical, having an anterior, posterior,

dorsal, and ventral end and two seemingly identical lateral surfaces.

However, flatworms is lacks of any type of body cavity, a fluid filled space

between the digestive tract and the body wall. The area of flatworm is filled

with tissue. Flatworms have primitive brains, comprised of clusters of nervous

tissue. Two long nerve cords run the length of the body, and they branch to

form small nerves running throughout the body. Another hallmark of

flatworms is a lack of a complete digestive tract. Flatworms take in food and

excrete waste from one single hole. The opening in the gastrula never fully

evolves in flatworms to make a tract that travels from one hole in the

anterior end to another hole in the posterior end, which would make the tract

complete. There are four classes that make up the phylum Platyhelminthes,

each with unique structures and living styles which is Turbellaria, Cestoda,

Monogenea.

In the experiment, Taenia pisiformis come from class Cestoda also known as

tapeworms while Dugesia sp Planaria common name planaria come from

class Turbellaria.

Experiment 2 : Roundworm of phylum Nematode

Ascaris (Figure 7.3) :

1. A female and male Ascaris was examined and identified the mouth and

anus.

2. By using a prepared slide of female Ascaris, ectoderm, muscle fibers,

intestine, oviducts, loops of ovaries, uteri, and pseodocoel was

identified.

3. By using a prepared slide of male Ascaris, ectoderm, muscle fibers,

intestine, loops of testis, seminal vesicle, and pseodocoel was

identified.

Figure 7.3 (A) Lateral view of female nematodes. (B) Lateral view of male

nematodes. (C) and (D) Cross section of the body.

Discussion

  The 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.   The

body cavity, if present, is a hemocoel derived from the blastocoel.  

            The body is covered with a thick extracellular cuticle secreted by a

cellular or syncytial epidermis that is molted during juvenile

development.   The 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.  

            The 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.  

            The 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.   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 the mouth.  

            Most nematodes lack cilia or flagella, even in the sperm.   There are,

however, ciliary derivatives in the amphids and cilia are present in the gut

epithelium of some nematodes.   Roundworms are ammonotelic and nitrogen

excretion is mostly by diffusion across the body wall.   Osmoregulation is

accomplished by an excretory canal system in some and perhaps by

excretory glands.   No nephridia are present.  

            Nematodes 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.   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 50 cm.  

Ascaris is an intestinal parasite in pigs. Parasitic nematodes tend to be

much larger than free-living nematodes. Digestive tract, when it is compare

this slide to the whole preserved Ascaris. If you dissect one of those

specimens, it will show that the internal organs (gut and reproductive organs)

are simply hanging free inside the body cavity, or pseudocoel. In this cross-

section, the intestine doesn't appear to be attached at all, and if you look

closely (at 40x magnification), will show that the wall of the intestine consists

of just one layer of epithelium, with no other tissue layers attached.

Reproductive organs are ovary is makes the eggs and uterus is stores eggs

until they are released for external fertalization.

The male differs mainly in terms of its reproductive organs is

where testis is a long tube, looping back and forth throughout the body; in

this cross-sectional view, there appear to be multiple separate testes. The

testis makes sperm. The vas deferens (also called ductus deferens) is

another part of the same tubular male reproductive tract; it is responsible for

storing and maturing sperm.

Post-lab Questions

1. How is bilateral symmetry different from radial symmetry?

Radial symmetry

Sea anemones display radial symmetry.These organisms resemble a

pie where several cutting planes produce roughly identical pieces. An

organism with radial symmetry exhibits no left or right sides. They have

a top and a bottom (dorsal and ventral surface) only.  

Most radially symmetric animals are symmetrical about an axis

extending from the center of the oral surface, which contains the

mouth, to the center of the opposite, or aboral, end. This type of

symmetry is especially suitable for sessile animals such as the sea

anemone, floating animals such as jellyfish, and slow moving

organisms such as starfish.  

Bilateral symetry

In bilateral symmetry (also called plane symmetry), only one plane,

called the sagittal plane, will divide an organism into roughly mirror

image halves. Thus there is approximate reflection symmetry. Often

the two halves can meaningfully be referred to as the right and left

halves, e.g. in the case of an animal with a main direction of motion in

the plane of symmetry.  

2. What are the advantages of a roundworm’s digestive system having

two opening?

The roundworm can continue eating, and the ingested material will be

processed appropriately, with different chemicals dumped into it at

different locations along the length of the alimentary canal to assist in

the digestion of the material. It provides for sequential processing of

the material in a spatial manner. The planaria, by contrast, has to use a

time-based manner for processing of ingested material.That is, for best

efficiency, it would eat its fill first, and then have time-based events in

the digestion rather than location-based.  

3. Describe several adaptations of parasitic flatworms to their external

environment.

A parasitic flatworm has less-evolved digestive system, since they

absorb the nutriens directly from the host, having no point to digest

further.  

They also have more complex reproductive life cycle, where they lay

eggs that will be put in the feces. Then a food will be contaminated

with eggs, new flatworms will develop to other hosts.   Also, parasitic

flatworms will usually have hooks on the scolex (anterior region) to

attach it to the wall of the gut, and they have an extra outer covering

(glycocalyx) to protect it from being digested.