KINGDOM ANIMALIA
BASAL LOPHOTROCHOZOA: PHYLUM
PLATYHELMINTHES – TRIPLOBLASTIC ACOELOMATES
INTRODUCTION
The
Platyhelminthes include free-living flatworms, like the planarians, and the
parasitic tapeworms and flukes. The
term flatworm refers to the fact
that the body is dorsoventrally flattened.
Flatworms have tissues organized into organs, and they exhibit bilateral symmetry. Bilateral symmetry means that one plane
passing through the longitudinal axis of an organism divides it into right and
left halves that are mirror images.
Bilaterally symmetrical animals typically move about actively and
exhibit cephalization, which is the
accumulation of nervous tissue and sensory structure at the anterior end of the
animal. This reflects the
importance to the organism of monitoring the environment it is meeting - rather
than that through which it has just passed - and results in the presence of
definite anterior and posterior ends.
However, the digestive tract is incomplete, with a single opening that serves
for both ingestion of food and elimination of wastes.
The
Platyhelminthes are triploblastic
and acoelomate. There are three primary germ layers: ectoderm, endoderm, and mesoderm. As with the Cnidaria, the ectoderm gives
rise to the outer epithelium and the endoderm gives rise to the lining of the
gut tract. The third germ layer,
the mesoderm, gives rise to the tissue between the ectoderm and the endoderm,
including muscle, excretory structures, and undifferentiated cells referred to
as parenchyma. Typically, different
tissues will come together to form organs and cooperate to perform body
functions. The term acoelomate
refers to the fact that there is no coelom
(fluid-filled body cavity) between the gut and outer body wall.
The Platyhelminthes are triploblastic which means that they develop from embryos with three primary germ layers: ectoderm, endoderm, and mesoderm. The ectoderm gives rise to the outer epithelium and the endoderm gives rise to the lining of the gut. The third germ layer, the mesoderm, gives rise to the tissue between the ectoderm and the endoderm, including muscle, excretory structures, and undifferentiated cells referred to as parenchyma.
Platyhelminths possess developmental features such as spiral cleavage that place them within the protostomes, and they have been shown on the basis of gene sequence information to belong to the lophotrochozoa. Most protostomes are coelomate, but the platyhelminths are acoelomate, lacking a fluid-filled space between the gut and outer body wall. Recent evidence suggests that platyhelminthes may have actually derived from a coelomate ancestor.
Two major groups of platyhelmithes, the tapeworms (Class Cestoda) and the flukes (Class Trematoda) are parasites whose bodies are highly modified for their specialized lifestyle. Structures that are well-developed in free-living flatworms are considerably reduced or even absent in these parasitic forms. Parasitic flatworms typically have reduced sensory structures, specialized structures for attachment to the host, enhanced reproductive capacity, and complex life cycles. Some platyhelminths such as the tapeworms have lost the digestive tract, which makes sense given that they live in the intestine of their host from which they can absorb nutrients directly. In this lab, we will focus on the free-living flatworms to investigate the basic body plan of the phylum.
Body Plan of Free-Living Flatworms (Class Turbellaria). Figure 1 shows the body organization of a typical turbellarian flatworm. The term flatworm refers to the fact that the body is dorsoventrally flattened. The flattened body shape results in a relatively large ratio of surface area to volume. Gas exchange and excretion of nitrogen-containing wastes like ammonia take place by diffusion between cells and water that surrounds the body or is present in the gut. Platyhelminths lack organs for gas exchange, but do possess a network of structures called protonephridia that function mainly in osmoregulation by removing excess water from the body. The nervous system consists of a pair of anterior ganglia, usually with two nerve cords running the length of the organism. Nerve cords are interconnected by transverse nerves to form a ladderlike structure. Free-living platyhelminths possess sensory structures such as ocelli (eye spots) which detect light and chemoreceptors that detect chemicals in the environment. Platyhelminthes have an incomplete gut, a digestive cavity with a single opening that serves for both ingestion of food and elimination of wastes.
Free-living flatworms live in marine, freshwater, and moist terrestrial habitats. They can move by ciliary gliding along a surface that is lubricated by mucous secreted by the ventral epithelium. Wave-like muscular movements are also used in crawling and, in some species, swimming. Most turbellarians are scavengers and or predators. Feeding takes place via a tubular pharynx that can be protruded from the ventral side of the animal and use to suction in food. The gut may be highly branched to help in the absorption of nutrients and to ensure that nutrients are available to all the cells of the body.
PROCEDURE
Class Turbellaria – the free-living flatworms
The
class Turbellaria contains free-living flatworms
(such as planarians) that retain the defining features of the phylum, such as
anteriorly located sense organs and a well-developed muscular system. The other
classes in this phylum are composed of specialized parasites that have lost
many features seen in free-living animals.
1. Place a live planarian (Dugesia) in a watch glass with a small
amount of water. Observe its
locomotion and behavior as it explores its environment. The planarian may respond to adverse
stimuli (such as touch, vibration, or bright light) by contracting its
body. As you observe your specimen, identify the head, auricles, and eyespots. Also look for
the pharyngeal tube projecting off
the ventral surface of the animal.
2. Obtain a prepared slide containing a cross section of a planarian. The central tube is the digestive cavity. Note that a body cavity is absent. The area between the digestive tube and body wall is filled with tissue called mesenchyme. Sketch the cross section and label the outer epidermis, gut, and mesenchyme.
3. Use a compound microscope at low power
to observe a prepared slide of Dugesia that prominently displays the digestive tract. Draw this specimen and label its pharynx, pharyngeal chamber, and highly branched gut. Label your drawing
with the total magnification at which your observation was made.
Class Trematoda – the digenetic flukes
Flukes
are all parasitic, primarily infecting vertebrates, including man. The flukes in the class Trematoda are
digenetic, meaning they have a life cycle requiring two or more hosts. In such a cycle, the final host is
termed the primary or definitive host, while the other (or others) are intermediate hosts.
Flukes of this group are typically endoparasites
(living inside the host). They have
highly specialized reproductive systems, very high reproductive capacity, and
complex life cycles in which most of the intermediate stages are capable of
(and exhibit) asexual reproduction.
Some of the body structures that are well-developed
in free-living flatworms are considerably reduced or even absent in these
parasitic forms.
3. Examine a prepared slide of Clonorchis sinensis, the
human liver fluke. This specimen
demonstrates typical parasite features, including the absence of sensory
organs, reduction of locomotor and digestive systems, expanded reproductive
system, and presence of holdfast organs.
Sketch the specimen and label the gut,
the prominent reproductive structures (uterus,
testes, and yolk gland), and the oral
sucker and ventral sucker. Label your drawing with the total
magnification at which your observation was made.
Figure 1: Anatomy of a turbellarian flatworm, a representative platyhelminth. These animals exhibit cephalization by the presence of anterior sensory structures (ocelli) and cerebral ganglion ("brain). Turbellarians are hermaphrodites, i.e. they possess both male and female reproductive organs.
Class Cestoda – the tapeworms
These
are highly adapted endoparasites which absorb their
nutrients directly through their body walls from the host gut. They have lost their own digestive
system and increased their reproductive capacity. A resistant cuticle protects them from the host's digestive enzymes. The anterior region of a tapeworm's body
is modified as a simple attachment structure known as the scolex. The rest of the
tapeworm body is composed of a series of segments called proglottids.
Proglottids are produced continuously behind the scolex, and therefore
the proglottids closest to the scolex are the least
mature. As the proglottids mature,
the male reproductive system develops before the female reproductive
system. The most mature proglottids
are little more than a uterus filled with eggs, and are called gravid
proglottids.
4. Examine a prepared slide of Taenia pisiformis. Find the scolex and also proglottids in different stages of development: immature, mature, and gravid
proglottids. Draw the scolex and label the hooks and the sucker discs. Draw a mature proglottid and label the testes,
ovary, uterus, and genital pore. (A diagram of a mature proglottid can be viewed here.) Also draw an immature proglottid and a gravid
proglottid. Label your drawings
with the total magnification at which your observations were made.
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