Download 2006 Bio153 Lab 5: Major Phyla of Invertebrates Before you leave

2006 Bio153 Lab 5: Major Phyla of Invertebrates
Before you leave the lab, please hand in the in-class assignment worth
2% of your total grade. You may wish to bring your textbook to lab.
Introduction
The organisms in today’s lab demonstrate the evolutionary trends in modification
of animal body plans, focusing on 6 phyla: Porifera, Cnidaria, Platyhelminthes,
Nematoda, Annelida and Mollusca. Porifera illustrates the simplest form of
animal organization; Cnidarians exhibit bilateral symmetry and diploblastic body
construction, and the remaining 4 phyla show variations on the bilateral,
triploblastic body plan.
Diversity in body plans: Symmetry
Sponges, given their relatively “loose” level of organization have no real
plane of symmetry. Cnidaria, which include animals such as sea anemones,
corals and jellyfish, are radially symmetrical, and
thus can perceive and respond to stimuli from all sides
of the body (fig. 1.1). Cnidarians do not have a central
nervous system; instead there is a network of nerve
cells (a nerve net) within the body wall.
Platyhelminthes are the first of the animals we
will look at today that exhibit bilateral symmetry (fig.
1.2), and this goes hand-in-hand with the possession of
an anterior/posterior axis in
the body, and a specialized region at the front end
called a head. Animals with a head are said to show
Fig 1.1. Radial symmetry
“cephalisation”. This implies the concentration at the
anterior end of the body of nerve cell bodies forming a brain or ganglia,
together with associated sensory organs for the perception of sight, hearing,
olfaction (smell), taste, etc. The animal responds to external stimuli by moving
either towards or away from the stimulus.
Fig 1.2. Bilateral symmetry, and planes of section
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Because bilaterally symmetrical animals have a distinct body axis (a dorsal
and ventral side, as well as an anterior and a posterior end), specific terms are
used to describe the location of structures relative to the body axis:
Table 1. Glossary of anatomical terms
dorsal- near or towards the back
lateral- near or towards the left /right side
anterior- near or towards the head end
proximal- near to a point of reference
pectoral- chest/shoulder region
viscera- internal organs
ventral- near or towards the belly
median- near or towards the middle
posterior- near or towards the hind end
distal- far from a point of reference
pelvic- relating to the hip region
Diversity in body plans: germ layers
Sponges exhibit a somewhat loose organization of cell types. Cnidaria
exhibit a more organized two-layered body (“diploblastic”), and a three-layered
body (“triploblastic”) is found in the Platyhelminthes, Nematoda, Annelida and
Mollusca. In diploblastic animals the two distinct germ layers of cells are the
ectoderm (outer) and endoderm (inner). A germ layer is a group of cells that
arise early in development and give rise to a set of tissues or organ systems.
Triploblastic animals have the ectoderm and endoderm layers and a middle layer
called the mesoderm.
Some groups of triploblastic animals show much greater diversity or variation
from the basic body plan than other groups, which may however be more diverse
physiologically. The phyla Nematoda, Annelida and Mollusca demonstrate this diversity.
The molluscs and annelids show tremendous morphological diversity or “adaptive
radiation”, which enables them to have lifestyles varying from aquatic (marine or
freshwater) to terrestrial, and from creeping, burrowing, actively swimming, or floating
(planktonic), to sedentary or attached to the substratum. This variety is in contrast to
the much more uniform body plan of the nematodes which, although very successful
numerically speaking, show little morphological diversity. However, many nematodes
are physiologically adapted to a parasitic lifestyle.
Two of the most important characters of nematodes, annelids and molluscs not
seen in Platyhelminthes are:
1. The development of a body cavity between the gut and the body wall which,
among other functions, can act as a hydrostatic skeleton.
2. The development of a complete, one-way digestive tract, with a mouth,
middle region of varying complexity, and an anus.
There are three basic forms of the body in triploblastic animals: those
without a body cavity (acoelomate) and those with a body cavity
(pseudocoelomate and coelomate). The distinction between
pseudocoelomate and coelomate is found in the presence of mesoderm derived
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tissue (mesentery) around the gut as well as mesoderm derived tissue
(peritoneum) lining the wall of the coelom. Thus, in a true coelomate animal
the entire body cavity is surrounded by mesoderm tissue. Note that in a
pseudocoelomate, only the body wall and not the gut is surrounded by
mesoderm. In today’s lab, you will be introduced to the acoelomate flatworms
of the Phylum Platyhelminthes, the pseudocoelomate roundworms of the Phylum
Nematoda and two phyla of coelomates, Mollusca and Annelida.
Fig 2. Body plans in triploblastic animals.
Diversity in locomotion: the hydrostatic skeleton
Fluid in a sealed body cavity can act as a skeleton. Its function is analogous to
the bones of vertebrates or the jointed exoskeletal sclerites of arthropods, which act as
levers and transmit forces between antagonistic muscle pairs. When one muscle
contracts, its pull on a bone or a sclerite simultaneously stretches another muscle
attached to the same skeletal element. If body fluid fills a flexible-walled cavity, then
fluid too can transmit forces between antagonistic muscle pairs.
In a segment of an earthworm, two bands of muscle invest the body wall
external to the coelom. The coelomic cavity is separated from that of adjacent
segments by thin septa that, like the body wall, are flexible. Since the coelomic fluid is
incompressible, i.e. it can be displaced but not reduced in total volume under pressure,
contraction of the circular or longitudinal muscles will alter the shape of each segment,
without affecting the volume of the coelomic cavity.
Many animals use a hydrostatic skeleton. A sea anemone (Phylum Cnidaria) with
its mouth closed, and its gastrovascular cavity full of seawater can assume a wide
variety of body shapes. Longitudinal, circular and other muscles operate antagonistically
via the trapped water, which thus functions as a hydrostatic skeleton. Some bivalves
(Phylum Mollusca) burrow and move within sand and silt substrates by means of a
flexible muscular foot that protrudes from between their shells. The bivalve’s bloodfilled body cavity extends into the foot. Blood can be pumped into the foot cavity, and
the foot muscles act against this fluid skeleton to alter foot shape for effective
burrowing movements.
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Lab Exercises
PART I: Phylum Porifera (Sponges)
The sponges are an ancient but abundant group of mostly marine animals that
lead a sessile existence. Their bodies are either asymmetrical - forming an encrusting
layer on the rock, or more or less radially symmetrical (often vase-shaped). As their
name implies, they have numerous pores leading into a complex system of chambers
and cavities within the body. They extract their food by a filter-feeding mechanism in
which water is drawn in, via the pores, into a central cavity or atrium, by the beating
of flagellated cells (choanocytes) lining the cavity. Food particles, including microorganisms, are trapped in mucus, and the water leaves through a hole at the top of the
sponge, called an osculum. Sponges belong to three main classes:
1. Class Calcarea: calcareous sponges, i.e. those having a skeleton of CaCO3
spicules (often Y-shaped). We will examine one member of this class (Scypha) in
some detail.
2. Class Hexactinellida: glass sponges with 6-rayed siliceous skeletons.
3. Class Demospongia: natural sponges with a skeleton of flexible spongin
fibres and/or siliceous spicules.
Examine the prepared slides of the body of a sponge and spicules using a
compound microscope.
PART II: Diploblastic Phyla
There are two diploblastic phyla (Cnidaria and Ctenophora). Today’s lab will
examine one of these (Cnidaria).
Phylum Cnidaria (Sea anemones, corals, jellyfish)
This group of animals is characterized by radial symmetry and the presence of
a sac-like gut or gastrovascular cavity with only one opening. The two tissue layers
of the body are the outer epidermis (derived from the ectoderm) and the inner
gastrodermis (derived from the endoderm). Between these two tissue layers is a
structureless jelly called the mesoglea. Cnidarians are unique in having special
stinging cells (cnidocytes) in their body wall, particularly in that of the tentacles.
These cnidocytes, contain a barbed, coiled up thread called a nematocyst, which is
explosively released when a trigger is touched. It often contains a paralyzing poison
that is injected into the prey, which is then brought to the mouth by contraction of the
tentacles. Members of this phylum have two distinct body forms: the polyp and the
medusa. Cnidaria is divided into three main classes:
1. Class Hydrozoa (hydras) in which the polyp stage predominates.
2. Class Scyphozoa (jellyfish) in which the medusa stage predominates, and in
which the mesoglea is much thicker than usual.
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3. Class Anthozoa (corals and sea anemones) which only have a polyp
stage.
Class Hydrozoa: Examine the living specimens of Hydra in the container on
your bench. Using a Pasteur pipette, carefully transfer one specimen to a watch
glass with a little of the water it is living in. Examine under the dissecting
microscope, preferably with a dark background for easier viewing of the
transparent animal. At the upper end of the body, observe several elongated
tentacles surrounding the mouth. Watch how the body and the tentacles
contract and gradually relax and elongate after stimulation. If available, add a
living Daphnia or small (pre-rinsed) Artemia (brine-shrimp) to the water with
Hydra, and see how it feeds. Make sketches showing the movements of the
Hydra.
• Class Scyphozoa e.g. Aurelia
These are marine, planktonic organisms that float near the surface of the ocean. They
range in size from a few mm to 2m in diameter.
Examine the preserved jellyfish, Aurelia on demonstration. Note the medusoid body
form - like an inverted polyp. The bell-shaped body has a ring of tentacles. The
mouth is central on the concave underside, and is surrounded by four oral arms,
between which lie the horseshoe - shaped gonads. There is a large mass of jelly, the
mesoglea, between the epidermis and the gastrodermis, which gives the animal
buoyancy.
• Class Anthozoa (e.g., Metridium)
This group of marine cnidarians, which includes the sea anemones and the corals, has
only the polyp stage in their life-history. They remain attached during most of their life
to underwater rocks or even the exoskeleton of larger Crustacea, or the shells of
marine molluscs. In this group, the gastrovascular cavity is divided internally by a
number of vertical partitions or septa.
Examine the preserved sea anemones on demonstration. Note the stout cylindrical
body expanded at its upper (oral) end into an oral disc around a slit-like mouth,
surrounded by several rows of tentacles. The body and tentacles are extended or
contracted by muscular action. The aboral end has a slimy pedal disc, on which the
anemone can slide around slowly, or remain firmly attached to the substratum.
Corals are animals with small polyps, which combine to form huge colonies. Each
member of the colony secretes a protective skeleton of limestone containing a pocket
into which the polyp can partially withdraw. Younger individuals build their skeletons
on the skeletons of dead polyps, so that huge reefs are eventually built, with the living
members near the surface of the ocean.
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Examine examples of coral skeletons on demonstration. Note the variety of forms, and
locate the small pockets that once contained the living polyps.
PART III: Triploblastic Phyla
A. Phylum Platyhelminthes (flatworms)
The animals in this phylum have a markedly dorso-ventrally flattened body, with
three distinct cell layers: an outer epidermis, an inner gastrodermis - surrounding
the gastrovascular cavity, and between these two, a third layer (mesoderm). The
epidermis is derived from the ectoderm and the gastrodermis is derived from the
endoderm. The mesoderm gives rise to a variety of tissues in triploblastic animals such
as muscles, circulatory system, gonads, and bone and cartilage (in vertebrates).
Platyhelminthes are acoelomates because they have no coelom or body cavity (see
Fig. 2). There are three main classes:
1. Class Turbellaria: free-living flatworms that mostly inhabit aquatic or moist
terrestrial habitats.
2. Class Trematoda: flukes: internal or external parasites
3. Class Cestoda: tapeworms: adults of these are intestinal parasites of
vertebrates.
Class Turbellaria (e.g., Dugesia)
Examples of this group are the small freshwater flatworms known as
"planarians". They are common in these habitats, either adhering to, or crawling over
stones and detritus on the bottom.
Examine the living planarians in the dish on your bench. Place one on a
watchglass with some water, and examine under the dissecting microscope.
From its movements, determine its anterior, posterior, dorsal and ventral sides.
On the "head", you should see a pair of eyespots, which give the animal a
distinctive "cross-eyed" look. Note how the animals react to the presence of
strong light. These simple, light sensitive organs will degenerate if the animal is
kept in total darkness for some time.
Examine a slide of a stained, whole mount preparation of Dugesia. Note
the extrusible pharynx on the ventral surface. This leads into the very elaborately
branched, but still sac-like, intestine. There is no anus. The nervous system
consists of two lateral nerve cords near the sides of the body, linked by nerve
connectives in a ladder-like fashion. The reproductive system is quite complex,
since the animals are hermaphroditic (i.e. each individual possesses both male
and female reproductive organs). During copulation, there is mutual exchange of
sperm.
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Members of the classes Trematoda and Cestoda are parasites. Parasitism is a
form of symbiotic (close, long-term relationship between different, unrelated types of
organism) association in which one species benefits and the other is harmed to a
greater or lesser degree. The parasite is usually much smaller than the host and
remains closely associated with it for a considerable period of time. In more extreme
and highly evolved parasites, the life cycles are very complex and involve the invasion
of more than one host species. Many trematodes and cestodes are internal parasites
(endoparasites) of vertebrates. Endoparasites invade the body of their host, where
they live for most of their life bathed in the body fluids of the host. As a consequence
of living internally in another organism, endoparasites tend to be highly specialized for
their particular mode of life. Other than the reproductive system that is quite elaborate
and hermaphroditic, they usually have a very simple body structure because they are
living in a constant environment with a continuous food source. Cestodes are among
the most highly specialized internal parasites, in which most of the internal organs,
other than the reproductive organs are lost. The adult tapeworm lives attached to the
gut of its vertebrate host, bathed in semi-digested liquid food that can easily be
absorbed without a specialized digestive system.
B. Phylum Nematoda (roundworms)
Roundworms are among the most numerous of multicellular animals. They are
unsegmented. Most species are free-living in fresh or salt water or in soil. Others are
parasitic in or on almost every species of plant and animal life. Many are barely visible
to the naked eye. However, some of the species which are parasitic on vertebrates (e.g.
Ascaris lumbricoides) can be up to several meters in length.
• Obtain a drop of culture solution containing "vinegar eels" (Anguillula aceti).
Mount on a slide, cover with a cover slip and examine under the microscope. Watch the
movement of these free-living nematodes, and compare it with the movement of an
earthworm.
• Study a preserved specimen of the parasitic nematode, Ascaris and note the
slender, cylindrical body shape and absence of segments.
• Examine a slide of the cross-section of Ascaris and note the presence of blocks of
longitudinal muscles adjacent to the body wall, but absent from the gut wall. The cavity
between the gut and the body wall is a "pseudocoelom" because it is not completely
enclosed in mesodermal tissue. This space acts as a hydrostatic skeleton. The
longitudinal muscles act against the incompressible fluid in the pseudocoelom, which is
prevented from expanding by the thick, inelastic cuticle.
C.
Phylum Annelida
These are coelomate, segmented worms. Segmentation allows for better
locomotion, due to localized muscles acting in each segment and movement of coelomic
fluid among segments. They are free-living or parasitic, inhabiting marine, freshwater
and moist terrestrial environments. There are 3 main classes:
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1. Class Polychaeta
- marine
- segments have parapodia (lateral appendages) bearing many setae (bristles)
e.g., fan worms and burrowing worms
2. Class Oligochaeta
- mainly terrestrial or fresh water
- conspicuous segmentation; few setae; no parapodia
e.g., earthworm (Lumbricus)
3. Class Hirudinea
- terrestrial, freshwater and marine species
- more rings or annuli on body surface than there are segments
- anterior and posterior suckers; no parapodia; no setae
e.g., leech (Hirudinea) - an external parasite
Examine members of the Phylum Annelida on demonstration. Compare slides of
the cross sections of an earthworm and a nematode.
D. Phylum Mollusca
No single class can be considered to be typical of the phylum -- each class has its
own unique features. However, a number of characters unite such different organisms
as squids, snails, slugs and clams in a single phylum. All molluscs are bilaterally
symmetrical. The body of a mollusc is arranged into three parts: a head, a foot and
an inner visceral mass (the digestive system, glands, reproductive system and
excretory organs). The head and foot are muscular. Molluscs have a coelom (i.e. a
true body cavity surrounded by tissue derived from the mesoderm), but their coelom is
greatly reduced in size. Molluscs have a circulatory system that includes a threechambered heart (two auricles and a single ventricle) that is found in a small portion of
the reduced coelom called the pericardium. Some molluscs use gills for aquatic
respiration (some have primitive “lungs”), and some use their gills for filter feeding.
Many molluscs have a unique feeding structure called the radula that can be used to
scrape food from objects. Molluscs have an extension of the dorsal epithelium known as
the mantle that may form either a pair of flaps or a wall that encloses the rest of the
body. The mantle secretes the shell that is present in many molluscs. The space
between the mantle and visceral mass is known as the mantle cavity. The gills of
molluscs are within the mantle cavity. There are 4 main classes:
1. Class Polyplacophora (radula present)
Chitons - see preserved specimen
2. Class Bivalvia (or Pelecypoda)
- use gills for filter feeding
e.g., Clams, scallops, mussels etc.
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3. Class Gastropoda (radula present)
e.g., Snails, slugs (preserved)
4. Class Cephalopoda (radula and beak-like jaws)
Octopus - 8 arms with suction discs
Loligo (Squid) - 10 arms, including 2 long tentacles
Nautilus - shell with air chambers for flotation
Demonstration: Anatomy of a Mollusc (the clam Mya arenaria)
The clam Mya arenaria is a member of the Class Bivalvia. These are aquatic
filter-feeding invertebrates whose body is unsegmented. The Bivalvia have bilateral
symmetry with two shells or valves enclosing the body. The valves usually remain
slightly open, but can be clamped tightly together by the contraction of powerful
adductor muscles. Most bivalve molluscs, such as the clam, can burrow in the sand or
mud, particularly in shallow water at intertidal regions, by using the large muscular foot
which becomes swollen with blood inside its large blood sinus. A few (such as the
scallop Pecten) can swim actively by clapping the two shells together.
The clam has large paired gills lying in the mantle cavity that are ciliated and
porous and are used for both respiration and filter-feeding. Water enters the mantle
cavity via an inhalant siphon and is drawn over the gills by beating of the cilia. Water
enters the pores in the surface of the gills and it is here that much of the respiratory gas
exchange takes place. The water is eventually expelled via the exhalant siphon. In
addition, the thin mantle wall itself functions as a respiratory surface. Microscopic food
particles in the water are trapped in a layer of mucus on the gill surfaces and are passed
to the mouth along food grooves at the edges of the gills and eventually to the mouth
via the labial palps. The clam has an open circulatory system with a threechambered heart and blood vessels that open into sinuses. One of these sinuses (the
pericardial sinus) surrounds the heart, and another sinus is in the foot. Bivalves have
no differentiated head region and, since they are filter feeders, lack a radula or rasping
tongue characteristic of other molluscs.
Fig. 3.1. External anatomy of the clam, Mya arenaria
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The two shells or valves of the clam are fastened along the dorsal surface by an
elastic hinge ligament. In living bivalves, the two valves can be tightly clamped
together for protection against predators or loss of moisture (for instance when exposed
at low tide). Note the concentric lines of growth. The oldest part is the umbo, a raised
region at the anterior end of each valve.
Fig. 3.2. Internal anatomy of the clam, Mya arenaria
Note the large anterior and posterior adductor muscles that hold the valves of
the clam closed. They were cut so that the valves could be opened fully. Next to the
valves is the semi-transparent mantle that hangs like a sheet attached dorsally and free
ventrally. Next to the large adductor muscles are the smaller anterior and posterior
retractor muscles. Also note the exhalent siphon (nearest the hinge) and inhalent
siphon of the
mantle.
Fig. 3.3. Internal anatomy of the clam, Mya arenaria (mantle removed)
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With the mantle removed or deflected, the uppermost pair of gills, the muscular
foot, and the labial palps near the mouth are visible. Removal of tissue dorsal to the
gill region exposes the heart, rectum and kidney. Trace the rectum posteriorly and
look carefully for the anus that opens into the exhalent siphon. A large digestive gland
is dorsal to the mantle and next to the anterior adductor muscle. Note the visceral
(nerve) ganglion against the ventral side of the posterior adductor muscle with nerves
running into the kidney.
The sexes are separate in the clam. The reproductive organs, either testes or
ovaries are similar in appearance and can be seen as light-coloured masses in the
visceral mass above the foot and extending partly into it.
The digestive system of the clam may be difficult to interpret. The mouth is
situated at the base of the labial palps and leads into an esophagus and stomach that
are embedded in the soft tissues of the digestive glands in the region between the
adductor muscles and the visceral mass. There are openings from the digestive glands
into the stomach. The intestine has an internal typhlosole (an infolding of the
intestine that increases surface area for absorption of digestive products). Most of the
intestine is embedded within the gonads in the visceral mass. The intestine joins the
rectum that runs through the pericardium before opening into the exhalant siphon at
the anus. A pericardium in molluscs and arthropods is an expanded part of the blood
system that contains the heart and supplies blood to it.
Assignment: Answer the following questions, and submit the following
drawing. (total = 17 marks, worth 2% of final grade)
Questions:
1) How can radial symmetry be an advantage to marine organisms that are slowmoving or sessile (i.e., permanently attached to the substrate)? In what
circumstances may radial symmetry be disadvantageous? (4 marks)
2) Parasites such as trematodes and cestodes are usually hermaphrodites, but two
individuals frequently cross-fertilize each other. What is the selective advantage
of hermaphrodism to a parasitic animal such as a fluke? In what other
circumstances may it be advantageous to be a hermaphrodite? (4 marks)
3) What are the differences between the feeding methods of sponges and Hydra? (4
marks)
Drawing:
1) Make a labeled drawing of the whole mount of Dugesia (5 marks).