Download LAB # 6: PHYLUM ANNELIDA 1. Overview The annelids comprise

LAB # 6: PHYLUM ANNELIDA
1. Overview
The annelids comprise the familiar invertebrates such as earthworms, clamworms and
leechs. The 12,000 or so species that make up this phylum are of great interest
biologically. It is often noted that members of this Phylum represent the ultimate in the
organisms that we generally call ‘worms’. They are found in all of the major habitats marine, terrestrial and freshwater. The main anatomical feature of these worms is that
their body is composed of cyclindrical compartments called segments or metameres.
Structural features of this group relative to others include 1) development of a true
functional coelom, 2) the presence of most of the organ systems found in the higher
inverts and the chordates and 3) true metamerism, the division of the body wall and
coelom into segments with the linear repetition of at least some of the internal organs. A
characteristic larval form, the trochophore, occurs in the class Polychaeta.
2. Essential features of the lab
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Annelid external and internal anatomy (dissection)
Annelid diversity (three major classes)
Annelid cladogram
Annelid ultrastructure and reproductive morphology (slide material)
Class Polychaeta Class Oligochaeta Class Hirudinae -
3. Classification
Glycera, Nereis, Chaetopterus, Arenicola, Sabella
Lumbricus
Placobdella, Hirudo
4. Special Features
a. Functional morphology and General Biology
Class Polychaeta
These are a diverse, marine group of segmented worms. Despite their wide diversity of
form and function, their single unifying feature is the presence of paired locomotory
appendages called parapodia. These possess bristle-like setae, also used for locomotion.
The anterior region (prostomium and/or peristomium) of polychaetes often possesses a
distinctive head, proboscis and variously modified jaws and antennae, or tentacles.
Compare the functional external morphology of each of the 5 species of Polychaete on
display. Without detailed knowledge of each of these species, you should be able to
relate their external morphologies with their life styles, feeding habits and reproductive
modes. In each case the body is comprised of a series of similar cylindrical segments,
each with a pair of parapodia that are important in both locomotion and respiration. The
coelom is compartmentalized by inter-segmental septa that are perforated to allow
coelomic fluid to pass from one segment to another. As you can see from the specimens
on display, not all polychaetes are uniformly segmented. Also, the parapodia may be
grouped into regions that differ in shape, size, and function. These specialized, functional
regions are called tagmata. Aim to understand the functional roles of the tagmata in
each of the 5 species shown on the side bench; in each case try to interpret the major roles
of the coelom.
For functional morphology, focus on the common marine clam worm, Nereis. These are
common in many coastal areas, particularly those with soft, muddy substrates. They are
important prey items for shorebirds, marine fish, and a variety of predaceous
invertebrates. Examine the external anatomy of this predatory carnivore, paying attention
to the head region. Make a wet mount of a parapodium, noting the complex structural
arrangement of this diagnostic polychaete feature. There is also a preserved example of
Nereis parapodia in your slide box.
The entire body is made up of similar metameres, each bearing lateral parapodia. This
consistent arrangement of sections differs during the reproductive season, as almost all of
the clam worms are epitokous. The non-reproductive individual is called an atoke;
reproductive individuals are called epitokes. In the latter case, the posterior 50% of the
body becomes modified with different types of parapodia (that now have a swimming
function) and a thinner body wall in each metamere.
Study the external and internal morphology of Nereis by referring to Figure 13.3 in your
text. Take note of the complex anterior region, noting the prostomium, eyes, palps,
antennae and peristomium (segments 1 and 2). The mouth is on the ventral side of the
peristomium. There is a posterior anus and caudal cirri. Depending on your specimen,
the pharynx will be extended or retracted. If muscles can only contract, how does the
animal extend its’ pharynx?
We will not spend too much time on detailed internal anatomy. In a demonstration
dissection, note the thick circular muscles below the epidermis. Depending on the
condition of the preserved specimens, we may see the longitudinal muscles that lie
internal to the circular muscles as 4 symmetrically placed masses, within the large
coelom. Running diagonally across the coelom are the oblique muscles that operate the
parapodia. We may also be able to see the dorsal and ventral blood vessels.
Class Clitellata
Sub-class Oligochaeta
This group includes 3000 or so familiar species of terrestrial and freshwater annelids.
They lack paropodia but possess setae. The term ‘earthworm’ is familiar to us, but
incorrect. Most species are freshwater, and there are many microscopic marine types.
They differ from the Polychaetes in their uniformity of structure and the presence of a
distinct clitellum (in terrestrial species). They lack parapodia and complex setae and are
always hermaphroditic. They range in size from less than 1mm to the giant Australian
earthworm that can reach a whopping 7m.
We will have a detailed look at the widely available Lumbricus, but take note that it is not
a typical Oligochaete.
First, pick-up a live specimen. You will immediately notice its’ capacity for mucus
production. Gently hold opposite ends of the worm in each hand and note the strength of
contraction. Run your fingers down the length of the worm while holding onto one end.
You should be able to detect its setae (4 pairs of 2 on each segment) and also the strength
of the hydrostatic skeleton. Locate the anterior end with its much-reduced prostomium
and peristomium surrounding the mouth. You will also note the distinct clitellum, used
to produce mucus during reproduction and to produce a cocoon to store eggs.
We will dissect freshly-killed specimens. Do the dissections under water. First, pin the
animal so that the ventral side is uppermost (see figure below). At the anterior end is the
mouth, located on the ventral portion of segment 1. The short knob-like projection is the
prostomum, which is not considered a true segment. Count posteriorly to segment 15.
Here you will find the paired male genital pores. Just anterior, in segment 14, you will
find the tiny female genital pores that lead to the oviducts. The paired openings of the
seminal receptacles are between segments 9 and 10, and 10 and 11. On the ventral side
you will also see the two sperm grooves leading from the male genital pores to the
clittelum that spans segments 32-37. The intent here is for you to note the complex
functional reproductive morphology of these hermaphrodites. Make sure that you can
match this morphology to the actual process of sexual reproduction that we discuss in
class.
Pin the specimen dorsal side up in dissecting pan, covered in water. Make a longitudinal
cut along the mid-dorsal line beyond the clitellum. The major internal structures to
observe are the digestive tract and septa, the pharynx, crop, gizzard, esophagus and
intestine. On the surface of the digestive tract can be seen the dorsal blood vessel. In
sections 7-11 this vessel expands to produce five pairs of hearts that pass around the gut
to connect to the ventral vessel. In an anesthetized worm, you should be able to see the
functioning of the dorsal vessel and pumping hearts.
Remove the digestive tract and you will see below it (near the hearts), a set (usually 3
prs) of large sacs - the seminal vesicles. The seminal receptacles or spermathecae are
located under the lobes of the anterior two pairs of the seminal vesicles. The testes are
located in segments 10 and 11; the ovaries in section 13. Some of the reproductive
structures might be difficult to find, depending on the time of year the worms were
collected.
The digestive system is a linear tube with few specialized regions. Use the appended
figure to identify as many of the features of the gut as you can. If you are especially
careful, observe the excretory system as represented by paired nephridia within each
segment. You should be able to find them with the dissecting microscope, especially by
examining the inner body wall surface in the region near the gizzard. They should be
present as convoluted tubules (see Fig. 13.2).
Dorsal to the buccal cavity (near the pharynx in segment 3), locate the brain. You should
also notice various ganglia leading anteriorly and laterally from the brain. The ventral
nerve cord extends posteriorly, branching as it passes past each segment.
You should note also the cross-section of earthworm in your slide box (and compare with
figure 13.18 in your text), taking note of setae, muscle arrangement and perhaps
nephridia.
Sub - Class Hirudinae
This group includes the common freshwater leeches. There are also many species of
marine and even terrestrial species. Many are adapted as blood-feeding ectoparasites.
Many are also scavengers and carnivores. These annelids lack parapodia and setae.
Although they do possess external segmentation, they are not internally segmented, like
the other two groups. They are also separated from the other annelids by having both an
anterior and posterior sucker, and they never reproduce asexually (but they are still
hermaphrodites). Their coelom is greatly reduced by the presence of muscular tissue and
the enlarged caecal sacs of the digestive system.
Alberta’s leeches are known to play an important role in our eutrophic waters. They play
an important part in decomposition and recirculating bottom sediments. They also act as
important vectors for trypanosomes (and also trematodes) of fish and amphibians. They
are also a major prey source for loons, grebes, walleye and pike. It was always a mystery
how loons could successfully rear their young on the 1000’s of fishless lakes that are
common in northern Alberta. It turns out that large leeches are one of their staple food
items.
We will have living specimens of the common leech, Glossiphonia. These are readily
available from bait stores, used primarily by walleye fisherman. Compare your specimen
with Fig. 13.22 in your text. Notice the large number of annuli (these are not true
segments and obscure the true, underlying, metameres). Observe the smaller anterior
sucker with the mouth at its centre. The posterior sucker includes the last 6-8 body
segments; the anus opens just anterior to it. Under the dissecting scope, you can observe
several tiny eyespots along the dorsal edges of the anteriormost segments. Turn your
specimen over so the ventral side faces up. Leech dissection is a difficult task – but
attempt it if you have time to note the highly specialized annelid body plan. Cut along the
body wall in midbody, and extend your incision anteriorly and posteriorly the length of
the worm. Carefully remove the tissue to expose the coelom and internal organs. At the
anterior end you will notice the jaws or a muscular proboscis depending on the species.
Follow the digestive tract backwards until you reach the point where it branches into
gastric cecae. Between the cecae are the rounded testes; lying between them on either
side are the elongated ovaries. There are usually 11 pairs of gastric cecae, all connecting
to a central crop. It is here where host blood is stored and ingested.
b. Annelid diversity
Observe the following specimens, noting the diversity of body shapes. Compare the
external anatomy of the various species. You should be at a point where you can make
reasonable guesses about their feeding biology, where they might live (hard vs soft
substrates), how they avoid predation, and their overall ecological role.
Glycera - This species is long and cylindrical. It has a huge pharynx armed with four
hooked jaws (each with an associated poison gland) used in prey capture. The large
proboscis is also used for burrowing. It jams the proboscis into the substrate, inflates it
(via mechanisms associated with the coelom) and then pulls the body along behind it.
Aphrodita - this is the common Sea Mouse. The body is oval and broad with a flattened
ventral surface. The dorsal surface is covered by thick hairlike projections, used in
defense. They are slow moving and omnivorous. Do you see any evidence of
tagmatization?
Arenicola - These are the so-called lugworms which are common in Europe and along
both coasts of the N.America. They are prized commercially as fishing bait. They have a
thick, fleshy body divided into 2-3 specialized tagmata. The pharynx is eversible and
aids in burrowing and feeding. They make characteristic U-shaped burrows and are
deposit feeders. The down-curved bill of long-billed curlews (which breed near
Lethbridge) are thought to be a specialization to remove worms such as these, which
build U-shaped burrows.
Chaetopterus - This species is fleshy and relatively large. It is divided into two or three
functional regions with highly modified parapodia. They live in permanent U-shaped
burrows lined with secretions from the worm. Their feeding strategy is bizarre! They
create a water current through the tube by using their fan-shaped parapodia. They secrete
a mucous-bag which captures food particles, and they remove useable food after they eat
the bag, mucus and all.
Lumbricus - this is the common terrestrial earthworm. They are direct deposit feeders,
well known for their role in maintaining soil fertility. They have complex reproductive
systems, often involving the ability of some segments to regenerate (although the
common Lumbricus we use for fishing has poor regenerating abilities). Many freshwater
species can undergo ‘cyclomorphosis’, similar to rotifers.
Tubifex - These are the so-called sludge worms or tubifex worms, common at pet stores.
They can be important as environmental indicators of heavily polluted, slow moving
water.
Chaetogaster - these are small, symbiotic annelids that we have seen on the shells of
Helisoma and Physa. Very little is known about the biology of this species in our area.
What is known however is that the annelids feed on cercaria from infected snails. Even
more amazing, they also are known to feed on miricidia which of course are looking to
infect the snail. This is an amazing way for the host to avoid becoming castrated!
Placobdella spp. – this is a common species of leech in our local waters. But it is very
usual to find a specimen such as this. This species is one of our few viviporous leeches,
and in this case it is clearly also a brooder.
5. Questions for discussion
1. What modifications of the ancestoral Polychaete body plan were required by the
Oligochaetes to colonize terrestrial habitats (especially in terms of respiration and
reproduction)? Which characteristics of the Polychaetes pre-adapted Oligochaetes for
successful invasion of land?
2. How have leeches adapted from sedentary or predaceous Polychaetes (or Oligochaetes
- their ancestory is controversial) to become parasitic?
3. Compare and contrast the various mechanisms used by the three Classes to avoid
predators.
4. Compare the locomotion of the motile polychaetes with oligochaetes and leeches.
5. How does the phenomenon of epitoky fit into our class discussions on the evolution
and maintenance of complex-life cycles?
6. Why does an earthworm have a well-developed gizzard, but not the leeches or
polychaetes?