Lab 3
Nematoda, Rotifera, and Annelida

    In this laboratory period you will study two interesting and important phyla of worms.  the first, Nematoda, is probably relatively unknown to you, although each of you has probably had (or is having) an intimate association with one or more taxa of the phylum.  The other phylum, Annelida, contains animals familiar to us all.  Both taxa show important evolutionary advances when compared with the flatworm and coelenterates.  The third phylum, Rotifera, contains creatures appropriately called "wheel animals".

LAB OBJECTIVE

    1.  To gain some exposure to the diversity within the taxa presented.

    2.  To learn simple dissecting techniques.

    3.  To identify the basic anatomical parts of the nematodes and annelids.

    4.  To begin to build a knowledge of animal systems and their functions.

I.    Phylum Nematoda (round worms)

The taxa included in this phylum have been classified in different ways by many authorities.  In some schemes, several of the taxonomic classes have been given phylum status, but more recently they have been brought into the phylum Nematoda due mostly to their unique type of body cavity, the pseudocoel, and the cuticle, a tough, non-living outer covering.  A pseudocoel differs from a true coelom, in that the internal organs are actually free within the cavity, since no membranes suspend the organs, nor is the cavity completely lined with mesoderm. 

This interesting phylum, along with two other obscure phyla of pseudocoelomates, is probably not an ancestral form leading to the coelomate phyla, but represents an evolutionary "dead end".

Members of the phylum Nematoda are among the most numerous animals on the globe.  Every fist full of soil, each rotten apple, each square meter of lake bottom, holds thousands.  While most are free-living, many are important parasites of plants and animals.  It has been estimated that when all species of Nematodes are properly classified, they will outnumber the Arthropods.

In addition to the pseudocoelom, important Nematode characteristics include a tough exterior covering called the cuticle, and a lack of circular muscles.  This gives the worms an awkward, thrashing method of motility.

Question 1:  Why do you suppose so few species of Nematodes have been classified and described?

Question 2:  Why would a lack of circular muscles result in such a strange method of locomotion?

ACTIVITIES:
1.    Select a specimen of Ascaris lumbricoides, rinse it in running water, and place it under a little water in a dissecting pan.  This round worm is quite large for a Nematode.  It is a parasite generally found in the intestines of pigs and children. 

In many invertebrate groups there is no distinction among the animals by sex, i.e., no obvious male and female.  Such is not the case with Ascaris.  The male worm can be distinguished from the female by its smaller size and the bend in the posterior part of its body.  Thus, Ascaris exhibits the phenomenon of sexual dimorphism.  Be sure you see both sexes.  The mouth of the smaller, anterior end is bordered by one dorsal and two lateroventral lips.  You may observe the mouth region at the demonstration microscope set up by your instructor.  Find an opening near the posterior end of the worm.  In the female, this is a true anus, in that it is the terminal opening of the digestive tract.  In the male, the anus also functions as the opening of the genital tract and is more properly called the cloacal opening.  Projecting from it you will see the copulatory spicules.  The genital opening in the female, called the vulva, is located midventrally about 10-12 cm posterior to the mouth.  The excretory pore is just posterior to the mouth on the ventral surface.  When your study is completed, return the specimen to the jar.

Question 3:  What is meant by sexual dimorphism?

Question 4:  Why is the Ascaris not digested in the intestine of its host?

Question 5:  What is a cloaca?

2.    Obtain a microscope slide showing a stained cross-section of Ascaris and identify the following structures, using figure 2: cuticle, epidermis, pseudocoel, digestive tract.

3.    Examine a slide of the muscle fibers containing specimens of encysted Trichinella spiralis.  This is a parasitic worm that infects man, rats and hogs.  At one stage of its life cycle, the larval worm forms a cyst in the striated muscle of its host.  In your slide, find a worm in its spindle-like cyst.  This cyst is formed of a fibrous tissue that eventually becomes calcified, surrounding the parasite in a wall of "reinforced concrete".  Larva may live 10-20 years in these cysts.  If eaten in poorly cooked meat, larvae are liberated in the intestine.

Question 6:  How may infections of Trichinella be avoided? 

II.  Phylum Rotifera (the wheel animals)

This class of microscopic pseudocoelomates is the most numerous and most characteristic animal taxon occurring in freshwater.  They derive their name from the motion of cilia typically found on the anterior end, which gives the impression of rotating wheels.  The more than 1500 species are mostly cosmopolitan (distributed world wide) and show a variety of shapes and modes of life.  Study figure 3 depicting representative rotifers.  A distinctive characteristic of the rotifers is possession of an unusual grinding organ, the mastax, in the digestive system. 

Question 7: What is a cosmopolitan species?

ACTIVITIES:
1.    Select a prepared slide of the common rotifer labeled Philodina.  Refer to figure 1 and locate the corona, mastax, yolk gland, intestine, and foot. 

2.    Your instructor may project a series of slides to illustrate the diversity within rotifers.

III.    Phylum Annelida (segmented worms)

The annelids are worms whose bodies are divided into similar rings or segments.  This biological principle of body segmentation is called metamerism and the divisions are known as metameres.  The metamerism in the annelids is manifested, not only in external body features, but also in the internal arrangement of organs and systems.  In addition to metamerism, the annelids show several advancements over the lower phyla.  They possess a true coelom, show specialization of the head region (cephalization), and have a complete digestive system (both mouth and anus present).  In all probability the arthropods evolved from an annelid-like ancestor.  The chief taxonomic classes of Annelida are Oligochaeta, Polychaeta, and Hirudinea.

ACTIVITIES:  

 A. Earthworms
    1.  The common earthworm, Lumbricus terrestris, is a representative oligochaete.  You should select an undamaged specimen, rinse it in running water, and cover it with a little water in a dissecting pan.  Note the form of the body.  What sort of symmetry does it exhibit?  Is the body perfectly round?  Distinguish between the anterior and posterior ends.  The clitellum, a group of enlarged metameres, is closer to the anterior end.  Count the number of metameres on your specimen and compare it with others.  The mouth is in the first segment and overhung by the prostomium, a fleshy lobe which is not considered a segment.  Draw the specimen between the fingers and feel the setae, tiny, bristle-like structures.  What is their function?  Locate the anus, a vertical slit in the terminal segment.  Refer to figure 4.

Question 8:  What is the function of the clitellum? (Consult your instructor.)

    2.    We will now dissect the worm to expose its internal structures for study.  Begin the dissection by placing a pin through each end of the specimen.  With scissors, make an incision slightly to the right of the blackish median dorsal blood vessel, ten segments anterior to the anus, and proceed anteriorly to the mouth.  The sides of the body wall should be pinned out progressively as the cut is made.  Avoid cutting the intestine; if black material oozes out, you are cutting to deep.  Note the coelom is not continuous, but is divided into a series of compartments by thin partitions called septa.  Cut the septa free from both sides of the body wall before pinning it down.  the dissection should be done with the specimen covered with about 1/4 inch of water.

Question 9:  What are some advantages accruing to the animal possessing a true coelom? (See text.)

        a.    Digestive system - The mouth cavity extends through the first three segments, terminating in a thick-walled pharynx.  Take care not to damage the brain which lies dorsal to the digestive tract in the third segment.  The esophagus is a tube leading to the crop, a large thin walled sack in segments 15 and 16.  Posterior to the crop, in segments 17 and 18, is the gizzard.  The intestines extends from the gizzard to the anus.  Refer to figure 5.

        b.    Circulatory system - There are two major blood vessels in the earthworm.  The dorsal vessel runs along the dorsal surface of the digestive tract, and the ventral vessel extends along surface of the gut.  These two vessels are connected by the "hearts", pulsating organs encircling the esophagus in segments 7 to 11.  Refer to figure 5.

Question 10:  What is the function of the circulatory system?

Question 11:  What is the function of the "hearts"?

        c.    Excretory system - Use a hand lens to examine an excised section of the body wall, cut from the region just posterior to the clitellum.  The minute, white, coiled tubes are the nephridia.  A pair of these is located in each segment, except for the first three and the terminal one.

Question 12:  What is the function of the nephridium? (See text.)

        d.    Nervous system - The two-lobed brain is located on the anterior dorsal surface of the pharynx in the third segment.  The brain is connected to another nerve center, the subpharyngeal ganglia, by two nerves which circle the pharynx.  From this center, a ventral nerve cord extends along the floor of the coelom to the last segment.  Refer to figure 5.

Question 13:  What is a ganglion? (See text.)

        e.    Reproductive system - The earthworm is hermaphroditic, with sex organs of both types located in the ninth through the fifteenth segments.  Most conspicuous of the male organs are the seminal vesicles, light colored bodies in segments 9-12.  The testes, located within the seminal vesicles, and the remainder of the male organs are to small or obscure to be examined in this procedure.  Of the female organs, only the small, white, spherical seminal receptacles in segments 9 and 10 are larger enough for satisfactory study.

Question 14:  What is the meaning of the term "hermaphroditic"?

WHEN YOUR STUDY IS COMPLETE, DISPOSE OF THE DISSECTED WORM AND ITS PARTS IN THE CONTAINER PROVIDED.

    3.    Select a prepared slide of the earthworm in cross-section.  Refer to figure 6 and identify the following parts.

        a.    The body wall is made up of five layers.  Outer-most is the non-living cuticle, secreted by the underlying epidermal cells.  Under this are two layers of muscle cells, an outer layer of circular muscles which contract to lengthen the worm, and an inner layer of  longitudinal muscles which serve to shorten the animal.  Last is a thin layer of mesodermal tissue which serves as the lining of the body cavity, making it a true coelom.

        b.    Within the body cavity lie the gut and its associated organs, the large dorsal blood vessel and the smaller ventral vessel.  Note that the intestine contains a large "T-shaped" fold called the typhlosole.

        c.    Other structures which should be identified are the nephridium, the ventral nerve cord and the setae.

B.  Sandworms

    To exhibit the structure of a generalized polychaete, we will view the sandworm Nereis.  This is a common marine worm found in the sand along the Atlantic coast.  Only the external anatomy of Nereis will be studied; consequently, the preserved specimen should be returned in good condition at the end of the laboratory period.

    Note the well-developed head, a characteristic that is easily discerned in the polychaetes.  Observe the triangular projection above the mouth, the prostomium, which bears a pair of tentacles, a pair of thick projections known as the palps and two pairs of small, beadlike eyes.  The first complete ring-like segment that surrounds the ventrally-placed mouth is the peristomium.  It possesses four pairs of peristomial tentacles.  The mouth is a ventral, transverse slit, but it may show a pair of black chitinous jaws.  Each of the segments back of the head possesses a pair of parapodia.  Note the bristle-like setae on these appendages.  Find the terminal anus.  Refer to figures 7 and 8.

C.  Leeches

    This class includes the blood-sucking leeches of which you have read.  Other leeches are free-living as either predators or scavengers.  Most have anterior and posterior suckers and lack the setae characteristic of most annelids.  Examine the leeches under the dissecting scope set up for demonstration.  Compare them with the other annelids you have seen.  Refer to figure 9.