LABORATORY EXERCISE #4

KINGDOM ANIMALIA - PART II

 

ROTIFERA, NEMATODA AND ANNELIDA

 

Introduction

 

The three phyla covered in this exercise represent several evolutionary advancements over the taxa examined last lab period.  Not only are they bilaterally symmetrical, but they all have a complete digestive system (a mouth at one end and an anus at the other) and a body cavity or coelom.  Most are elongated, worm-like creatures.

 

Phylum Rotifera - The Wheel Animals

 

The rotifers constitute the most numerous and most characteristic animals inhabiting freshwater.  They derive their name from the cilia typically found on the anterior end, the motion of which gives the impression of a rotating wheel.  The body cavity of these minute animals is termed a pseudocoelom, since it is not lined with the mesodermal tissue typical of most animals.  A unique feature, found only in the rotifers, is the muscular mastax, an organ located in the digestive tract and which bears tiny jaws used by the animals in a variety of ways.  Most rotifers are female and the usual method of reproduction involves the hatching of unfertilized eggs, a type of parthenogenesis ("virgin birth").

 

The more than 1500 rotifer species are mostly cosmopolitan (distributed world wide) and show a variety of shapes and modes of life.  Rotifers typically inhabit the waters of lakes and ponds, but may live in moist moss and on beaches among the grains of sand. Some rotifer taxa can withstand near total desiccation (drying out) and be "reconstituted" by the addition of water.  They are remarkable little animals indeed!

 

 

ACTIVITIES

 

a. Select a prepared slide of the common rotifer.  Examine it on high power and, referring to Figure 96b in the Photo Atlas identify the wheel organ (corona), foot, and mastax.  Figures 96c and d illustrate two other rotifer taxa.

 

 

Phylum Nematoda - The Roundworms

 

The phylum Nematoda is a ubiquitous group, which, never the less, is relatively unknown to most people.  These rather unspectacular-looking worms are nearly everywhere.  It has been suggested that if all matter on earth except nematodes was eliminated, a ghostly outline of the planet and most organisms would remain!  These worms are mostly small and free-living, but some are important internal parasites of humans and most other animals, as well as plants.  The diversity of nematodes is such that the total number of species on earth may exceed that of the arthropods!

 

Nematodes are characterized by bilateral symmetry, a complete digestive system, a pseudocoelom and a tough, non-living outer covering, the cuticle.  Unlike the annelids, which we will examine shortly, the nematodes lack the layer of circular muscles, which allow animals such as earthworms to elongate and shorten their bodies.  This gives the nematodes an inefficient, thrashing method of movement.  The nematodes seem to represent an evolutionary dead end and probably did not give rise to higher forms of life.

 

 

ACTIVITIES

 

b. Obtain a slide of the free-living nematode Anguillula oceti, often called the vinegar eel.  This small worm shows the simple anatomy typical of the nematodes.  The nematode in Figure 94a of the Photo Atlas may be useful as you view the anatomy of this organism.

 

c. Select a specimen of Ascaris, the pig roundworm, and place it in a dissecting pan in a little water.  Referring to Figure 94b in Perry and Morton, determine the sex of your specimen.  Be sure you see both sexes and can identify both.  Find the anterior end by locating the mouth, surrounded by three "lips".  These large nematodes live in the intestines of pigs and humans.

 

d. Take a slide of Trichinella and examine it, first on low, then high power.  This slide shows larval worms encysted in muscle tissue.  Does yours look like Figure 94a?  It should!  These worms are usually contracted by eating inadequately cooked pork.

 

e. Now look at a slide of the dog hookworm, Ancylostoma.  These blood-sucking parasites live in the gut of dogs and are responsible for the deaths of many puppies each year.

 

 

Phylum Annelida - The Segmented Worms

 

To most people, the word "worm" conjures up an image of an annelid, since the common earthworm or night crawler is a member of this phylum.  The annelids take their name (meaning "little rings") from the repeating units, which make up the body of the worm.  These rings or segments are quite similar, both inside and out.  In addition to being segmented, members of this phylum possess a coelom different from those encountered thus far.  This body cavity is lined with mesodermal tissue, which makes it a eucoelom (the prefix "eu-" means "true").  All animals studied subsequent to the annelids will be eucoelomate. The annelids continue the trend toward specialization of the anterior end of the body, with some members of the phylum having a well-developed head.  There are three classes in Phylum Annelida:

 

1.     The earthworms are the annelids most familiar to us.  They are commonly used as fish bait and are frequently seen along sidewalks and roads after a rain, having been flushed from their burrows.  These animals seem to be quite simple, since they lack eyes or any of the other obvious structures we generally associate with a head.  In fact, there seems little to distinguish one part of their body from another.  If one were to examine these animals internally, however, the story would be different.  The earthworms have a well-developed digestive system and a closed circulatory system reminiscent of our own.  They also have two full sets of reproductive organs, a characteristic which makes them functionally bisexual (hermaphroditic).

 

 

2. The clamworms are marine worms, which do not bear much resemblance to the earthworms, other than their segments.  They are predaceous worms with a well-formed head equipped with eyes, jaws, and tentacles.  Most segments of the clam worm bear a pair of leg-like appendages, the parapodia, with which it swims and breathes.

 

3. The leeches include the blood-sucking external parasites of fact and fiction.  Other taxa are free-living and function as predators or scavengers.  Leeches possess one or two suckers.  The segmentation of these creatures is not as complete as in the previous classes of annelids.

 

 

ACTIVITIES

 

f.  View the dissected specimens of the roundworm, Ascaris and Lumbricus, the earthworm.  Note the presence of septa, which divide the earthworm coelom into its segments.  Are such septa present in the roundworm?  Referring to Figures 95c and d in Perry and Morton, note that the Ascaris is little more than an elongate bag containing an intestine and reproductive structures.  Can you see how much more complex the earthworm, shown in Figures 99b and c, appears?

 

g.  Take an earthworm, rinse it briefly, and place it in a dissecting pan in a little water.  Pick up the worm (it's NOT yucky) and gently run your fingers along the ventral (bottom) surface of the worm.  Can you feel the tiny, bristle-like setae?  These allow the worm to move inside its burrow.  Using Figure 99a, locate the following parts: Mouth, anus, clitellum, segments.  The clitellum is used to secrete the cocoon, which houses the fertilized eggs.

 

h. Now replace the earthworm with a clamworm.  Referring to Figures 100a and b, locate the parapodia, setae, and tentacles.  Gently squeeze the anterior end of the worm to evert the jaws.

 

i.  Examine a slide of the leech on 40x magnification.   Locate the suckers.  Can you identify this animal as a leech, an annelid and an animal?

 

J. Examine the preserved specimen of the medicinal leech.  These large leeches have been used by the medical profession to lower blood pressure, treat black eyes and reduce post-operative swelling.  Figure 100c illustrates the medicinal leech.

 

 

 

LABORATORY EXAM NEXT MEETING

 

This practical examination will cover material in Labs 1, 2, 3, and 4.  Your instructor will announce when the exam is to begin.