LABORATORY # 8

INVERTEBRATE ANIMALS – II

LABORATORY OBJECTIVES

Upon completion of this laboratory the student will:

1. Be acquainted with trends in animal evolution.
2. Know the features that characterize each animal phylum.
3. Be familiarized with representatives from each phylum.

REFERENCE

Textbook:  chapter 21, pp 422 -- 443, and 446 – 454

Photo Atlas:  chapter 7, pp 151 – 155, and 162 – 177

INTRODUCTION

The Arthropods, Mollusks and Echinoderms are the invertebrate Animal phyla that are of greatest complexity.  Also, in terms of diversity, numbers of species, and numbers of individuals, they are the most successful organisms on earth.  In this exercise you will be familiarized with the most common classes of each of these three phyla.

Phylum Arthropoda – The Insects, Crayfish, Spiders and Their Kin

Contrary to the opinion of most humans, self-centered as we are, it is probably the arthropods, not the mammals that represent the pinnacle of evolutionary advancement.  Arthropods occupy nearly every conceivable habitat on the earth and exceed all other animal and plant phyla combined in both number of species and number of individuals.  Arthropods were the first land animals.

Arthropods are of great importance to human society.  Animals such as crabs, shrimp and lobsters are important food items, while many insects play a critical role in the reproduction of valuable plant species.  On the negative side, arthropods may be vectors of deadly diseases and one group, the insects, are humankind's chief competitors for food, fiber and other important commodities.

Arthropods are characterized by a more-or-less hard exterior skeleton (exoskeleton) made of a material called chitin.  Your hair and fingernails are composed of a similar material.  To allow movement, this exoskeleton is jointed, much like a suit of medieval armor.  To allow growth, this covering must be cast off from time to time and replaced by a larger exoskeleton.  This process is termed molting and is indulged in by all arthropods.

Like the annelids from which they probably evolved, the arthropods are segmented.  Unlike their ancestors, however, the arthropod body has several segments fused and otherwise specialized to form two or three body regions.  Animals such as the insects have three body regions; an anterior head, a medial thorax and a posterior abdomen.  Crustaceans, however, have only two; a cephalothorax, which represents the fused head and thorax, and the posterior abdomen.

The following are major groups or classes within the phylum Arthropoda:

1. The trilobites are an ancient group of organisms, all of which have been extinct for nearly 250 million years.  They dominated the oceans of the world during the 350 million years of their existence, however, and thus merit our consideration.  Trilobites get their name for the three lobes of their bodies, one medial and two lateral.  One can readily identify the three body regions of the trilobite, these being the anterior cephalon, a medial thorax and the posterior pygidium.  There was considerable variety in shape, habitat, and way of living among the trilobites.

2. The insects are the most diverse group of arthropods.  They first appeared on earth around 400 million years ago and the basic design hasn't changed much.  As mentioned earlier, the insects have three body regions.  Each of the three thoracic segments has a pair of jointed legs.  Unique among the arthropods are the wings, which appear on the last two thoracic segments of most insects.  The head of the insect is characterized by the presence of paired mouthparts, a pair of compound eyes and a single pair of antennae.  Respiration among the insects is carried out by a system of air tubes or tracheae.  These open to the exterior through valved openings termed spiracles.  Some insects undergo a radical change in form (metamorphosis) during their development.  The forms are called larva, pupa and adult.  An example is the butterfly, an adult form that began life as a caterpillar (larva), and then formed a cocoon in which the pupa reorganized itself into the adult.  There are more species of insect than all other animals combined.  Examples include bees, ants, moths, aphids, houseflies, dragon flies, grasshoppers, and all types of beetles.

3. The crustaceans are to the water what the insects are to the land.  These organisms have only two body regions; a cephalothorax covered with a hard carapace, and the abdomen.  The number of legs varies among the Crustacea, but they usually have a stalked pair of compound eyes and two pairs of antennae.  Since they are water-dwellers, they respire with gills.  Many of our favorite "shellfish", like shrimp, crab, and lobster, are crustaceans, but there are many others also.  An important category of crustacean is the micro-crustacean or planktonic variety.  These serve as important food sources for a variety of organisms, from small fish to whales.  Another interesting crustacean is the barnacle.  These secrete hard shells, which form crusts on virtually anything in the ocean.

4. The arachnids are a misunderstood and much-maligned group of organisms.  Largely due to the fact that a small number of their species constitute a threat to the well being of humans, these animals are often indiscriminately killed on sight.  This is unfortunate, for the arachnids are some of our chief allies in the ongoing battle against pestiferous insects.  These animals are characterized by having two body regions, a cephalothorax and an abdomen.  They have four pairs of walking legs and at least one pair of simple eyes.  They lack antennae, but may have special body parts associated with poison glands, which they use in getting food (they are all predators) and in defense.  Included in this taxonomic class are the spiders, harvestmen, scorpions, mites and ticks.

Activities:

1. Fossils of Trilobites

Examine the trilobite fossil specimens provided and see if you can identify the three lobes and the three body regions.

2. Insect Collection

Observe the provided insect collection and note the great diversity within this Class of Arthropods.  You should be sure that if you see any of these specimens again you will be able to recognize them as insects.  How can you recognize them?

3. Preserved Specimens of an Insect:  the Grasshopper

Select a specimen of the large lubber grasshopper provided and compare it to the labeled photos in your atlas.  Be able to identify the following parts:  head, thorax, abdomen, antennae, femur, compound eyes and spiracles.

4. Slide of an Insect:  the Hog Louse

Using the 4X objective of your microscope, examine the slide of the hog louse, an ectoparasite of swine.  Locate the three body regions, the antennae, and count the legs.

5. Representative Crustaceans 

Examine the various crustacean specimens provided and be able to identify them by common name, class, phylum and kingdom.  These will include crabs, shrimp, barnacles and crayfish.

6. Preserved Specimens of a Crustacean:  the Crayfish

Select a crayfish, rinse it well in water and place it in a dissecting pan.  Using illustrations in your atlas you should be able to identify all external parts labeled.

7. Slide of a Microcrustacean:  Daphnia

Examine the slide of the common microcrustacean, Daphnia, under the 10X objective of your microscope.  Compare it with the figure in your atlas.

8. Plastimount of an Arichnid:  the Spider, Tarantula

Examine the large spider (Tarantula) in the plastic block.  Count the number of walking legs.  Identify the two body regions.  Turn the spider over and note the prominent appendages just anterior to the first pair of walking legs.  These are the pedipalps.  In male spiders these are modified for use in transferring sperm to the female.  Anterior to the pedipalps are the "jaws" with which the spider injects venom into its prey.  Note the spinnerets projecting posteriorly from the abdomen.  These organs allow the spider to spin silk threads, which may be used in many ways, depending on the spider’s mode of life.  Compare the tarantula with the more-common garden spider.

9. Plastimount of an Arachnid:  the Scorpion.

Find the plastimount containing the scorpion.  How many pairs of walking legs do you count?  Note that the pedipalps are modified into "pincers".  Perhaps the most distinctive feature of the scorpion is the stinger at the tip of its abdomen.  This stinger is raised up over the back and plunged into the prey being held by the "pincers".

Phylum Mollusca – Clams, Snails, Octopi and Their Kin

Phylum Mollusca is made up primarily of marine and freshwater organisms with a long history on the earth.  While they are mostly bottom dwellers, the mollusks occupy a wide variety of habitats and exhibit a diversity of life styles.  This has led to Mollusca having more species than any phylum except the arthropods.  Characteristics common to most members of the phylum are:

A. The internal organs are confined to a visceral mass, which is enclosed in the mantle.

B.  The organism is supported and/or protected by a shell secreted by the mantle.

C. A prominent organ is the muscular foot, which has been modified to perform several functions within the phylum.

D.  In the mouth is a radula, a sort of spiked tongue, which is used in feeding.

Although there are several minor taxonomic groups within Phylum Mollusca, the following classes are considered most significant:

1. The bivalves include some of the most familiar invertebrate animals.  Among them are clams, oysters and scallops.  These filter-feeders lack a head or radula.  Oysters and scallops also lack the typical foot, but most bivalves have a very prominent hatchet-shaped foot with which they burrow into the bottom.  All, however, have a two-part, hinged shell, which encloses most of the visceral mass.  One interesting member of the class, the shipworm, has small shells, which are used to bore into submerged timbers.  In the past they were greatly feared by sailors of wooden ships and even today they cause much damage to peers, wharves, etc.

2. The gastropods probably show more diversity than any group of mollusks.  Snails, slugs and nudibranchs (sea slugs) are members of this class.  Gastropods may be herbivorous, carnivorous or omnivorous.  The cone shells are fearsome predators, which utilize a poisonous stinger to kill their prey, usually other snails.  The gastropods are the only mollusks to evolve land-dwelling species.  A spiraled shell characterizes all the snails.  Their digestive systems have become twisted to allow them to expel feces out the one opening of the shell.  Gastropods have a well-developed head with functional eyes, tactile tentacles and a radula.  Slugs are similar to snails, but they typically lack a shell and have not undergone torsion.  Like most gastropods, they move about on a flat, muscular ventral foot.  The nudibranchs are large, brightly colored marine animals.  They are active swimmers and feed on jellyfish and other cnidarians.

3. The cephalopods represent the highest evolutionary development among the Mollusca.  In this class, the foot has become modified into a group of suckered tentacles, which surround the mouth.  In most cephalopods, the mouth is a horny, parrot-like beak with a radula.  These animals have the most highly developed eyes of all the invertebrate groups.  Probably the most numerous of the cephalopods are the squids.  These creatures have a streamlined body, modified for swimming, with a much-reduced internal shell.  Like other cephalopods, they swim by water propulsion.  The giant squids, up to 18 meters in length, are the largest of today’s invertebrates.  The octopus, with its eight equal-sized tentacles and its sack-like body, is adapted to life on the bottom, where it lives as a scavenger.  The chambered nautilus is the last in a long line of shelled cephalopods, most of which have become extinct.  The shell of the nautilus is coiled but not spiraled and is composed of a number of separate chambers.  The animal lives in the largest chamber and uses the remainder as a sort of balloon to maintain its buoyancy in the water. 

Activities:

1. Bivalve Collection

Consider the collection of bivalved shells.  If you see any of these again, can you give their kingdom, phylum and class?  Do not continue until your answer is “yes

2.   Gastropod Collection

Examine the collection of gastropod shells.  What common features may be observed in all the shells?  Be able to identify each as an animal, a mollusk and a snail.

3.  Cephalopod Models and Shells:  the Squid, Octopus, and Nautilus

Consider the Biocast model of the squid, as well as the preserved specimens.  Using figures in your photo atlas, identify the arms, tentacles, mantle and eyes.  Which end goes first when this organism swims?  If this animal has a shell, where is it located?

Provided is a Biocast model of a typical octopus.  Compare this to the squid.  How are they alike?  How do they differ?  Can you tell them apart?

Examine the shells of the chambered nautilus.  Can you see the remains of the tiny canal connecting the chambers?  Where did the adult live?  How many times did it extend the shell?

Phylum Echinodermata – Starfish, Sand Dollars and Their Kin

Some of the most familiar sea creatures, the starfish, are included in Phylum Echinodermata, along with several other interesting animals.  They are of particular interest to us because studies of their embryology indicate a close kinship with the chordate phylum to which we belong.  Most members of the phylum show the following characteristics:

A.  They have radial symmetry and their construction is based on a plan of five, with major body parts typically occurring in fives or multiples of five.  A bilateral larval form indicates non-radial ancestors, confirming a separate lineage from the other, more primitive, radically symmetrical invertebrates.

B.  The skeleton is internal and made up of calcareous plates.  These may articulate so as to give some flexibility to the organism, or they may be fused into a rigid shell.  Hard spines may extend from these plates to give the organism a distinctive “spiny” appearance, hence the name Echinodermata (“spiny skin”).

C. Unique to this phylum is a sort of hydraulic apparatus, the water vascular system, which is used for locomotion by most echinoderms.

This interesting phylum has a long-standing and significant role in the history of the earth’s oceans.   We will examine the following classes of Echinoderms:

1. The crinoids or sea lilies are the most ancient and primitive of the echinoderms.  They are best known from the fossil record, being widely distributed in the rocks, where their stem segments are locally known as “Indian money”.  These animals may be envisioned as upside down starfish, anchored to the bottom by a long, jointed stalk.  A few crinoid taxa still exist, but in nowhere near the number and diversity that they once enjoyed.

2. The sea star or starfish is the most familiar animal in the phylum.  These organisms consist of a central disk with a number of arms or rays arranged around it.  At the center of the bottom or oral surface of the sea star is the mouth, with distinct ambulacral grooves extending to the tips of the arms.  Within those grooves are two rows of tube feet.  The upper or aboral surface is marked by little more than the sieve plate, a sort of filter connected to the water vascular system.  The carnivorous starfish move on their tube feet and feed on mollusks and other small invertebrates.

3. The sea urchins, sand dollars and sea biscuits have shells made of fused calcareous plates. The bodies of sea urchins do not have arms like the starfish and brittle stars; rather, they are more-or-less spherical, with movable spines projecting outward from the shell.  Sand dollars are strongly flattened, and the sea biscuits are intermediate in thickness.  While the urchins maintain the radial symmetry characteristic of the phylum, the other two taxa show signs of developing a secondary bilateral symmetry.   The ambulacral areas of the sea urchin continue onto the aboral portion of the animal, dividing the shell into ten regions.  The oral surface is flattened and bears a central mouth enclosing jaws with five teeth.  On the sand dollars, the aboral ambulacral areas are shaped like the petals of a flower and radiate from the aboral center.  Most of the organisms in this class are scavengers or detritus feeders.  Some are capable of burrowing.

Activities:

1. Fossils of Crinoids

Examine carefully the plaster cast of a fossil crinoid “head”.  Note the plates of which it is composed.  See the arms.  How many of them can you count?  Observe the fossils of crinoid stem segments.  Have you ever seen similar fossils elsewhere?

2.   Preserved Specimens of Starfish

Select a preserved starfish, rinse it well and place it in a dissecting pan. Using the labeled photographs in your photo atlas, identify the following external parts:  Central disk, arms, madreporite, mouth, ambulacral grooves and tube feet.  Note the spiny skin and the fact that the sea star is somewhat flexible.

3.   Preserved Specimens of the Sea Urchin

While referring to your photo atlas, examine the preserved sea urchins, as well as the sea urchin shell.  See if you can find the following parts:  Oral and aboral surfaces, spines, ambulacral areas, sieve plate, mouth and teeth.

4.   Specimens of the Sand Dollar and Sea Biscuit

Consider the specimens of the sand dollar.  Do you see evidence of a developing bilateral symmetry?  Note the ambulacra on the aboral surface.  Can you distinguish between these animals and the “fatter” sea biscuit?