Lab 4
Arthropoda, Mollusca, and Echinodermata

    In this laboratory exercise, you will become familiar with several arthropod, mollusk, and echinoderm representatives.  All are invertebrates and exhibit an organ system level of development.  The larvae of many mollusks go through a trochophore larval stage, indicating the close relationship of this group with the annelids.  Arthropods are related to the annelids and mollusks.  The echinoderms are most closely related to the vertebrates, with which they share many similarities in embryological development.

LAB OBJECTIVES:

        1.  To become familiar with the major characteristics that distinguish arthropods, mollusk, and echinoderms.
        2.  To be able to identify various preserved specimens as either arthropod, mollusk, or echinoderm.

PROCEDURE:  Carefully read and work through the lab performing all of the prescribed activities.

I.  Phylum Arthropoda (jointed foot)

    Of all the animal groups, the arthropods have had the greatest "biological success", that is, they have the greatest numbers of species, the greatest number of individuals, and the greatest ecological diversity.  They represent more than 80% of all known animal species, more than 900,000 species have been described.  The presence of a chitinous exoskeleton, which is relatively insenstitive to environmental influences, and pairs of jointed appendages have contributed to their success by providing protection, support, and a framework for locomotion.  The inner layer of the exoskeleton provides areas for muscle attachment which are necessary for movement of the arthropod's specialized appendages.  Arthropod movement, as in vertebrates, is based on leverage, that is, the pulling action of muscles against the rigid elements of the exoskeleton.

    Arthropods are bilaterally symmetrical, possess a true coelom, a complete digestive tract, a ventral nerve cord with paired ganglia, and exhibit metamerism.  Unlike metamerism of the annelids, there is a tendency toward fusion of the segments resulting in specialized body parts.  In insects, the body parts are fused into three regions: head, thorax, and abdomen.  In arachnids and crustaceans, two body parts are evident:  the cephalothorax and abdomen.  Arthropods also show a great degree of cephalization.

    Respiration and excretion of nitrogenous wastes in aquatic arthropods are accomplished by the gills.  In terrestrial arthropods, respiration is accomplished by the spiracles in the abdomen; and the excretory organs called malpighian tubules remove nitrogenous wastes.  In most Arthropods, the sexes are separate (dioecious) and fertilization is usually internal.

Question 1:  What major characteristics do all arthropods share?

Question 2:  What anatomical similarity in muscles and skeletal arrangement is shared by arthropods and vertebrates?  What is the evident dissimilarity?

Question 3:  Explain what is meant by cephalization.

        A.  Examine the preserved arthropoda specimens.  Note the various adaptations of the aquatic and the terrestrial arthropods.

Question 4:  List the preserved examples and classes of Arthropods provided in lab.

        B.  Obtain one grasshopper (a terrestrial arthropod) and one crayfish (an aquatic arthropod) in a dissecting pan.  DO NOT DISSECT.  Examine the external characteristics by identifying the major body regions and locating all structures labeled in figures 1 and 2.

Question 5:  Describe the differences and similarities between these two arthropods.  (Compare and contrast the two arthropods.)

II.  Phylum Mollusca (soft body)

    Mollusca constitutes the second largest phylum in the animal kingdom and contains the largest of all invertebrates, the giant squid.  Mollusks do not exhibit metamerism, and cephalization is absent in the bivalves.  They are characterized by the presence of a large muscular foot, which serves for locomotion; a visceral mass above the foot, where most of the internal organs are located, and a mantel which is a heavy fold of epithelial tissue that is glandular and secretes a calcareous shell.  Also in cephalopds, a radula, or "parrot like beak".  The mantle covers the visceral mass and also encloses an area called the mantle cavity that contains the gills and is used for locomotion by the squid and octopus (cephalopods).

    In many species, fertilization occurs internally with the larva first developing within protective egg cases deposited by the adult.  Some mollusks are dioecious, some are monoecious, and some are first one sex, then the other during their life cycles.

Question 6:  List the three (four for cephalopods) characteristics of mollusks.

Question 7:  Most mollusks have a shell for protection.  What replaces this protective function in those mollusks lacking shells (squids and octopi)?

        A.  Examine the preserved mollusk specimens.

Question 8:  List the preserved examples of the phylum Mollusca.
 

Question 9:  How do the structural characteristics of the squid and octopi differ from the general mollusk characteristics?

        B.  Obtain the dissected model of a bivalve mollusk.  Note carefully the parts labeled.  Compare the model with figure 3.

Question 10:  List the labeled parts of the model your instructor addresses and describe their function.

III.  Phylum Echinodermata (spiny skin)

    Echinoderms are marine coelomates that begin life as bilaterally symmetrical larvae.  The larva mature into adults with secondary, five-parted, radial symmetry.  Radial symmetry is not unique to echinoderms, for other phyla share this characteristic: but no other group with such complex organ systems has radial symmetry.  Echinoderms have small bony plates of mesodermal origin embedded in their body wall creating a calcareous endoskeleton.  This endoskeleton is considered to be the first indication of the endoskeleton which is so well developed among vertebrates.

    Echinoderms possess a spacious coelom into which project elements of the digestive and reproductive systems.  Their water vascular system is unique among members of the animal kingdom providing for locomotion and aiding in feeding.  The nervous system consists of a single oral nerve ring, which is connected with a radial nerve running beneath the radial canal of the water vascular system in each ray.  They possess sac-like dermal branchiae (skin gills) responsible for respiration.

    Echinoderms are dioecious and ferilization is external.  However, some have the ability to produce new organisms asexually through the process of autonomy and regeneration.  Through the process of autotomy (to cut self) an arm may be automatically broken off near its base if injured or bruised, and any arm with part of its central disk may regenerate an entire body.  A sea star, having lost one arm, will regenerate the lost appendage.

Question 11:  Why are echinoderms said to exhibit secondary radiality?

Question 12:  What is regeneration?

        A.  Examine the preserved echinoderm specimens.

Question 13:  What characteristics identify these organisms as echinoderms?

Question 14:  List the preserved specimens of this phylum. 

        B.  Dissection of the Sea star
    Obtain a sea star in a dissecting pan.  On the aboral (dorsal) side of the sea star note the central disk; the rays or antimeres (arms); and the madreporite, a light-colored circular area near the edge of the central disk at the junction of two rays.  These two rays comprise the bivium and the other tree the trivium.  The ray opposite the madreporite is called the anterior ray.  On the oral (ventral) side note the mouth; the ambulacral (walking tears) groove which extends from the mouth along the middle of each ray: and the tube feet.

Question 15:  Draw the aboral side of a sea star and label: the central disk, ray, bivium, trivium, madreporite, and anterior ray.

    With strong scissors, cut off 1/2 inch of the tip of the anterior ray.  Then carefully cut through the aboral wall of the ray along each side.  When you reach the central disk, cut around its outer edge to the outside of the madreporite.  Lift the flap and examine the body cavity being careful not to injure the stone canal to which the madreporite is attached.

    The majority of the ray is occupied by a very large pair of digestive glands.  The central disk is occupied largely by the two stomachs (pyloric and cardiac) of the sea star.  Remove the digestive glands exposing the radial canal and ampullae of the tube feet.  Locate the stone canal connected to the madreporite.  Remove the stomachs and expose the ring canal to which the stone canal is connected.  The water vascular system is now exposed.

    Examine the dissection of the water vascular system noting the ampullae within the ray that constitute the ends of the tube feet opposite the suction cups.  The ampullae function like the bulb of a medicine dropper.  When they contract, hydraulic pressure extends the foot until the sucker at the tip contracts a surface where it sticks like a bathroom plunger.  Subsequent contractions of the tube feet pull the organism forward.

    Obtain the dissected model of the sea star and compare all labeled parts with your own sea star.  Be able to identify all of the structures labeled in figures 4 and 5.

Question 16:  Name the parts of the water vascular system beginning with the madreporite, and explain how the system works.

Question 17:  Sea stars frequently feed on mollusk bivalves.  How does the water vascular system make this feeding possible?

DISPOSE OF SEA STARS IN TRASH CANS.  RINSE DISSECTING PANS WELL.