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Biology 172L – General Biology Lab II Laboratory 10: Echinoderms

Introduction Echinoderms, including sea stars, brittle stars, sea urchins, sea cucumbers, and crinoids, are bilaterally-symmetric deuterostomes whose ancestors adopted a sessile way of life (similar to today's stalked crinoids). As a consequence of this sessile life style, the ancestral echinoderm evolved a pentamerous radial symmetry that was superimposed upon their fundamental bilateralism – their bilateral heritage is most apparent in the larval stages. Unique to the echinoderms is a water-vascular system, derived from the coelom, composed of a network of hydraulic canals that operates their tube feet in locomotion. In this laboratory exercise, you will become familiar with the diversity, anatomy, and morphology of the echinoderms. To facilitate your investigations please be sure to read the appropriate materials in your BIOL 172 textbook (pp. 665-667) and bring your Photographic Atlas for the Biology Laboratory to the class session. PHYLUM CHARACTERISTICS 1. deuterostomes with secondarily-obtained

pentamerous radial symmetry superimposed upon an underlying bilateral symmetry; free-swimming larval stages are bilaterally symmetric

2. most with a complete digestive system (except in ophiuroids)

3. well-developed coelomic space formed enterocoelously: partly modified into a system of water canals (water-vascular system) with external projections (tube feet) used in feeding and locomotion and papulae ("gills") that function in metabolic exchange with the environment

4. no head nor brain; nervous system consists of a circumoral ring with radial nerves that typically extend into the arms; few specialized sensory organs present

5. endoskeleton of calcareous ossicles formed in the mesoderm of the body wall; calcareous spines often project from the body wall as well

6. no circulatory system, but coelomic circulation substitutes for a circulatory system in most; holothurians with a well-

developed hemal system that may function as a circulatory system

7. excretory system absent 8. sexes are separate; many are capable of

regeneration of lost parts and can reproduce by asexual fragmentation

9 6,000 living species plus 20,000 known fossil species that are now extinct; exclusively marine forms; benthic as adults

ECHINODERM SYSTEMATICS (Table 33.6, p. 666) Class Asteroidea

1. sea stars (Figs. 33.40a, p. 667; 33.39, p. 665)

2. five rays or arms (sometimes more than five) which are not well-defined from the central disc of the body

3. each arm contains paired branches of the digestive system (pyloric ceca) and gonad

4. skeletal ossicles in body wall only partly fused, thus endoskeleton is slightly flexible

5. tube feet with suckers project from ambulacral grooves on the oral surface

6. pedicellariae (modified spines that act as tiny pincers) present

7. metabolic exchange via dermal papulae 8. anus and madreporite open on aboral

surface 9. digestive system divided into cardiac

stomach and pyloric stomach 10. primarily predators on sessile benthic

animals, typically by everting stomach through the mouth and digesting prey outside the body of the sea star

Class Ophiuroidea 1. brittle stars and basket stars (Fig.

33.40b, p. 667) 2. five rays or arms (sometimes more &

sometimes branched) which are distinct from the central disc

3. ambulacral grooves are closed 4. arms appear jointed because of

articulating skeletal ossicles, allows for great flexibility in movement

5. tube feet lack suckers

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6. internal sacs, the bursae, opening to the exterior on the oral surface, function for gas exchange and for gamete release

7. pedicellariae absent 8. secondarily derived sac-like digestive

system lacks an anus 9. mouth armed with five teeth 10. tend to be deposit feeders or

scavengers, some (e.g., the basket stars) are suspension feeders

Class Echinoidea 1. sea urchins (Fig. 33.40c, p. 667), sand

dollars, and heart urchins 2. skeletal ossicles fused to form a rigid

circular or oval test 3. lack radiating arms 4. movable spines and tube feet with

suckers arising from ambulacral areas of the test function in locomotion

5. pedicellariae present 6. digestive is complete with specialized

Aristotle's lantern for masticating food 7. anus, madreporite, and genital pore

open on aboral surface 8. sea urchins feed primarily on algae,

while sand dollars are suspension feeders

Class Holothuroidea 1. sea cucumbers (Fig. 33.40e, p. 667) 2. body elongate along the oral-aboral axis

with oral tentacles (= modified tube feet) surrounding the mouth

3. many exhibit secondary bilateral symmetry superimposed upon the radial plan (which is superimposed upon a primary ancestral bilateral symmetry)

4. skeletal ossicles microscopic and not fused

5. possess a pair of internal respiratory trees which connect to the cloaca near the anus

6. lack pedicellariae 7. madreporite lies in the coelom and does

not connect to the exterior 8. primarily deposit feeders, although a few

are suspension feeders

Class Crinoidea 1. feather stars and sea lilies (Fig. 33.40d,

p 667) 2. five branching arms with ambulacral

grooves (as in sea stars) and bearing lateral pinnules

3. ambulacral grooves on oral surface which projects upwards into the water column

4. both mouth and anus open on the oral surface

5. calyx may be attached to the bottom via a stalk

6. lack spines, madreporite, and pedicellariae

7. suspension feeders usually in deep water

Procedures and Assignment

I. Echinoderm Diversity Review the echinoderm classes on demonstration in the laboratory. Compare these classes by filling in the details into a table like that illustrated below. Note: prepare your own table answering the questions for each category present below. Rays or Arms Are rays or arms present? How many? Are they fused or distinct from the central disk? Tube Feet Are the tube feet long or short? Do they have suction cups (sucker disks) at their distal ends. Skeleton Does the skeleton consist of ossicles fused into a solid test? Or do they permit movement of the body wall? Are the ossicles microscopic? Pedicellariae Are pedicellariae present or absent? Respiratory Structures Briefly describe the structures involved in gas exchange. Specialized Feeding Structures Briefly the structures involved in obtaining food. Feeding Mode Describe the mode of feeding (e.g., filter feeder, deposit feeder, carnivore, algal grazer, etc.)

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Structures or

Aspects to

Describe

Echinoidea Asteroidea Ophiuroidea Holothuroidea Crinoidea

Rays or Arms

Tube Feet

Skeleton

Spines

Pedicellariae

(present or

absent)

Respiratory

Structures

Specialized

Feeding

Structures

Feeding Mode

II. Sea Star Anatomy

A. Sea Star Dissection 1. Examine the external

anatomy of the sea star. Draw labeled diagrams of the oral and aboral surfaces that illustrate the following structures: central disk, madreporite, anus, arms, spines, tentacle (if present), tube feet, ambulacral grooves, and mouth.

2. Examine the surface of the sea star under a dissecting microscope. Identify the spines, papulae, and pedicellariae. Remove some pedicellariae for examination under the compound microscope (make a wet mount). Draw a diagram illustrating a pedicellarium.

3. Begin your dissection of the sea star by carefully cutting

and removing a circular section of the body wall of the central disk of your sea star. It may be easiest to perform this dissection while holding the sea start in your hand. Start by pushing the tip of one of your scissors blades through the starfish test immediately to the right of the madreporite. Cut away from the madreporite (counter clockwise) by slowing making small snips. Continue along the perimeter of the central disk. As you cut with one hand, try to hold the newly cut edge of the disk between your fingers of the other hand. Every few cuts, separate the disk from the internal organs by scraping your scissors gently along the underside of the disk. The internal organs

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are attached to the test by a mesentery. Internal organs will be damaged if you are impatient; take your time. When the circular cut is nearly complete and you have reached the madreporite’s left side, cut around the madreporite to the inside (toward the center) and complete the circular cut. At this point the central disk should come off easily. Locate the two rectal glands that are situated on the top of the pyloric stomach. The thin membrane of the pyloric stomach may be torn from your first cut.

4. Next you should remove the aboral surface of an arm. Find the arm opposite the madreporite and snip off the very tip of this arm. From this point, cut the body wall on one side of the arm up to where the arm joins to the central disk. Then make a similar cut along the other side of the arm. Carefully pull back the aboral body wall of the arm, exposing the internal organs in the arm. Note the spacious coelom lined with a ciliated peritoneum. Using the same procedure, remove the aboral body wall from the two adjacent arms as well.

5. Identify the following internal features of your sea star (be able to identify these in the sea star model as well): madreporite, anus, stone canal, pyloric caeca (digestive glands), pyloric duct, gonads (will need to remove pyloric caeca in one of the arms to see these), cardiac stomach, ring canal, radial canal, ampullae, and ambulacral ridge.

6. Draw a labeled diagram illustrating the major internal organs of the sea star

B. Sea Star Arm Cross-Section 1. Examine the prepared slide

that illustrates the cross-section of a sea star arm. identify the pyloric ceca, gonads, podia, coelom, and epidermis. Depending on where the cross-section was cut, your slide may also show ampullae, spines, dermal gills, or pedicellariae.

2. Draw a labeled diagram that illustrates your observations

III. Echinoderm Development Examine the prepared slides that illustrate the various stages of echinoderm (sea star) development (refer to pages 992 & 995 in your BIOL 172 textbook, as well as to appropriate pages in your Photoatlas): unfertilized egg, fertilized egg, early cleavage, late cleavage, blastula, gastrula, bipinnaria larva, and brachiolaria larva. Draw labeled diagrams that illustrate these stages of development.

VOCABULARY ossicles pedicellaria abulacral groove pinnule tube foot cardiac stomach pyloric stomach pyloric caeca pyloric duct dermal papula madreporite bursa Aristotle's lantern test oral tentacle respiratory tree cloaca calyx stalk water-vascular system stone canal radial canal ring canal ampullae fertilization membrane cleavage blastula blastocoel gastrula

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archenteron, blastopore bipinnaria brachiolaria SYSTEMATICS TO KNOW Phylum Echinodermata Class Asteroidea Class Ophiuroidea Class Echinoidea Class Holothuroidea Class Crinoidea

Lab Summary Your lab summary should consist of the following:

1. Descriptive title. 2. Short introduction identifying main

objectives of the lab activity. 3. Brief description of methods employed. 4. Results and Discussion section

including information requested in the Procedures and Assignment section above and following the appropriate protocols for presenting figures and tables in a laboratory report. Be sure to follow all of the rules from producing figures for lab reports (one figure per page). Corresponding to each figure, there should be a short paragraph that describes the significant features of the figure. Be sure to answer all questions asked throughout the Procedures and Assignment section of this lab description.

5. Short conclusion summarizing what you learned by carrying out this study.

Starfish: Oral Body Surface

Starfish: Aboral Body Surface

Starfish: Dorsal (Aboral) View With Arms In Various Stages of Dissection