Starfish: Part 2 of 3: External Identification

By Kathy Egbert

Last updated over 5 years ago

21 Questions

Lab Rules Apply
You will need a writing utensil and your chromebook; all other personal items are to be on your desk and off the floor. 
No goofing around.
Treat your starfish respectfully.  It gave its life for you to learn.
Clean up begins about 10 minutes prior to the end of class.  Everyone participates.  No one is dismissed until every lab station is clean and picked up properly.

Be sure you have completed Part 1: Introduction prior to beginning Part II: External Identification Your Name: Class Period: Date:

General Lab Rules:
1.  All personal items are to be put on the sideboards and out of walk ways.  This includes beverages.
2.  Must have shoes on at all times during labs.
3.  Must have gloves on when handling specimens.  Be sure you throw your gloves away after their use.  Please do not take extra gloves and play with them!
4.  It is your job to wash and dry any utensils you use that touch your specimen.  Once dried, return them to the same location as you picked them up.
5.  Fold up the plastic wrap around the wet newspaper inside your dissection try and then place it in the trash can.
6.  Return your dissecting tray to the sideboard and stack it the same way you received it - stack them criss-crossed so that they won't stick to the wax in the next tray.
7.  Use sanitary wipes to clean your lab station at the end.
8.  No one will be excused until everything is cleaned up and in order for the next class.
9.  Ten minute cut-off at end of class for clean-up.

A.  Prepare a bag to keep your speciment in prior to obtaining your specimen.
     >Get a gallon/quart plastic zip lock bag and a sharpie from the black cart.  
     >Write your names and class period on the outside of the plastic bag.  
     >Return the marker to the front table.
     >Keep your bag to use at the end of class to keep your specimen in.  All parts of the starfish are returned to the plastic zip lock bag. 

B.  Preparing your Dissection Tray:
     >>I will demonstrate. Get a tray, a roll of plastic wrap, piece of newsprint, and scissors if desired.  
     >>Pull off a piece of plastic wrap that will go completely around the tray lenghtwise.  
     >>Now, take one of the pieces of newspaper print and place it on top of the plastic wrap inside the tray.  
     >>Now, all you have to do to return a clean tray at the end of class is to fold up the plastic wrap around the newsprint and throw it away in the trash can. 
          <<The newsprint has 2 purposes: 
               >first, it helps hold down the plastic wrap inside the tray, and 
               >second, it absorbs liquid from the starfish which aids you with dissection and handling your specimen.

Tip:  You may want to pull off a long piece of plastic wrap to put around the keyboard on your chromebook so you won't get any starfish liquid or pieces on your keyboard inadvertently.

C.  Getting your Specimen:
     >Bring the plastic bag you've prepared for storing your starfish and your prepared dissection tray to the front to obtain your specimen.
     >You will be checked whether you have done the 2 preparatory steps prior to receiving your specimen.  
     >Be sure to rinse your specimen in water briefly to help remove the preservative before you take it back to your table.

Materials for External Dissection:
     - dissecting pins about 6
     - abt 10 pieces of paper to label structures with pins
     - probe (optional)
     - hand lens (optional)

Watch the Video prior to starting the questions/identififying of external structures.  Use the pictures provided to help you with identification.

A. Dorsal Side (aboral surface or non-mouth side) observations and structures

1. Begin with the starfish in your tray with the dorsal side up.

2. Observe that the body of the starfish is radially symmetrical. The body consists of a central disc, which is surrounded by five blunt arms or rays.

3. Locate the small, round yellow or red hard plate on one side of the central disc. This is the madreporite, a perforated plate that functions as an intake valve for the water-vascular system within the body. The madreporite is also called the sieve plate. Therefore, the madreporite’s job is to bring water into the body for respiratory purposes.

4. Two rays are nearer to the madreporite than the rest; these are called the bivium. The other three rays are called the trivium. In the very center of the aboral surface is the anus, which may be hard to see. You may be able to use a probe to locate it. If you locate it, the probe will easily go down inside the starfish through the opening.

5. At the end of each arm or ray is an eyespot containing red pigment which allows the starfish to sense and respond to light. In preserved specimens, the eyespot can be located with a magnifying glass or dissecting scope by spreading the tube feet at the tip of the ray.

Use these photos to help you identify the structures on your starfish

Use these sketches of starfish to help identify structures

More photos and sketches to help you identify structures

The purpose of the madreporite is to control the movement of water in and out of the starfish.

The starfish has bilateral symmetry.

The sieve plate is

Label the following on the dorsal side of the starfish: 1) central disc (CD) 2) madreporite (M) 3) bivium (B) 4) trivium (T) 5) eyespot (E) 6) ray (R) and 7) general location of anus (A) Use the abbreviations given after the structure to make it easier to insert the names. Be sure to use a line that points to the structure. You only have to identify where one eye spot is located and where one ray is located.

Demonstrating your knowledge - tagging your specimen

Now that you know where all these structures are located, use 4 dissecting pins and 4 pieces of paper to tag the following:
1. central disc
2. ray
3. eyespot
4. madreporite
When completed, raise your hand so your work can be checked off for credit.
Be prepared to demonstrate what constitues the bivium and trivium and where the anus is generally located. (6 pts) Enter "completed" after you have been checked.

B. Observations and structures of the starfish's skin

Examine the skin on the aboral surface with a hand lens. Notice the many coarse spines that cover the entire aboral surface. These spines project from the endoskeleton that is found just below the skin.
The spines are an adaptation for the burrowing existence of many starfish. Surrounding the spines, forming a collar-like ring, are the small fingerlike processes, dermal brachia. These thin-walled structures are the major structures in contact with the aquatic environment, and function in respiration.
In addition, small jaw-like pincers, or pedicellariae, rise from the surface for use in food handling and for protection against small larvae or animals that settle on the starfish.

Pictures of spines, dermal brachia, and pedicellariae to help you locate them on the skin of your starfish. A hand lens can be helpful.

Select a small area on the skin about 2 cm squared. Enlarge what you see to fit inside the drawing circle. Sketch/draw the pattern of the spines in the circle. If you can see any of the other skin structures, include them in your drawing as well. Label structures. There is a metric ruler placed in your lab station tub to help you identify a 2 cm space on your starfish. A hand lens may help you see the structures better.

The "top" of the starfish is also known as the dorsal side, aboral surface, and non-mouth side.

The prefix ab- on the term aboral means "no". So, aboral literally means "no mouth".

10.

Select the statement(s) that is/are true about the structures of the skin.

C. Ventral side (also called oral and mouth side or the "bottom") observations and structures

Turn the starfish so that the ventral, or oral surface faces up in the dissecting pan.
Observe the mouth opening in the middle of the central disc. Examine the ring of small, protective oral spines surrounding the mouth. A hand lens might be useful here.
Find the ambulacral groove that extends from the mouth to the tip of each ray.
Locate and examine the numerous soft, small tube feet that project from and line each side of the ambulacral groove. The tube feet are part of the water-vascular system and are used for locomotion and holding prey. At the end of the tube feet are suckers that help with movement. Once again, your hand-lens might be useful here.
Watch how the tube feet work

11.

Identify and label the 1) mouth and 2) ambulacral groove in the picture. Then tag the same two structures on your starfish using dissecting pins and pieces of paper. Raise your hand and wait for your teacher to check off your work. Be ready to explain and show where the tube feet and suckers are located.

12.

Review: Give the name of the center of the starfish where the "arms" are attached.

use lower case letters and separate the two words with a space

13.

Review: Give the name of the large "arms" that usually number five on a starfish.

use only lower case letters

14.

Review: A small yellow or red structure that is used for the intake of water is called the ...

use only lower case letters

15.

Review: What are the hard, blunt projections covering the entire surface of the starfish and are part of the skeleton?

use lower case letters

16.

Review: What is the name of the opening near the center of the dorsal side of the starfish? It is difficult to see and it is where waste removal occurs.

use lower case letters

17.

Review: Name the opening in the center of the ventral side of the starfish that is surrounded by a protective circle of spines.

use lower case letters to answer

18.

Review: What is the name of the long groove running along the center of each ray on the ventral side of the starfish?

use lower case letters and put one space between the two words

19.

Review: Name the soft, small dimpled structures in the ambulacral groove on the ventral side of the starfish. These structures are used for movement, help catching prey and moving the prey to the starfish's mouth.

use lower case letters and one space between the two words

Good review video about what you learned in Part I and II and transitioning information into Part III - Internal Structures

20.

At the end of the vido there is a list of vocabulary words. The last vocabulary word on the video is one that wasn't used in your introductory Part I lab. Describe how this term relates to the growth of the starfish and how that growth is different from other chordates like you and me.

It is the vocab word that starts with "a"

21.

Sometimes larger starfish will eat smaller ones. Describe the condition(s) that usually occur for this to happen.

Watch the video clip to prepare your starfish for internal dissection during the next lab period (Part III- Internal)
Two important steps that are different than what you will see in the video:
    1st - cut a larger tip off of each arm of the trivium than what is shown in the video - cut off about an inch (2.54 cm)
    2nd - cut up the sides using scissors with a sharp point - do not cut the flap of skin off of the arm until next lab period; otherwise you might destroy the internal structures when you store your specimen in the plastic bag between Lab II and Lab III

Using the scissors, cut off the tips of the three rays called the trivium. Cut off a length of about 2 cm. These tips need to go into your plastic bag to store with your starfish.
Do not cut off the tips of the two rays next to the madreporite (bivium).
Examine the cross-section of the rays and identify structures. Study the picture below to become familiar with some of the structures that can be seen in the cross section.

Cutting tips off the starfish rays; different cross sectional views of the ray with labelled parts

Clean up from Part II - Put your starfish and the tips of the rays that you cut off inside your zip lock plastic bag.  Close the bag tightly.  (Your name & class period should already be on the outside of the plastic bag.)  Take your "bagged" starfish and store it where directed by the teacher.

To continue with Part III: Internal Dissection you need to have completed Part II: External Observations and Identification