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Invertebrates (Clam, Sea Star, and Worm) Anatomy & Dissection Lesson for Middle School Biology

Updated on February 21, 2019
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I am a Christian. I was an 8th-grade American History teacher. I am currently a freelance writer, public speaker, & homeschooling mom of 8.

Invertebrates Anatomy & Dissection Lesson for Middle School: Clam, Sea Star, & Worm
Invertebrates Anatomy & Dissection Lesson for Middle School: Clam, Sea Star, & Worm

This is the 20th lesson in a series of 30 hands-on lessons covering middle school biology. This lesson focuses on the study of invertebrates through dissections of clams, starfish, and earthworms. Also included is the observation of live earthworms. I used this plan while teaching a 55 minute middle school biology class. Each lesson plan includes homework assignments and a variety of hands-on activities to make each lesson engaging & memorable. Use these fun lessons with your classroom, homeschool, after-school program, or co-op!

These lessons are written for a class that meets once a week. If your class meets 5 days a week, simply do this lesson one day a week and use the homework assignments (at the bottom of the page) for the work for the other days of the week.

Homework Review

1. Pass out tickets for students who did their homework and who did the extra credit. Go over the homework questions from the book and allow students to share what they found interesting or important when studying the clam, starfish, & earthworm. (I give out tickets for students who volunteer to answer the questions.)

Why do 3 dissections in 1 day?

We'll be doing 3 dissections in 1 day because I bought a dissection kit that includes all the specimens in 1 bag. Once you open the bag, you need to dissect all the specimens within 4 weeks. I failed to note that when ordering the set. I wouldn't have purchased it had I realized that. That's why the next few classes will include dissections. If I could do it again, I would have purchased separate specimens and spread the dissections out over the course of the lessons.

Clam Anatomy & Dissection

Dissecting the Clam
Dissecting the Clam

2. Quickly review about clams:

  • In what kingdom is a clam? (Animalia) phylum? (Mollusca) class? (Bivalva)
  • What makes it a bivalve? (2 hinged shells)
  • What else do you think is inside this clam?

3. External Observation: Find the hinge. Now find the beak-shaped bump. That's the umbo, which is the oldest part of the clam. It grows out from there. The shell has growth rings just like a tree has growth rings.

4. Internal Observation

  • Can you open the clam with your hands? Why not? The clam's posterior and anterior adductor muscles are still holding it shut tightly. You'll need to slide your knife or scalpel between the two shells and try to slice it open.
  • What do you notice inside the clam?
  • Starting on one side you'll find the gills, which are thin, feathery pieces of tissue. They serve the same function as gills on fish. What's that? (extracting oxygen from water)
  • Next you'll find the visceral mass and the foot. They're slightly different colors and have completely different textures. The visceral mass is squishy. The foot is rubbery. Can you feel the difference? The visceral mass contains & protects the digestive system and reproductive system. What does the foot do? (Helps it move)
  • On the other side of the visceral mass and foot is another set of gills. Let's look at those gills. What texture are they? They're striated. Why? (Increases the surface area)
  • Moving on from the squishy parts, let's look inside the shell at the mantle. The mantle secretes the shell. This is the shell. The mantle is the brown stuff. It makes that shell, and it goes all the way to edge of the shell.
  • Who has eaten a clam before? There is a lot more to a clam than I ever realized. Let's slice open the visceral mass and find out what you can actually see.
  • Inside the visceral mass you'll find the mouth, esophagus, stomach (like a tiny balloon), intestine (long pearl-colored tube that wraps around the stomach), heart (which is sometimes challenging to see), & gonads (reproductive organs). It also has an incurrent and excurrent siphon, lips (labial palps), kidney, rectum, & anus but we won't try to find those.

5. What surprised you about the clam or what did you think was neat about it?

6. Put all the pieces of the specimen in the trash bag.

You will need:

  • freshwater clams (I got one for each group of 4-5 students) - I purchased these preserved from on online science site, but if I was to do this again, I'd simply buy them from the seafood department at the grocery store. The fishy smell combined with the preservative smell was unbearable to some of the students.
  • dissection kits such as a hard, plastic disposable plate and a paring knife or scalpel
  • disposable gloves

Sea Star Anatomy & Dissection

Dissecting a sea star or starfish
Dissecting a sea star or starfish

7. Quickly review about sea stars:

  • In what kingdom is a sea star? (Animalia) phylum? (Echinoderms)
  • Does anyone remember what Echinoderm means? (spiny skin) That's a pretty accurate name to describe the sea stars, wouldn't you say? Does anyone know of another echinoderm? (sand dollar, sea cucumber, sea urchin)
  • Many animals are bilaterally symmetrical, which means they're the same on the left and right side. Sea stars are one of the few animals that are radially symmetrical. What you do think that means? (they're the same if you dissected any part in the circle. You could dissect any arm & would see the same thing.)

8. External Observation:

  • Rays: What you do first notice about the sea star. It's arms, right? They're also called rays. How many rays does this starfish have? (5) They can have up to 40 rays. If you see a starfish with 2, 3, or 4 arms, what does that mean? (Some were broken off) Can they grow back? (Yes, they can regenerate rays or other body parts as long as it has some ring canal attached.)
  • Oral Surface: Flip it over to the bottom. What do you notice? In the middle is the mouth. Along each ray you'll see ambulacral grooves which contain lots of tiny tube feet which help the sea star do what? (move) At the end of each ray are eye spots that allow it to sense light, but we can't really see those.

9. Internal Observation:

  • What do you think we'll see inside this sea star?
  • Flip it back over to the top side, which is the aboral surface.
  • Do you see a tiny whitish dot in the middle? That's the madreporite. Pick a ray that isn't next to the madreporite. Use scissors to cut the tip off and the end of a ray and then cut up the side of the skin until you get to the center. Cut a small circle around the middle, but avoid the madreporite because I don't want you to accidentally cut the little tube, the stone canal, which attaches to it. Try not to cut too deep.
  • Are you seeing a thin, parachute-like membrane in the middle? What do you think that is? Yes, it's the stomach. Using your tweezers, pull out all the stomach but be really careful not to hit the stone canal so we can see the hopefully undamaged water vascular system. While you're removing the stomach, tell me why it's made of a parachute-like material. Yes, when it eats a clam, it uses the tube feet to pull open the clam, pushes its stomach out of its body, secretes digestive juices, slurps up the clam, & then pulls the stomach back inside for further digestion. In case you were wondering, it does have an anus, but it's too tiny for us to see.
  • With the stomach out of the way, we can view the full water vascular system: madroporite, stone canal, ring canal, radial canal, & tube feet. That's how the starfish moves and feeds. It's pretty neat, working in a similar manner to a hydraulic power system. Water is brought into the system through the madroporite and passes through the stone canal into the ring canal. Next the water goes to the radial canals. Each ray has a radial canal. Do you see all those tiny bulbs attached to the tube feet? They're those small bumps that feel like a zipper. They're called ampullae. The ampullae contract, and the water is forced through the tube feet to make them extend like little tentacles. The starfish even secrets a sticky chemical that helps it to grip on to surfaces. Scientists are amazed at its adhesive, especially how it works underwater.

  • Let's go back to that ray you cut open. Clip off rest of skin on ray. Remember that every ray will have the same things. If this was alive, as long as you had some of the ring canal, new rays can develop. Starfish can reproduce through asexual reproduction.
  • That's not the only way starfish reproduce, though. Toward top of the ray are gonads, which will develop into eggs. Most of those eggs will become food, in the form of plankton, to other creatures in the sea.
  • In each ray is also a digestive gland. It's a really cool looking gland. It’s what releases the enzymes, the digestive juices that help break down the clam, or whatever the starfish is trying to make into a meal.

You will need:

  • sea stars (I got one for each group of 4-5 students) - I purchased these preserved from homesciencetools.com
  • dissection kits such as a hard, plastic disposable plate, a paring knife or scalpel, sharp scissors, and tweezers
  • disposable gloves

Live Worm Observation

Observing live worms
Observing live worms

10. Before dissecting dead worms, let's observe live worms. [Allow students to hold them if they'd like. Have them crawl across a piece of paper on the tables.]

  • Did you notice how hard it was to pull that worm out of the soil? It was holding on using its tiny bristles called setae. Lightly rub your finger toward the posterior end and feel the setae.
  • What do you notice about how they move? It's kind of like a slinkie, isn't it?
  • Which part is the mouth, or anterior end?
  • If you can't tell, find that large band, the clitellum. It's closer to the mouth. What is a clitellum used for? Yes, reproduction,
  • Which part is the anus, or posterior end? Is anything coming out of the anus? Yes, you'll need to wash your hands afterward, but that is great for gardens!
  • The top, or dorsal, side is darker because of the larger blood vessel that runs across the top. Which side is the dorsal side?
  • Notice all those segments and how each one is separated by septa.
  • (Optional) Use a magnifying glass to look at the setae and also the tiny pores in each segment. There are larger pores in the front that you should be able to see without a magnifying glass. They're used in reproduction. The smaller ones are used to expel liquid waste.

You will need:

  • at least 1 live worm (either from students bringing them in or purchased from a store, such as Walmart, sold in the sports & fishing section) - Make sure they're in damp but not wet soil in a dark container that allows them to get air.
  • (optional) magnifying glasses or lenses

Worm Anatomy & Dissection

Worm anatomy & dissection
Worm anatomy & dissection

11. External Observation: We went over these parts with the live worm, but let's see if you can figure them out on a worm that isn't moving.

  • Anterior end – front (mouth – food goes in)
  • Posterior – back (anus – food goes out)
  • Dorsal – back/top (darker with more blood vessel) – Where we’ll make the incision
  • Ventral – bottom -little setae/bristles, feels like sandpaper (lighter in color)
  • Prostomium – lip on mouth
  • Clitellum – big band = used in reproduction – closer to mouth
  • Seminal Receptical – where worm gets sperm from opposite worm - Worms carry both eggs and sperm. They keep their eggs & exchange sperm with another worm in order to reproduce.
  • Anus

12. Internal Observation: (Note: In the below video the instructor pins down the worm. We didn't do that but were still able to easily see all the parts.)

  • What do you think we'll see inside this worm that will be different from what we saw in the clam or star fish?
  • Cut very gently & not too deep from the anterior (front) to posterior (end). Some fluid might come out, but it's just the preservative liquid. Cut through the walls of the muscles & connective tissue. Go all the way down past the clitellum and then stop.
  • [As students were cutting open their worms, I pointed out the parts drawn on the board and spoke through what they were seeing.]
  • Cerebral ganglia is part of the nervous system – 2 white dots - Make up the "brain" - Might need magnifying glass to see them
  • Pharynx – light colored just inside the mouth - muscular pump sucks in food
  • Esophagus – tube for food to go to crop
  • Aortic arches (hearts) – 5 that go across – the blood vessels that act as "hearts" of the worm
  • Seminal vesicles – white – make sperm – receptacles (smaller ones connect to the outside to receive the sperm from another worm)
  • Crop - soft & gushy - food is temporarily stored here
  • Gizzard – hard & muscular – grinds up food – rocks act as grit to grind up food
  • Intestine – runs entire length of body to anus - food is digested & stored here
  • Dorsal blood vessel (over area between crop & gizzard) – very fragile – from 5 hearts & runs along top side all the way down
  • Ventral blood vessel & nerve cord (under intestines) - (swollen regions are ganglia – white cord)
  • If you want, you can cut all the way down to see how the intestine and ventral nerve cord go all the way down to the anus.

You will need:

  • preserved worms (I got one for each group of 4-5 students) - I purchased them from homesciencetools.com .
  • dissection kits such as a hard, plastic disposable plate, a paring knife or scalpel, sharp scissors, and tweezers
  • disposable gloves

Clean up

13. Clean up. Answer any further questions students may have.

A Beka's Science: Order & Design science textbook
A Beka's Science: Order & Design science textbook

Homework

Friday: Read the Grasshopper Dissection Guide. Highlight or underline 5 bits of information you think are interesting or important.

Monday: Complete the Grand Grasshopper Worksheet which includes External Grasshopper Coloring and the Internal Structure of a Grasshopper (on p. 7)

Tuesday: Read the Crayfish Dissection Guide. Highlight or underline 5 bits of information you think are interesting or important.

Wednesday: Label as many parts as you can on the Cool Crayfish Worksheet which includes the External Crayfish picture (#18 on the worksheet) and the Internal Structure of the Crayfish (the last activity) from https://www.biologycorner.com//worksheets/crayfishinternal.html . (You will need to cut and paste the second link as this site limits how many live links I can include.)

*Extra Credit (up to 8 tickets): Participate in the Great Backyard Bird Count from Feb. 15-18. You will need to register at http://gbbc.birdcount.org/ . Spend at least 15 minutes each day counting how many of each type of bird you see and then enter the amount online at the above site. You will get 2 tickets for each day that you participate.

Looking for all my lessons?

(I'll be posting a new lesson each week.)

© 2019 Shannon

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