B. Biology: Tissues, Organs, and Systems of Living Things · Tissues, Organs, and Systems of Living Things

Dissection: Seeing How Organ Systems Connect

B2.6 B2.1 B3.5 A1.2 A1.4 A1.6 A1.11About 75 minutes

Learning goals

I can follow safe and respectful procedures for a laboratory or virtual dissection.
I can identify the main organs of an earthworm, a fish and a frog, and of a plant's root and shoot systems.
I can explain how organ systems are interrelated, such as the digestive and circulatory systems, or the root and leaf systems.
I can compare the hearts and circulation of a fish, a frog and a human.

Words to know

dissection
Carefully cutting open and separating the parts of an organism to study its structure.

virtual dissection
A computer simulation of a dissection, using images, video or 3D models.

dorsal / ventral
Toward the back / toward the belly.

anterior / posterior
Toward the head / toward the tail.

crop and gizzard
In an earthworm, the crop stores food and the muscular gizzard grinds it.

swim bladder
A gas-filled sac in many fish that lets them float at different depths without swimming.

gills
Organs with thin, feathery filaments full of capillaries where fish exchange gases with water.

single / double circulation
Blood passes through the heart once per circuit (fish) / twice per circuit (frogs, birds, mammals).

Looking inside

Diagrams and models are useful, but they tidy everything up. Real organisms are crowded, connected and surprising: organs are tucked behind each other, blood vessels thread through every system, and the gut is much longer than you'd expect. Dissection (real or virtual) lets you see how systems fit and depend on each other. The goal is not to cut things up; it is to observe, carefully and respectfully, the way an anatomist, a veterinarian or a surgeon learns.

Respect, choices and alternatives

Students in Ontario may have the choice of a virtual dissection: interactive software, videos or 3D models that show the same structures. Many people choose virtual dissection for ethical, cultural or personal reasons, and that choice deserves respect. When real specimens are used, they are usually preserved animals bred or collected for teaching, and they should be treated with care: no playing, no unnecessary cuts, and proper disposal.

Safety and tools

Tools: dissecting tray, scalpel (very sharp), dissecting scissors, forceps (tweezers), a blunt probe, and pins.
Wear safety goggles, gloves and an apron. Preserved specimens may contain chemicals that irritate skin and eyes; work in a ventilated room and follow the WHMIS label and your teacher's instructions.
Cut away from yourself and your partner. Use scissors with the tips lifted up so you don't slice organs underneath. Pass tools handle first.
Report any cut immediately.
Dispose of specimens and waste as your teacher directs, clean the tools and tray, and wash your hands well.

The earthworm: a tube within a tube

An earthworm's body is a long tube (the body wall, made of muscles in segments) with a digestive tube inside it.

Digestive system: mouth → muscular pharynx (sucks in soil) → esophagus → crop (stores food) → gizzard (grinds food with the help of soil particles) → long intestine (digests and absorbs) → anus.
Circulatory system: a closed system. Blood flows forward in the dorsal blood vessel (on top) and back in the ventral vessel, pumped by five pairs of aortic arches near the front.
Respiration: no lungs. Oxygen diffuses through the moist skin into capillaries just beneath it. This is why worms need damp soil and come up when the ground is flooded.
Nervous system: a small "brain" (cerebral ganglia) at the front and a ventral nerve cord running along the belly.
Excretory system: pairs of tiny tubes called nephridia in most segments.

Systems connect: nutrients absorbed by the intestine are picked up by blood vessels wrapped around it and carried to every segment; oxygen entering through the skin is carried by the same blood.

The front part of an earthworm, opened along the top (simplified). Follow the digestive tube from mouth to intestine and notice how closely the blood vessels run beside it, and how the aortic arches connect the dorsal vessel to the ventral vessel.

The fish

A bony fish such as a perch or trout shows how an animal is adapted to life in water.

Respiratory: water enters the mouth and flows over the gills. Their feathery filaments have a huge surface area and are packed with capillaries, so oxygen dissolved in the water diffuses into the blood.
Circulatory: a heart with two main chambers (one atrium, one ventricle) pumps blood to the gills, then on to the body, then back: single circulation.
Digestive: mouth → esophagus → stomach → intestine → anus, with a large liver. Many fish also have small finger-like pouches (pyloric caeca) near the stomach that help digest and absorb food.
Swim bladder: a gas-filled sac that adjusts buoyancy.
Excretory: dark red kidneys run along the underside of the backbone.
Nervous/sensory: the lateral line along each side detects vibrations and movement in the water.

Internal organs of a bony fish (simplified, not to scale). The heart sits just behind the gills, so freshly pumped blood goes straight to the gills to pick up oxygen. The swim bladder (dashed) lies above the gut.

The frog

A frog lives both in water and on land, and its organ systems show it:

Respiratory: simple lungs, plus gas exchange through its thin, moist skin (and the lining of its mouth). Under water, skin breathing is enough when the frog is resting.
Circulatory: a three-chambered heart (two atria, one ventricle) and double circulation; some mixing of oxygen-rich and oxygen-poor blood happens in the single ventricle.
Digestive: a long, sticky tongue attached at the front of the mouth, esophagus, stomach, coiled small intestine, large intestine and a cloaca, a single opening shared by the digestive, excretory and reproductive systems. The large liver is easy to spot.

In a frog dissection, follow the blood vessels from the heart: they branch to the lungs, the skin and the gut. That's the circulatory system linking the respiratory and digestive systems.

Hearts compared. The more chambers, the better oxygen-rich and oxygen-poor blood are kept apart. Mammals (and birds) have four chambers and double circulation, which supports a high, steady body temperature and an active lifestyle.

Dissecting a plant

Plants have organ systems too. Pull up a whole young plant (a bean seedling, a weed such as a dandelion or clover) and gently wash the soil off the roots.

Root system: main root and branching side roots; root hairs near the tips (use a magnifier).
Shoot system: stem, leaves, buds, maybe flowers.
Cut across the stem and the main root (grown-up): look for the vascular tissue.

Interrelationship: the root system's surface area must supply enough water for all the leaves' transpiration, and the leaves must make enough sugar to feed the roots. Scientists often measure the root-to-shoot ratio (mass of roots ÷ mass of shoots) to study this balance.

Worked example

Let's connect two systems in a frog

During a frog dissection, a student sees that the small intestine is covered in a net of tiny blood vessels, and that a large vessel carries blood from the intestine toward the liver. Explain what this shows about the interaction between the digestive and circulatory systems.

Digestive system's job: break food down and absorb nutrients through the wall of the small intestine.
Observation: a dense capillary network surrounds the intestine. Absorbed nutrients diffuse into these capillaries.
Circulatory system's job: carry the nutrients away. The large vessel takes nutrient-rich blood to the liver, which processes and stores some nutrients (such as glucose) before the blood goes on to the heart and body.
Conclusion: the systems are interdependent: the digestive system can't deliver nutrients to cells without the circulatory system, and the circulatory system has nothing to deliver without the digestive system.

The capillaries and the vessel to the liver show nutrients passing from the digestive system into the circulatory system, which distributes them to the rest of the body.

Worked example

Let's write up an observation like a scientist

Write the observation and analysis sections for a plant "dissection" in which a student measured a bean seedling: root mass 1.8 g, shoot mass 3.0 g; she counted 6 leaves and saw root hairs only near the root tips.

Observations (organized data): a table with structure, number or mass, notes: roots 1.8 g, root hairs present near tips only; shoot 3.0 g, 6 leaves, one stem.
Calculation: root-to-shoot ratio = 1.8 g ÷ 3.0 g = 0.60.
Analysis: the root system has about 60% of the mass of the shoot system. Root hairs at the tips give a large surface for absorbing water to supply the 6 leaves; the leaves make sugar that is sent to the roots.
Communication tips: use correct terms (root system, shoot system, root hairs, vascular tissue), units on every number, and a labelled diagram with a title.

A clear table, the ratio 0.60, and an analysis linking root hairs (absorption) and leaves (photosynthesis), written with correct terms and units.

Just for fun

Funny but true: people often say an earthworm has five hearts. Strictly speaking, it has five pairs of aortic arches that squeeze to pump blood, and no heart like ours. So if a worm ever sends you a valentine, it may be overdoing it.

Hands-on activity

Kitchen dissection: a whole fish or a whole plant

Choose one option.

Option A: a whole fish (with a grown-up). You need: a small whole fish that has not been gutted (ask at a fish counter for a "whole, uncleaned" fish such as trout, perch or smelt), a cutting board, kitchen scissors, a sharp knife used only by the grown-up, gloves, paper towel, a magnifier.

Look at the outside: fins, scales, the lateral line, the gill cover. Lift the gill cover and look at the red, feathery gills.
The grown-up cuts from the anus forward along the belly to the gills, cutting shallowly with scissor tips pointing up.
Identify the liver, stomach, intestine, heart (just behind the gills), the swim bladder (a shiny sac above the gut) and the dark kidney along the backbone.
Sketch and label what you see. Then clean up: wrap the remains for the garbage (or cook the fish properly if it was bought for dinner and handled hygienically), sanitize surfaces and wash hands.

Option B: a whole plant. Pull up a dandelion, clover or bean seedling with its roots. Wash the roots gently. Separate the root system from the shoot system with scissors. Count leaves, measure the longest root, and if you have a kitchen scale that reads to 0.1 g, find the masses and the root-to-shoot ratio. Look for root hairs with a magnifier.

What to look for: in the fish, how close the heart is to the gills and how the gut fills most of the body cavity; in the plant, how much of the plant is underground. In both, describe one way two systems depend on each other.

Your turn

Give three safety rules for a dissection.

In what order does food pass through an earthworm's digestive system?

How does an earthworm get oxygen?

What is the function of a fish's swim bladder?

Compare the number of heart chambers in a fish, a frog and a human.

Explain how the respiratory and circulatory systems of a fish work together.

Give one reason a student might choose a virtual dissection.

More practice: warm-up

What does "dorsal" mean?

What does an earthworm's gizzard do?

Where are a fish's gills?

What is a cloaca?

Name the two organ systems of a plant.

More practice: on your own

Why do earthworms come to the surface after heavy rain?

Explain why the heart of a fish is located right behind its gills.

Why is a four-chambered heart an advantage for a mammal?

A seedling's roots have a mass of 2.4 g and its shoots 4.0 g. Find the root-to-shoot ratio.

A frog sits under water in a pond in winter. How can it get oxygen?

Challenge

A plant grown in dry soil has a root-to-shoot ratio of 1.2; the same species grown in wet soil has 0.5. Explain this difference.

Explain why single circulation in fish delivers blood to the body at lower pressure than double circulation in mammals.

Design a short procedure to compare the gut length of a herbivorous fish and a carnivorous fish of similar body length (using pictures, video or real specimens). What would you predict, and why?

Check your understanding

Quiz

Which is the safest way to cut during a dissection?

How many chambers does a frog's heart have?

Which earthworm organ grinds food?

Where does a fish exchange gases?

Which observation best shows the digestive and circulatory systems interacting?

A plant has roots of mass 3.0 g and shoots of 6.0 g. What is its root-to-shoot ratio?

For parents and teachers

Big idea: expectation B2.6 asks students to investigate, through a laboratory or computer-simulated dissection of a plant, worm, fish or frog, the interrelationships between organ systems (for example, root and leaf systems; digestive and circulatory systems). Virtual dissection fully meets the expectation.

Sensitive topic: some students and families object to animal dissection for ethical, cultural or religious reasons. The plant option and virtual dissections are complete alternatives; it's fine to choose them.

How to help: If you try the fish option, handle the knife yourself and follow food-safety cleanup. The plant option needs only a weed and a magnifier. Ask your teen to point out one connection between two systems in whatever they examined.

Common difficulties:

Listing organs without explaining how systems interact.
Mixing up the crop (stores) and gizzard (grinds).
Thinking all animals have four-chambered hearts.