Setup

Living things need nutrients, gases and hormones to be transported all around their bodies.

In most animals, they’re carried by fluid in a circulatory system, which is hidden from view.

So to observe one in action we need...

Asellus aquaticus

A crustacean known as Asellus aquaticus, or water louse.

pipette

A plastic pipette with the end cut off.

cavity slide

A cavity slide.

microscope

And a microscope.

Let’s look closely at the circulation of fluid in Asellus.

How does the circulatory system of Asellus aquaticus work?

How does Asellus aquaticus obtain oxygen from water?

Method

We use a plastic pipette to withdraw a young Asellus from a holding container...

And place it in a drop of water in the middle of a cavity slide.

We place another slide on top and secure it with sticky tack, taking care not to squash the Asellus.

The cavity slide ensures that the specimen will not be harmed, but will be held in position for examination.

We flip the slide over so that we can examine the underside of the creature.

Now we can examine the specimen under the microscope, which is set to a low power.

10x magnification

First we focus on one of its large, rear limbs and observe the movement of materials inside it.

Next we examine the posterior part of the Asellus and look closely at the thin structures sticking out from both sides of its body.

After the examination, we quickly and carefully return the Asellus to its container.

Analysis

We examined the circulatory system of the small freshwater crustacean Asellus.

We select a young Asellus because there is less pigment in its skin, and its transparency means we can see its circulatory system.

Crustaceans have an open circulatory system, in which fluid flows around cavities in the body and bathes the cells to supply them with oxygen.

Open circulatory system:
Fluid flows around cavities in the body and bathes cells to supply them with oxygen.

Humans, and most other vertebrates, have a closed circulatory system, in which blood flows between organs through enclosed vessels.

Closed circulatory system:
Blood flows through enclosed vessels to supply organs with oxygen.

We focus on the limbs of the water louse because they are large enough for us to be able to observe the movement of fluid inside them.

This fluid is called haemolymph.

haemolymph

It consists of plasma, blood cells and lymph cells.

In humans, the lymph cells circulate in a separate system.

In the Asellus, we see at least two streams of haemolymph circulating in fixed directions – one stream flows directly towards the body and the other flows away from it, so the fluid is flowing through the entire limb.

Streams of haemolymph flow in opposite directions.

The Asellus aquaticus's abdomen is covered with small appendages called pleopods.

These increase the surface area of the crustacean’s body.

PleopodsWhen we examine the pleopods under the microscope – we can see haemolymph flowing mainly towards the body.

The open circulatory system of the Asellus allows oxygen dissolved in the water to diffuse through its skin directly into the haemolymph, which then transports the oxygen throughout its body.

Asellus aquaticus has an open circulatory system.

In the Asellus’ open circulatory system oxygen is delivered directly to the cells through cavities in the body.