What is this part next to the seawater strainer?

[Krumelur]

Sorry if I'm double posting this - but something went wrong here.

I'm trying to understand what the highlighted part in the attached image is for.

From what I understand:

* Water is coming in through the saildrive
* Big parts are filtered out by the strainer
* From there it goes into the unknown thingy
* Makes its way to the seawater pump and into the heat exchanger

On top of the thingy there's a small hose that ends behind the engine in the bilge. What's that for?
Is the whole installation set up/laid out in a way that it is above sea level so if no pump is running, no water will be pushed into the boat?

[Yngmar]

Anti-siphon valve. On top is a tiny little air valve that closes under (water) pressure from the pump, but opens under vacuum from the engine trying to commit suicide by siphoning itself full of seawater. Instead it just gets air and the siphon effect is interrupted.

[Krumelur]

How would the engine suck in seawater? Isn't this just the outer cooling ring?
Or are you saying it would (potentially) suck it in through the exhaust elbow's connection?

And what about my idea of the valve and sea strainer being placed above the water line? True or false?

[symphony2]

Essential bit of kit as it vents the seawater feed pipe when the engine stops so eliminating any chance of syphoning. as you know the seawater exits via the exhaust an if any water gets syphoned into there it will enter the engine via the exhaust valve that is open. Makes a horrible mess of the engine, often terminal!