Sea squirts are strange animals in their own right. One of our most distant relatives, they have a “notocord”, like a primitive backbone or spinal cord when they’re in their juvenile stage. At this point, they look a bit like a tadpole.
When they find a place to call home, they land, and root themselves in place – and lose their notocord as they change into an adult sea squirt.
To look inside, the best way is not to shine your torch from the front – this tends to light up the sea squirt too much, and can cause it’s openings to contract.
Place your torch very close to the side of a transparent / semi-transparent sea squirt and ensure its brightness is all the way up. If done just right, the sea squirt and the amphipod should light up.
Photographers – make sure you have spot focus turned on, otherwise you’ll just get a lot of shots of the opening of the sea squirt.