astronomy-to-zoology:

How do certain invertebrates know if they are rightside up?

the science of statocysts 

certain types of invertebrates like bivalves,echinoderms and cnidarians lack sensory receptors, buy yet they know if they are upside down or not and can right themselves up if knocked over. This is due to a special balance sensory receptor called a statocyst, a statocyst can be easily described as a marble inside a basketball, as the statocyst consists of a sac-like structure with a small ball-like mass inside of it called a statolith. The walls of this fluid filled sac are lined with small setae (hairlike structures). Which when the animal moves or is overturned the statolith inside the sac will respond to gravity and fall into the hairs which in turn will activate neurons that tell the animal what orientation it is in so it can adjust accordingly.

Further reading

Image sources: 1,2

(via mad-as-a-marine-biologist)