The Four Sub orders of Nudibranchia are;
Doridina || Dendronotina || Arminina || Aeolidina
Doridina have a wide range of species, and by far tend to be the most photographed of all the sub orders.
This sub order tends to follow the same format in each species too, whereas there are some bizarre looking species in the other sub orders that I found difficult to relate as nudibranchs.
Most easily identified follow the template of the Chromodorididae family
Key physiological points are shown using this example of Chromodoris lochi below.