They range in size from 2cms to species that can grow to 70cms ! we have seen some small & up to 20cms in our local waters.They have a head with a pair of rhinophores [ maybe they look like Hare ears ?] and enrolled oral tentacles beside the mouth…..
The shell[ they are Molluscs]when presents thinly calcified internal plate over the gill & heart.the photo above shows the parapodial lobes which are very noticeable when they swim.
Their egg masses resemble pale green spaghetti.
Information from Seaslug forum.