Comb Jellies, such as this Sea Walnut (Mnemiopsis mccradyi) are truly in a class of their own.  To be more precise, biologists have assigned them their own phylum:  Ctenophora.  They are not true jellyfish, since they do not have the nematocysts (stinging cells) which are characteristic of Phylum Cnidaria (jellyfish, anemonies, corals, etc.)  There are two species of comb jellies I see frequently on Little Cayman:  this Sea Walnut, and another called the Venus' Girdle.  They tend to be in the top 10' of the water column where they prey on tiny planktonic animals.  They are transparent and so nearly invisible.  That makes them quite a challenge to photograph.  The Sea Walnut is about 2" long, and at times they can be quite numerous.  I suspect most divers swim right through them and never notice them.  There's nothing dramatic about watching comb jellies, but they are quite beautiful in their own way.