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Beroids eat almost nothing but other Comb Jellies, which is pretty rude. Some of them even focus their attentions on particular species of Comb Jellies, which is ruder still. They have the same rows of cilia used for swimming as other Comb Jellies do, but they also have other more unique ones. too.
Lining that gigantic mouth of theirs, rows of hair-like cilia are fused together to create what one could well call "teeth". This means that the Beroid can take bites out of Comb Jellies that are too big to swallow whole. This is good because while some Beroids might be 15 cm long, others can be just 3 cm.
When not being fed, that abyssal mouth can be shut. Furthermore, some Beroids have sticky lips, meaning that it can be tightly closed to offer a more streamlined shape when giving chase to prey. You need every advantage you can get when you're a ravenous mouth and almost nothing else whatsoever!