Invertebrate Animals

This forum is for all discussions of Invertebrates only . These organisms comprise 99% of all animals, so there are many things to talk about in this forum! Keep in mind, Invertebrates are not a group of closely related animals, but instead comprise many different evolutionary lineages. Remember to add something that hasn't been touched on already - when in doubt, pick an obscure group of invertebrates and describe it. (Include source link underneath each response!)