Cystoids were animals that lived in the shallow salt water seas. This one is from the famous Rochester Shale of Middleport New York. It is Silurian in age.
The Cystoid is made up of a “stem” using a “holdfast” to attach itself to a rock or bryozoan. At the top of the stem is a round theca, the main body part of the cystoid. On that are the brachiols, the branching arms of the cystoid on that are very small pinnules that do the work of capturing food for the cystoid. These are NOT plants, they are animals.