Seaweed
"Seaweed" is a colloquial term and lacks a formal definition. A seaweed may belong to one of several groups of multicellular algae: the red algae, green algae, and brown algae. As these three groups do not have a common multicellular ancestor, the seaweed are in a polyphyletic group. In addition, some tuft-forming bluegreen algae (Cyanobacteria) are sometimes considered to be seaweed.
Ulva intestinalis
Ulva intestinalis, formerly referred to as "Enteromorpha intestinalis" (Linnaeus) Nees, is a green alga in the family Ulvaceae, of the genus "Ulva" (sea lettuce), also known by the common names gutweed and grass kelp. Until they were reclassified by genetic work completed in the early 2000s, the tubular members of the genus "Ulva" were in the genus "Enteromorpha".