Ernst Heinrich Philipp August Haeckel (February 16, 1834 – August 9, 1919), also written von Haeckel, was an eminent German biologist, naturalist, philosopher, physician, professor and artist who discovered, described and named thousands of new species, mapped a genealogical tree relating all life forms, and coined many terms in biology, including phylum, phylogeny, ecology and the kingdom Protista.

This is from the 85th plate of Haeckel's Kunstformen der Natur, Ascidiacea.

Ascidiacea (commonly known as ascidians or sea squirts) is a class in the Urochordata subphylum of sac-like marine invertebrate filter feeders. Ascidians are chraacterised by a tough outer "tunic" made of the polysaccharide tunicin, as compared to other tunicates which are less rigid.

Ascidians are found all over the world, usually in shallow water with salinities over 2.5%. While members of the Thaliacea and Larvacea swim freely like plankton, sea squirts are sessile animals: they remain firmly attached to substrata such as rocks and shells.