Taxonomy of species is particularly complex. Many of us know the animal kingdom as divided simply into vertebrates and invertebrates. Phylum Chordata included vertebrates, or those organisms with a backbone or vertebral column, encasing a spinal or nerve cord.
However, the Subphylum Urochordata, includes strangely shaped animals called tunicates, which do not have backbones but once exhibited characteristics of Phylum Chordata at some point in their life cycle. Tunicates are often overlooked, given their tiny size or mistaken identity for sponges (Phylum Porifera) but they have a fascinating anatomy.
There are stationary (attached to a substrate) tunicates and pelagic (free swimming in open water) tunicates. All tunicates have a cellulose tunic covering the body, hence their name. With a similar appearance to sponges (though much smaller), tunicates also have incurrent and excurrent siphons, functioning in water intake for oxygen extraction and outflow, respectively.
Class Ascidiacea includes stationary tunicates, comprising solitary (also called simple ascidians) or colonial/compound types. Many compound tunicates are extremely colorful and are found at various depths, in bulbous, often translucent clusters, joined at one base or sharing a common tunic.