Tunicates are marine invertebrates belonging to the Tunicata subphylum. It belongs to the phylum Chordata, which contains all creatures with dorsal nerve cords and notochords. Some tunicates live alone, but others reproduce through budding and form colonies, with each unit known as a zooid. They are marine filter feeders with a water-filled, sac-like body structure and two tube openings, known as syphons, via which they draw in and expel water. Tunicate colonies come in a variety of shapes and sizes, and the degree to which individual organisms, known as zooids, interact with one another varies. Individual animals are widely apart in the simplest systems, but they are joined together by horizontal connections called stolons that grow along the seafloor. In other species, the zooids form a tuft or are crowded together and share a similar foundation. The most advanced colonies include zooids into a shared structure surrounded by the tunic. An ascidiacean's body is surrounded by a test or tunic, which gives rise to the subphylum's name. This varies in thickness according on the species, however it can be stiff, like cartilage, thin and fragile, or transparent and gelatinous. The tunic is made of proteins and complex polysaccharides, and includes tunicin, a form of cellulose. The tunic is unique among invertebrate exoskeletons in that it may grow with the animal and does not need to be shed on a regular basis. The body wall or mantle inside the tunic is made up of connective tissue, muscle fibres, blood arteries, and nerves.