Ascidians, like the ones pictured above in Ernst Haeckel’s “Kunstformen der Natur”, are part of the taxon Tunicata - a group of invertebrate, sac-like, marine filter feeders that include sea squirts, sea tulips and salps. They are an incredibly diverse group of organisms, including those that are attached to the sea bed and those that are free-swimming; some solitary and some that form colonies; some that look like unassuming semitransparent jellies and some that explode with colour. Many have very complex life cycles that include both asexual and sexual reproduction.
As a sister-group of vertebrates, and likely our closest relatives, they are absolutely essential to our understanding of vertebrate origins and evolution, including the origin and development of the vertebrate nervous system. In their larval phase tunicates have a dorsal nerve cord and notochord, structures related to parts of our central nervous system like the spinal cord. Some free-swimming tunicates never out-grow the larval phase and conserve these structures as adult organisms.
The genomes of the tunicates Ciona intestinalis, Ciona savignyi and Oikopleura dioica have been sequenced, allowing us to analyse this group of organisms on a genomic and molecular level. Tunicates have many of the same genetic components as vertebrates, including the developmentally important Hox genes, as well as many genes that encode components of the nervous and endocrine systems.
Kunstformen der Natur (Art Forms of Nature)
by Ernst Haeckel
If you go here you can see 100 of these in enormous sizes. In German, but you can figure it out.