Chordate Zoology - Textbook - Y. V. Tsaryk - 2013

Chapter 2. SUBPHYLUM TUNICATA, or UROCHORDATA, TUNICATA, seu UROCHORDATA

2.3. SYSTEMATICS AND ORIGIN

The subphylum Tunicata comprises three classes: ascidians Ascidia, salps Salpae, and appendicularians Appendiculariae. Ascidians are divided into three orders: Monascidiae, Synascidiae, and Pyrosomata; salps are divided into two orders: Cyclomyria (doliolids) and Desmomyria (true salps). Together with appendicularians, they are grouped into pelagic tunicates. Some researchers assign the rank of Class to this group.

Today, there are about 1,400 species of tunicates in the seas and oceans, including 1,300 species of ascidians, 75 of appendicularians, and 25 of salps.

The assignment of tunicates to the phylum Chordata was primarily established by the works of O. O. Kowalevsky. It is believed that this group of animals diverged from sluggish, primitive Chordates and adapted to life on rocky substrates due to a sessile lifestyle. The Development of a tunic and a powerful filtering apparatus, asexual reproduction, and other adaptations ensured their widespread distribution.

Subsequent evolution, particularly the increased intensity of filtration, allowed tunicates to colonize the marine pelagic zone: the jet forces generated during intensive filtration enabled first salps, and later other groups, to regain mobility. Movement became possible through the action of ciliated Cells, which is atypical for chordates. Notably, even among invertebrates, cilia of this size are not used as a means of locomotion. Pyrosomes in tube-like colonies, through the close integration of zooids and The Effect of collective filtration, modified The Mechanism of jet propulsion. In both cases, the free-dispersing larva disappeared. In ascidians, such a larva is present. The same jet propulsion mechanism, but in their own way, is utilized by appendicularians, in which the adult stage has essentially disappeared, and neotenic reproduction has become established.

Much remains unknown about THE ORIGIN OF urochordates. However, despite their regressive evolutionary path, this group of animals is not in decline, as evidenced by their high species diversity.