The class Thaliacea is made up of three orders, five families, and 73 species. The three orders are Pyrosomatida, with one family and 10 species; Doliolida, with three families and 23 species; and Salpida, with one family and 40 species. Recent work indicates the thaliaceans are an artificial group because each of the orders evidently arose from a different group of benthic tunicates (class Ascidiacea). Evidence for this poly-phyly is in comparative embryology and anatomy, where each of the orders show their ascidiacean ancestry to greater or lesser degrees. Their similar body forms are an example of convergent evolution in which animals look alike despite their differing phylogeny. Molecular work being done on thali-aceans should help determine relationships more precisely; consequently, this group may be dismantled and reorganized into separate groups in the future. Salps are related to the benthic tunicate class Ascidiacea.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.