Van der Hoeven, 1850
Thaliacea are pelagic tunicates with a permanent and transparent test; a tail is absent. They vary in size from very small ( about 1 mm) to very large (20 m) in case of the colonial pyrosomid forms. In contrast to the Class Ascidiacea, the Thaliacea have in common that they have the buccal and atrial siphons at opposite ends of the body. The musculature consists of loop-like bands in the body wall; rhythmic contractions of the muscles provokes a water circulation, which is not only used for gas exchange and feeding but serves also as a sort of propulsion. The life cycle comprises an alternation of generations. The sexually reproducing stages (blastozooids) are derived by budding from the individuals (oozooids) that develop from the egg. The Thaliacea contain three orders: the Order Doliolida, the Order Salpida and the Order Pyrosomida. It seems probable that the group is not monophyletic (Godeaux et al., 1989; Christen and Braconnot, 1998).
The key to the thaliacean orders and species of the North Sea starts at Page 454: Tunicata. The following species are included:
Class Thaliacea
Order Doliolida
Family Doliolidae
Dolioletta gegenbauri
Doliolina muelleri
Doliolum denticulatum
Doliolum nationalis
Order Salpida
Family Salpidae
Salpa fusiformis
Thalia democratica
Pegea confoederata
Thetys vagina
Ihlea punctata
Soestia zonaria
Order Pyrosomida
Family Pyrosomatidae
Pyrosoma atlanticum