TUNICATA 



657 



History. Ascidians have been known since the time of Aristotle, 

 but it was not until the first years of the last century that Cuvier, Savigny, 

 and Lamarck first accurately described them. The name Tunicata was 

 introduced by the latter author in 1816. Salpa was first described by 

 Forskal in 1775, and in 1819 its interesting life history was first made 

 known by the German poet Chamisso. Steenstrup in 1842 more fully 

 described this and first called it an alternation of generations. It was 

 in 1826 that Milne-Edwards discovered and correctly interpreted the 

 ascidian tadpole, while Kowalevsky in 1866 and 1871 published his cele- 

 brated memoirs giving the complete embryology of the animals and dem- 

 onstrating the common relationship of tunicates and vertebrates. Up to 

 that time the tunicates were almost universally classified with mollusks. 

 Milne-Edwards had, however, in 1841 placed them in the Molluscoidea, 

 a phylum he created to contain them and the Bryozoa. 



Tunicates are all marine animals, the ascidians being sessile and the 

 others pelagic. The number of species is about 1,300, which are grouped 

 in 3 classes. 



Key to the classes of Tunicata. 



dj Pelagic animals. 

 Z>! Animals minute and with a 



long tail 1. LABVACEA 



6 a Animals not minute, and more 

 or less cylindrical or flat- 

 tened 2. THALIACEA 



o a Sessile (except the Pyrosomi- 

 dae), sac-shaped animals.. 

 3. ASCIDIACEA 



CLASS 1. LARVACEA.* 



The appendicularians (Fig. 

 1,000). Minute, transparent, 

 free-swimming tunicates in which 

 the body consists of a trunk (2) 

 and a long tail (10), having es- 

 sentially the same organization 

 as the larval ascidian. The trunk 

 contains the large pharynx and 

 the viscera. The tail is a solid 

 structure, the axis of which is 

 structure is the principal nerve 

 muscle band. The tail is twisted 



Fig. 1,000 Diagram of an appendicu- 

 larian (Oikopleura) (Delage et H6rouard). 



I, mouth ; 2, trunk ; 3, pharynx ; 4, anus ; 

 5, stigmata ; 6, heart ; 7, stomach ; 8, 

 ovary ; 9, testis ; 10, basal portion of tail ; 



II, notochord ; 12, principal nerve ; 13, 

 endostyle. 



the notochord (11) ; dorsal to this 

 (12) and on each side of it a broad 

 90, so that its dorsal side lies at the 



* "fitudes sur les Appendiculaires du dtroit de Messine," by H. Fol, Mm. Soc. 

 Phys. Hist, nat., Gneve, Vol. 21, 1872, 



