4io UROCHORDA OR TUNIC ATA. 



ici) Ascidice Simplices. Solitary fixed forms which rarely bud ; 

 when colonial, each individual has a separate test. Ascidia, 

 Phallusia, Ciona. 



(b) Ascidise Compositae. Fixed Ascidians which reproduce by gem- 

 mation, the individuals being embedded in a common investing 

 mass. Botryllus, Polydinum. 



{c) Pyrosomidse. Free-swimming Ascidians which reproduce by 

 gemmation to form a colony, having the shape of a hollow 

 cylinder, open at one end. Pyrosoma. 



Order 3. Thaltacea. 



Free-swimming pelagic forms, which may be either simple or com- 

 pound, and in the adult are never provided with tail or notochord. 

 The muscles are in the form of distinct bands. The life history exhibits 

 distinct alternation of generations, but true colonies are not formed. 



(a) Cyclomyaria. Muscle bands form complete rings. Doliolum, 

 Anchinia. 



{/>) Hemimyaria. Muscle bands are in the form of incomplete rings. 

 Salpa, Octacnemus. 



The questions as to the origin of the Tunicates and the relations of 

 the orders are too difficult to be discussed here, but we may note that 

 there are two possible views as to the position of Appendicularia and 

 its allies. They may be regarded as the slightly modified descendants 

 of the primitive Tunicates, from which the Ascidians have diverged in 

 the direction of degeneration, or as prematurely sexual larvee derived 

 from an already degraded Ascidian-like form. Both views have had 

 supporters, and the one adopted materially affects the general method 

 of regarding the group. The question as to which is correct is not 

 likely to be decided until the embryological data are more complete. 



The relations of the Tunicates to Amphioxus will be discussed in the 

 next chapter. 



