514 UROCHORDA 



branchial sac opening by many slits into the peribranchial chamber' 

 which communicates with the exterior by a single opening. There is 

 a permanent and well-developed cuticular test into which cells from 

 the body migrate. Many have the power of budding, and there is 

 sometimes alternation of generations. 



Sub-order i. Ascidiae Simplices. Solitary fixed forms which rarely 

 bud ; when colonial, each individual has a separate test. 

 Ascidia, Phallusia, Ciona. 

 Sub-order 2. Ascidiae Composita?. Fixed Ascidians which repro- 

 duce by gemmation, the individuals being embedded in a 

 common investing mass. Botryllus, Polyclinum. 

 Sub-order 3. Ascidiae Luciae. Free-swimming Ascidians which re- 

 produce by gemmation to form a colony, having the shape of 

 a hollow cylinder, open at one end. There is one genus, 

 Pyrosoma, widely represented, especially in tropical seas. 

 They are brilliantly phosphorescent, and some attain a length 

 of twelve feet. 



Order 3. Thaliacea 



Free-swimming pelagic forms, which may be either single or 

 ** social," and in the adult are never provided with tail or notochord. 

 The muscles are in the form of distinct circular bands, which effect 

 locomotion by squirting out the water from the body. The test, 

 which may be well or ill developed, is always transparent. The life- 

 history exhibits distinct alternation of generations, and there is some- 

 times polymorphism. 



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

 Anchinia. 



(6) Hemimyaria. Muscle bands are in the form of incomplete rings. 

 Salpa, Octacnemus. 



Relationships 



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 larva? 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. 



In any case the Larvacea retain persistently a number of characters 

 which were probably possessed by the primitive Tunicata. 



There are several resemblances between Tunicates and Lancelets 

 (see the next chapter), e.g. the relatively large respiratory pharynx 

 and the peribranchial cavity, but this probably does not mean more 

 than that both groups arose from a common stock of primitive chordate 

 animals. 



