XVIII] 



CLASSIFICATION 



411 



Class II. The ACOPA. 



Forms which have lost the tail with its nerves and muscles. 

 These are divided into 



Order I. The Ascidiacea, fixed forms. 



Order II. The Thaliacea, 

 which have secondarily acquir- 

 ed the power of swimming by 

 contractions of the whole body 

 carried out by transverse bands 

 of muscle. 



The Ascidiacea constitute 

 the great bulk of the Uro- 

 chorda. Some of them, such 

 as the form taken as a type 

 in the general description given 

 above, remain solitary through- 

 out life, but others bud and 

 form colonies embedded in a 

 common test ; these are called 

 Compound Ascidians. But the 

 group is not . a natural one. 

 since budding is carried out 

 in different ways in different 

 families, and has therefore pro- 

 bably originated several times. 

 The commonest method is by 

 the outgrowth of a hollow 

 finger-shaped process of the 

 pharynx, called a stolon, aris- 

 ing at the hinder end of the 

 endostyle, which becomes 

 divided into pieces, each form- 

 ing a bud. On the other hand 

 in Botryllus a different method 

 is followed, since in this case 

 the buds originate simply as 

 little pockets of the atrial wall 

 of the parent. Botryllus is 

 one of the most beautiful 

 colonial forms ; in it the buds are arranged in circles ; the atrial 

 openings of* the members of a circle open into a common pit in 



FIG. 205. Dorsal view of a fully-grown 

 specimen of the solitary form of Salpa 

 democratica x about 10. From Brooks. 



1. Muscle bands. 2. Narrow band re- 

 presenting the dorsal .wall of the 

 pharynx, the so-called " gill." 3. En- 

 dostyle. 4. Peripharyngeal band. 

 5. Brain. 6. Ciliated pit. 8. "Nu- 

 cleus," consisting of stomach, liver, in- 

 testine. 9. Stolon or row of young. 

 10. Processes of mantle. 11. Mouth. 



