TUNICATA 677 



balance. Presently the vesicle acquires an opening into the dorsal 

 side of the pharynx, near the mouth. The pericardium arises towards 

 the end of larval life as an outgrowth from the ventral side of the 

 pharynx. It does not form the heart until metamorphosis. The front 

 end of the body is a prominent chin, and bears three fixation papillae 

 of glandular cells. Except for the tips of these papillae, the animal is 

 entirely covered with test, which even closes the mouth, so that feed- 

 ing is impossible. After swimming for a short time, the larva fixes 

 itself to some solid object by the papillae, and proceeds to undergo a 

 metamorphosis (Fig. 472 B, C), by which it assumes the adult form. 

 The tail is devoured from within by phagocytes. By growth in the 

 region between the chin and the mouth, the latter and the atrial 

 opening are shifted back until they point upwards from the region of 

 fixation. Meanwhile, the central nervous system degenerates, save 

 for certain portions of the cerebral vesicle, which forms from its 

 hinder region the ganglion of the adult and from its ventral and an- 

 terior region the subneural gland and the ciliated funnel ; the pharynx 

 develops in the way described above ; the heart is formed ; the epicar- 

 dial diverticula grow out from the pharynx; and the gonads arise 

 from a mass of mesoderm. 



Ciona is a solitary animal. Some other tunicates resemble it in this 

 respect, but a large number establish by budding colonies of zooids, 

 each zooid having the essential features of an individual of Ciona. In 

 a few cases {Perophora, Fig. 476), the zooids are free from one another 

 save at their bases, where they are united by the stolon from which 

 they were formed. In most genera, however, the zooids of a colony 

 are imbedded in a common test, with only the mouths and cloacal 

 openings at the surface (Figs. 473, 477). In such cases the original 

 connection between the zooids is lost, though their atrial openings 

 usually join in a common cloaca. In the pelagic genera Salpa and 

 Doliolum and their relatives buds are formed but, instead of remaining 

 together as a colony, eventually become free and lead a solitary 

 existence. 



Budding is accomplished in various ways, (a) It most often takes 

 place from a ^/o/o«, which is a median ventral, tubular outgrowth of the 

 visceral (abdominal) region of the parent, usually containing an inner 

 tube that consists of the united distal portions of the two epicardial 

 diverticula of the pharynx, and some mesenchyme cells in a blood 

 space between this tube and the stolon wall. The epicardial tube will 

 form the alimentary canal of the buds. Often it also forms the atrium 

 and the nervous system. In the class Thaliacea, however (see below), 

 the stolon is more complex and contains special tubes or strands of 

 cells for the atrium and gonads and sometimes also for the nervous 

 system. (/>) In other cases (Perophora and Clavelina) the stolon con- 



