TUNICATA 599 



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 {Clavelina, Fig. 451), 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. 448, 452). In 

 such cases the stolonial connection between the zooids is lost, 

 though their atrial openings usually join in a common cloaca. 



Budding usually takes place from a stolon,which. is a median ventral, 

 tubular outgrowth of the visceral (abdominal) region of the parent, 

 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, 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 Botryllus and its allies budding is effected in a 

 different way. These genera, which, unlike Ciona but like most 

 solitary ascidians, possess no epicardium (epicardial diverticula), 

 form their buds by paired outgrowths that are of quite a different 

 kind from the stolon, for they arise from the atrial wall and each 

 contains an inner vesicle which is a prolongation of the epithelium 

 that lines the atrium of the parent : this vesicle forms the internal 

 organs as well as the atrium of the bud. It should be noticed that 

 in budding the origin of the organs takes place without regard to 

 the germ layers from which they arise in the development of the 

 ovum, for the endodermal inner tube of stolonial budding usually 

 forms atrium and nervous system, which should be of ectodermal 

 origin, and the ectodermal (atrial) inner vesicle of the "pallial" 

 budding of Botryllus forms the alimentary canal, which should be 

 endodermal. In the ascidians and Pyrosoma (Fig. 454) the buds 

 remain together to form a colony. In the thaliaceans Salpa and 

 Doliolum they become free and live an independent existence. 



