xii PHYLUM CHORDATA 319 



distinguishable into pharynx, oesophagus, stomach and intestine. The 

 pharynx opens on the exterior by the mouth : in its ventral floor the 

 endostyle (end. ) has become developed ; its walls are pierced by 

 stigmata (stig. ), the number of "which varies ; a ciliated sac opens into 

 it below the trunk part of the nerve-cord. The atrial cavity has be- 

 come formed round the pharynx, and opens on the exterior by a single 

 aperture (atr.). The heart and pericardial cavity have become de- 

 veloped. In this tailed free-swimming stage the larva remains only a 

 few hours ; it soon becomes fixed by the adhesive papillae, and begins 

 to undergo the retrogressive metamorphosis by which it attains the 

 adult condition. 



The chief changes involved in this retrogressive metamorphosis (Fig. 

 1 88) are the increase in the number of pharyngeal stigmata, the 



FIG. 187. Free-swimming larva 6f Ascidia mammillata, lateral view. adh. 

 adhesive papillae; ali. alimentary canal; atr. atrial aperture; cil.gr. ciliated 

 groove ; end. endostyle ; eye, eye ; med. nerve-cord ; noto. notochord ; oto. 

 otocyst ; sens. ves. sense-vesicle ; stig. earliest stigmata. (From Korschelt and 

 Heider, after Kowalewsky.) 



diminution, and eventually the complete disappearance, of the tail with 

 the contained notochord and caudal part of the nerve-cord, the dis- 

 appearance of the eye and otocyst, the dwindling of the trunk part of 

 the nervous system to a single ganglion, and the formation of the 

 reproductive organs. Thus, from an active, free-swimming larva, with 

 well-developed organs of special sense, and provided with a notochord 

 and well-developed nervous system, there is a retrogression to the 

 fixed inert adult, in which the parts indicative of affinities with the 

 Vertebrata have become aborted. 



A remarkable feature of the Ascidians is that, though many remain 

 simple, others give rise to colonies by a process of budding. In some 



