UROCHORDA. 



The changes in the Ascidiozooids are even more considerable 

 than those in the Cyathozooid. A nervous system appears as a 

 fresh formation close to the end of each Ascidiozooid turned 

 towards the Cyathozooid. It forms a tube of which the open 



A & 



FlG. 14. TWO STAGES IN THE DEVELOPMENT OF PYROSOMA IN WHICH THE 

 CYATHOZOOID AND FOUR ASCIDIOZOOIDS ARE ALREADY DISTINCTLY FORMED. 

 (After Kowalevsky.) 



i'}'. Cyathozooid ; as. ascidiozooid ; nt*. nervous groove ; lit. heart of Cyathozooid ; 

 iL cloacal opening. 



front end eventually develops into the ciliated pit of the 

 mouth, and the remainder into the actual nervous ganglion. 

 Between the nervous system and the endostyle an involution 

 appears, which gives rise to the mouth. On each side of the 

 primitive alimentary cavity of each Ascidiozooid branchial slits 

 make their appearance, leading into the atrial tubes; so that the 

 primitive alimentary tract becomes converted into the branchial 

 sacks of the Ascidiozooids. The remainder of the alimentary 

 tract of each zooid is formed as a bud from the hind end of the 

 branchial sack in the usual way. The alimentary tracts of the 

 four Ascidiozooids are at first in free communication by tubes 

 opening from the hinder extremity of one zooid into the dorsal 

 side of the branchial sack of the next zooid. At the hinder end 

 of each Ascidiozooid is developed a mass of fatty cells known 

 as the elseoblast, which probably represents a rudiment of the 

 larval tail of simple Ascidians. (Cf. pp. 30 32.) 



The further changes consist in the gradual atrophy of the 

 Cyathozooid, which becomes more and more enclosed within the 

 four Ascidiozooids. These latter become completely enveloped 



