22 



PYROSOMA. 



In the following stages the Ascidiozooids grow with great rapidity. 

 They soon cease to lie in a straight line, and eventually form a ring 

 round the Cyathozooid and attached yolk sack. 



While these changes are being accomplished in the external form 

 of the colony, both the Cyathozooids and the Ascidiozooids progress 

 considerably in development. In the Cyathozooid the atrial spaces 

 gradually atrophy, with the exception of the external opening, which 

 becomes larger and more conspicuous. The heart at the same time 

 comes into full activity and drives the blood through the whole colony. 

 The yolk becomes moi^e and more enveloped by the Cyatiiozooid, and 

 is rapidly absorbed; while the nutriment derived from it is transported 

 to the Ascidiozooids by means of the vascular connection. The 

 nervous system retains its previous condition ; and round the Cya- 

 thozooid is formed the test into which cells migrate, and arrange 

 themselves in very conspicuous hexagonal areas. The delicate ali- 

 mentary tract of the Cyathozooid is still continuous with that of 

 the first Ascidiozooid. After the Cyathozooid has reached the 

 development just described it commences to atrophy. 



The changes in the Ascidiozooids are even more considerable than 

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



Fig. 14l. Two stages in the development of Pyeosoma in which the Cyathozooid 

 AND FOUR Ascidiozooids ake already distinctly formed. (After Kowalevsky.) 



cy. cyathozooid; an. ascidiozooid; ng. nervous groove ; ht. heart of cyathozooid ; 

 cl. cloacal opening. 



formation close to the end of each Ascidiozooid turned towards the 

 Cyathozooid. It forms a tube of which the open 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 alimen- 

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



