28 PYROSOMA. 



in a common test, and form a ring round the remains of the 

 yolk and of the Cyathozooid, the heart of which continues how- 

 ever to beat vigorously. The cloacal opening of the Cyathozooid 

 persists through all these changes, and, after the Cyathozooid 

 itself has become completely enveloped in the Ascidiozooids and 

 finally absorbed, deepens to form the common cloacal cavity of 

 the Pyrosoma colony. 



The main parts of the Ascidiozooids were already formed 

 during the last stage. The zooids long remain connected to- 

 gether, and united by a vascular tube with the Cyathozooid, and 

 these connections are not severed till the latter completely atro- 

 phies. Finally, after the absorption of the Cyathozooid, the 

 Ascidiozooids form a rudimentary colony of four individuals 

 enveloped in a common test. The two atrial tubes of each 

 zooid remain separate in front but unite posteriorly. An anus 

 is formed leading from the rectum into the common posterior 

 part of the atrial cavity; and an opening is established between 

 the posterior end of the atrial cavity of each Ascidiozooid and 

 the common axial cloacal cavity of the whole colony. The 

 atrial cavities in Pyrosoma are clearly lined by epiblast, just as 

 in simple Ascidians. 



When the young colony is ready to become free, it escapes 

 from the atrial cavity of the parent, and increases in size by 

 budding. 



Doliolidae. The sexually developed embryos of Doliolum have been 

 observed by Krohn (No. 23), Gegenbaur (No. 10), and Keferstein and Ehlers 

 (No. 17); but the details of the development have been very imperfectly 

 investigated. 



The youngest embryo observed was enveloped in a large oval trans- 

 parent covering, the exact nature of which is not clear. It is perhaps a 

 larval rudiment of the test which would seem to be absent in the adult. 

 Within this covering is the larva, the main organs of which are already 

 developed ; and which primarily differs from the adult in the possession of a 

 larval tail similar to that of simple Ascidians. 



In the body both oral and atrial openings are present, the latter on the 

 dorsal surface ; and the alimentary tract is fully established. The endostyle 

 is already formed on the ventral wall of the branchial sack, but the bran- 

 chial slits are not present. Nine muscular rings are already visible. The 

 tail, though not so developed as in the simple Ascidians, contains an axial 

 notochord of the usual structure, and lateral muscles. It is inserted on the 

 ventral side, and by its slow movements the larva progresses. 



