UROCHORDA. 23 



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 Ascidio- 

 zooid is developed a mass of fatty cells known as the elgeoblast, 

 which probably represents a rudiment of the larval tail of simple 

 Ascidians. (Cf. pp. 25 and 26.) 



The further changes consist in the gradual atrophy of the Cyatho- 

 zooid, which becomes more and more enclosed within the four Asci- 

 diozooids. These latter become completely enveloped in a common 

 test, and form a ring round the remains of the yolk and of the 

 Cyathozooid, the heart of which continues however to beat vigor- 

 ously. The cloacal opening of the Cyathozooid persists through 

 all these changes, and, after the Cyathozooid itself has become com- 

 pletely enveloped in the Ascidiozooids and finally absorbed, deepens 

 to form the common cloacal cavity of the Pyrosoma colony. 



The main parts of the Ascidiozooids ware already formed during 

 the last stage. The zooids long remain connected together, and 

 united by a vascular tube with the Cyathozooid, and these connec- 

 tions are not severed till the latter completely atrophies. 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 estab- 

 lished between the posterior end of the atrial cavity of each Ascidio- 

 zooid 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 SIZL J by budding. 



DoliolidaJ. 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 im- 

 perfectly investigated. 



The youngest embryo observed was enveloped in, a large oval transparent 

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

 rudiment of the test which would seein 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 

 branchial 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. 



