214 THE ASCIDIANS. 



It has long been recognised that the development of an 

 Ascidian is much abbreviated in comparison with that of 

 Amphioxus, since in the former it neither comes to the 

 formation of a ciliated embryo nor to the production of 

 archenteric pouches for the mesoderm. One of the chief 

 evidences, however, of abbreviation in the Ascidian devel- 

 opment is the precocious formation of the larval tail. 



Formation of Alimentary Canal and Hatching of Larva. 



When the enteric cavity of the Ascidian embryo begins 

 to grow in length so as to give rise to the stomach and 

 intestine, which it does shortly after the appearance of 

 the atrial involutions, there is only one resource open to 

 it on account of the limited space in which it lies, and that 

 is to double round upon itself. This it accordingly does. 

 As the growth progresses, the posterior dorsal angle of 

 the enteric cavity bends sharply downwards on the right 

 side, and then upwards and slightly forwards on the left 

 side, ending at first blindly in the vicinity of the left atrial 

 sac. In this way the four divisions of the alimentary 

 canal become established ; namely, pharynx or branchial 

 sac, oesophagus, stomach, and intestine. (Cf. Fig. 105.) 



By the time these changes have taken place, the embry- 

 onic development is at an end, and the larva is ready to 

 hatch. By spasmodic jerkings of its tail, the larva finally 

 succeeds in bursting the egg-follicle or vitelline membrane 

 in which it has been hitherto enclosed, and so escapes 

 into the open sea. 



Clavelina and Ciona. 



While the development of most forms of Tunicata is re- 

 ducible to a common type, yet the details vary within very 

 wide limits in different genera. The tendency here, as 



