222 THE ASCIDIANS. 



to form a lining membrane round tlie cavity, remain 

 independent of one another and scattered about inside tlie 

 cavity. 



Fixation of the Ascidian Larva. 



Wiien the larva first fixes itself to some available surface, 

 the tail remains for a time stretched straight out and 

 almost motionless, giving perhaps an occasional twitch. 

 Soon the tail is observed to become shorter and to finally 

 disappear, having been drawn within the body proper of 

 the young Ascidian. The entire tail, with the whole of 

 the notochord, musculature, and caudal portion of nerve- 

 tube, becomes thus retracted and invaginated into the 

 posterior region of the body-cavity, where it forms a coiled 

 amorphous mass, which goes through a gradual series of 

 histolytic changes, and is finally absorbed by being dissolved 

 in the fluid of the body-cavity (Fig. 105 B). 



By the time the tail has been completely drawn up into 

 the body, the organ of fixation or snout, as we have called 

 it above, becomes drawn out into a long probosciform 

 structure in a line with the long axis of the body. Its 

 cavity is no longer completely filled with mesoderm-cells 

 as it was at first (Fig. 105 A), but it has become so volu- 

 minous that its contained cells are loosely scattered about 

 (Fig. 105 E). In the concluding chapter we shall endeav- 

 our to show, what has been already implied, namely, 

 that the organ of fixation is seen to the best possible 

 advantage from a morphological point of view in the 

 species now under consideration, viz. Ciona intcstiiialis, 

 and that it is homologous with the prasoral lobe (snout) of 

 Amphioxus, including under that term both the prasoral 

 body-cavity and the praeoral pit, and further that it is 

 homologous with the proboscis of Balanoglossus. 



