METAMORPHOSIS. 223 



arises an individual which, long before it has com- 

 pleted its development, sends out a comparatively 

 large bud or stolon composed like all such buds 

 of entoderm, mesoderm, and ectoderm, and com- 

 parable to the first bud of the single Ascidian. 

 When the stolon of the young Perophera has at- 

 tained a considerable length, there appear at inter- 

 vals along it, several small swellings, comparable 

 to the subdivision of the young bud of Botryllus. 

 Each of these little swellings of the stolon de- 

 velop into sexual individuals, which remain united 

 in one colony, and never have an independent 

 existence. In external appearance all the individ- 

 uals of the Perophera colony are alike. Let us 

 compare now the development of the Perophera 

 colony with what seemed like the two generations 

 of Salpa. In each case there is a first individual 

 which produces a stolon. From the different por- 

 tions of the stolon are produced the other individ- 

 uals which make up the colony. Suppose a fixed 

 ' Perophera-like colony to change its habit of life 

 and become free-swimming. Evidently a weak 

 immature individual dragging a long stolon would 

 be at a great disadvantage. The probable result 

 would be that the single individual would attain 

 its full growth before the appearance of the stolon, 

 and the stolon would be much diminished in size. 



