NOTES. 241 



2. (p. 211.) In Clavelina the atrial involutions do not merely 

 arise as minute circular invaginations of the ectoderm, but at first 

 they appear as short, though quite distinct, longitudinal grooves. 

 Compare also the remarkable longitudinal atrial tubes of Pyrosoma. 



3. (p. 238.) There is another possible way of interpreting the 

 structure and systematic position of Appendicularia which may 

 perhaps be nearer the truth than either of the views mentioned in 

 the text. It is not absolutely necessary to suppose that the 

 ancestors of Appendicularia were fixed Ascidians ; but both 

 Appendicularia and the fixed Ascidians may have descended from 

 a common free-swimming stock, and have undergone certain 

 modifications in common, such as loss of true vascular system and 

 coelom. Then, while the Ascidians proper became adapted to a 

 sessile existence, Appendicularia may be supposed to have gone 

 to the opposite extreme, and have become adapted to an absolutely 

 pelagic existence. In becoming adapted to such a purely pelagic 

 or oceanic environment as that of Appendicularia, it is eminently 

 conceivable that an animal would have to undergo as radical a 

 modification of structure as it would in becoming adapted to a 

 sessile existence. (Compare Salpa, Doliolum, etc.) 



