178 W. K. BROOKS. 



chordata, are the U-shaped folds of the digestive cavity, the sharp 

 separation of the tail from the body, and the differentiation of the 

 nervous system into a caudal nerve and anterior vesicle. 



We have little basis for speculation as to the path by which the 

 reproductive organ acquired its present position, and it is by no 

 means certain whether the tunicate heart is homologous with that 

 of the other chordata. 



The conditions of pelagic life are so permanent that we may 

 safely make use of the structure and habits of the modern pelagic 

 forms to reconstruct this part of the ancestral history of the tuni- 

 cates, for time writes no wrinkles on the azure brow of the ocean. 



As regards the later history the case is diiferent. Between ap- 

 pendicularia and the ascidians there is a great gap which we can 

 bridge only in imagination. The transitional animals are totally 

 unknown, and the conditions of life on the bottom of the modern 

 ocean may, possibly, be very different from those which j)revailed 

 when the fixed ascidians were first evolved. 



It is easy to imagine changes which might have gradually con- 

 verted an ancestor like appendicularia into a descendant like the 

 fixed ascidians, through successive adaptations to a sedentary life, 

 but in the absence of all evidence we cannot feel implicit confidence 

 that the imaginary picture bears any minute and detailed resemblance 

 to the actual history. 



It seems probable that after the bottom of the ocean became fit 

 for life, some of the descendants of the primitive pelagic tunicates 

 gradually acquired the habit of sometimes swimming upon or near 

 it in an inclined position with the mouth downwards to suck up the 

 organic sediment, and that they also acquired the habit of resting 

 upon the bottom in this position. 



We may well doubt whether these animals obtained any more 

 food than their pelagic ancestors, but it is well known that it is not 

 the amount of food, but the ratio between the supply and the amount 

 of expended energy which affects size. As this new habit econo- 

 mized energy both during rest and during activity, it permitted an 

 increase in size, and it is interesting in this connection to note that 

 Chun has found at great depths appendicularias which may well be 

 called gigantic as compared with all which are known to exist at 

 the surface. 



