SALPA IN RELATION TO EVOLUTION OF LIFE. 175 



On the dorsal middle line the threads of slime are gathered up 

 and guided along the epipharyngeal band or dorsal lamella, beyond 

 the influence of the current of water which sets backwards, on each 

 side of the ventral velum, to the pharyngeal clefts, and the food is 

 thus safely conducted into the oesophagus while the water escapes. 



Up to this point I believe that the ancestral history of the tuni- 

 cates was identical with that of the vertebrates, for the hepatic 

 caecum, the dilated pharynx, the pharyngeal clefts, the hypo- 

 pharyngeal gland and the peripharyngeal bands have been in- 

 herited by all the chordata, and have impressed themselves so 

 firmly in their organization that even the highest vertebrates still 

 retain them, either as vestiges, or as organs which have been fitted 

 to new functions. 



I believe, however, that while they were acquired before the 

 tunicates diverged from the chordata stem, they were acquired by 

 an organism whose environment and habits of life were essentially 

 like those of the modern appendicularia. 



All the parts of the pharynx of appendicularia are so beautifully 

 co-ordinated for effecting a purpose so useful and so well adapted 

 to the conditions of its simple pelagic life, that we find it difiicult 

 to resist the belief that its ancestors had essentially the same habits, 

 and that they lived under essentially the same conditions, and that 

 this simple organization was directly acquired in adaptation to 

 these conditions. 



If this view involved any great or unusual difficulties we might 

 well distrust it, notwithstanding its simplicity ; but I shall try to 

 show that it does not. In the preceding chapter I have shown 

 that it accords with our knowledge of the fundamental principles 

 of the general biology of the ocean, and further on I shall try to 

 show that it is equally in accord with the principles of morphology. 



At present we must devote our attention to the history of the 

 evolution of the tunicates from this primitive chordata stem. 



Section 2. — The Origin of the Tunicates. 



Like most recent students of the tunicates, I believe that we 

 have in appendicularia a persistent representative of the primitive 

 tunicata ; but, unlike many of them, I fail to find in its structure 



