182 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



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

 cates diverged from the chordata stem, they were acquired by an organ- 

 ism 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 difficult 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 organi- 

 zation 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 evolu- 

 tion 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 any evidence of 

 degeneracy, or in its habits any basis for the assumption that it is 

 degenerated. In most respects its structure is like that of the hypo- 

 thetical ancestor whose evolution we have traced. It has an unseg- 

 mented notochord, and a capacious lumen throughout the whole course 

 of the digestive tract from mouth to anus. This lumen is permanently 

 distended and food is carried through it by cilia. It has a blind diverti- 

 culum from the stomach, and the greatly expanded pharynx opens 

 laterally through two ciliated pharyngeal clefts, through which the 

 water escapes while the food passes into the oesophagus. There is a 

 ventral slime-gland just inside the mouth, and its excretion is conveyed 

 upwards around the pharynx by the cilia of the peripharyngeal bands, 

 and is then swept into the oesophagus with the entangled food. 



This increasing complexity and perfection of the pharynx is accom- 

 panied by an increase in its size, so that in the primitive tunicates it soon 

 comes to be the most important and dominant organ of the body, and 



