172 W. K. BROOKS. 



excuse that the study of phylogeny is impossible without the use 

 of the imagination, and that the field is already occupied by a 

 phylogeny of the tunicata which cannot be set aside until a more 

 satisfactory one has been found. 



Appendicularia is a very simple organism, and while much 

 ingenuity has been expended in the negative task of accounting 

 for the absence of all the structures which it lacks, I hope that the 

 more positive attempt to account for its actual structure will not 

 lead us into any great difficulties. 



In the belief that the sequel will justify the assumption, I shall, 

 as my starting-point, picture the ancestor of appendicularia as a 

 simple, minute, unsegmented, chordate animal, leading a free, 

 locomotor, pelagic life and subsisting upon the micro-organisms 

 of the ocean. I siiall also assume that this ancestor had an 

 elongated, unsegmented body stiffened by an axial, unpaired, 

 unsegmented notochord, like that of amphioxus, appendicularia, 

 and the ascidian larva; that it had a simple, elongated, dorsal, 

 nervous system, and an elongated, ventral, digestive tube, without 

 pharyngeal clefts ; that this tube was nearly straight ; that it had a 

 capacious lumen, and that,; as in amphioxus and the tunicates, 

 this was permanently distended and ciliated, and that the water, 

 with the micro-organisms that float in it, was swept through it by 

 endodermal cilia and not by muscular contractions. 



In order to entangle the floating particles of food and to hold 

 them while the water swept on through the intestine and out of 

 the anus, gland-cells for the excretion of slime were scattered 

 among the ordinary ciliated endoderm cells of the digestive tract. 

 In origin, these slime-cells may have been modified or specialized 

 digestive gland-cells. 



As particles which are entangled and held captive near the oral 

 end of the gut are more perfectly exposed to its digestive action 

 than those which continue to float with the stream, the most 

 anterior slime-cells are most efficient and valuable, and as each 

 variation in this direction gave its possessor an advantage, the 

 slirae-cells gradually, through the action of natural selection, 

 became localized in the pharyngeal region, and this region 

 gradually became enlarged and was thus set apart, at a very early 

 period, as a specialized tract of the gut. 



