W. K. BROOKS ON THE GENUS SALPA. 179 



far from the beaten paths of observation, I must plead as my 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 appendicular-ia as a simple, 

 minute, unsegmented, chordate animal, leading a free, locomotor, pelagic 

 life and subsisting upon the micro-organisms of the ocean. I shall 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 slime-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. 



It is also probable that, at a very early stage in the phylogeny of 

 these primitive chordata, a blind pouch was developed, behind the 

 pharynx, to catch the food-particles as they were hurried past with the 

 stream of water and to retain them long enough for perfect digestion, 

 and that the rudiment of the organ which has in the higher vertebrates 

 become the liver was thus established. 



