SALPA IN RELATION TO EVOLUTION OF LIFE. 173 



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 per- 

 fect digestion, and that the rudiment of the organ which has in 

 the higher vertebrates become the liver was thus established. 



In these primitive animals the current of water through the 

 digestive organs was most useful as the vehicle for floating food, 

 but Avhile necessary, it was a necessary evil, for the large dis- 

 tended lumen which furnished it a channel also permitted undi- 

 gested food to be swept away and lost. 



The immovable, permanently distended, ciliated digestive tract 

 of a modern lamellibranch is very similar to that of these primi- 

 tive chordata, but the lamellibranchs have acquired an apparatus 

 for straining oif the water from the captured food, so that the 

 digestive tract is relieved from this disadvantageous current. 



If, after the pharynx had been established, a secondary opening 

 from it to the exterior were to be formed, this opening would per- 

 mit the water to escape without passing through the intestine, and 

 as the advantage of this new arrangement is obvious, there can be 

 no doubt that after an opening of this sort was once formed, it 

 would be preserved and perfected by natural selection, as a channel 

 for the escape of the water after the food has been strained out and 

 entangled by the excretion of the pharyngeal slime-glands. 



I shall show, further on, that if an useful opening of this sort 

 were to be fixed and preserved by natural selection on one side of 

 the body, the laws of growth would soon cause it to be duplicated 

 on the other side. These two openings are the so-called gill-slits 

 of appendicularia, although they are beyond question much older 

 than the modern appendicularia, dating back to a time before this 

 animal had acquired the features which distinguish it from its more 

 primitive chordata ancestors. 



I am not able to suggest what led to the first establishment of a 

 secondary opening into the pharynx ; but, once formed, its preser- 

 vation and gradual improvement, by natural selection, as a channel 

 for the escape of superfluous water, and its duplication on opposite 

 sides of the body, are easily intelligible. 



