180 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



In these primitive animals the current of water through the diges- 

 tive organs was most useful as the vehicle for floating food, but while 

 necessary, it was a necessary evil, for the large distended lumen which 

 furnished it a channel also permitted undigested 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 primitive chordata, 

 but the lamellibranchs have acquired an apparatus for straining off 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 permit 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 pharyn- 

 geal 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 a of 

 secondary opening into the pharynx ; but, once formed, its preservation 

 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. 



If we accept the view that the chordata type was evolved under 

 purely pelagic influences, we are forced to believe that the first chordata 

 were minute, and that their small bodies were soft, and unprotected by a 

 hard covering. If we also admit that their digestive tract was a channel 

 for a current of water, we can hardly believe that they needed respiratory 

 organs, or, for that matter, excretory organs, for all the tissues of a 

 minute soft animal, bathed within and without by pure water, must 

 have been sufficiently aerated and purified without any organs for this 

 purpose. 



