174 W. K. BROOKS. 



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

 purely pelagic iufluences, 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 di- 

 gestive 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 auy organs for this purpose. 



It is not at all probable, then, that the pharyngeal clefts were origi- 

 nally either gills or renal organs, and we have seen that the conditions 

 of pelagic life furnish a much more simple explanation of their advan- 

 tage, and I believe that the view that they were originally concerned 

 in nutrition rather than in respiration will commend itself to all 

 who approach the subject without any philosophical preconception. 



After they were once established they gradually eifected a rear- 

 rangement of the slime-cells and ciliated cells of the pharynx, for 

 as it now became important that all the food particles should be 

 entangled by the product of the slime-cells before it reached the 

 pharyngeal clefts, the slimfe-cells were gradually restricted to the 

 anterior part of the pharynx, while the ciliated cells gradually be- 

 came specialized to carry the entangled food past the openings and 

 to convey it safely into the oesophagus. 



All the parts of the pharynx of append icularia are beautifully 

 constructed for this purpose. The pharyngeal clefts are situated 

 far back in the pharynx, and are separated by nearly its whole 

 width from the oesophagus. They are fringed by large cilia to 

 expel the water, and they are separated from each other by a 

 vertical shelf or velum on the ventral floor of the ])harynx, so 

 placed as to prevent cross-currents. 



In front of this shelf the slime-cells are brought together in two 

 rows, near the middle line, just inside the mouth, to form the hypo- 

 pharyngeal band or endostyle. Between these two rows of slime- 

 cells there is a median row of large ciliated cells, so placed that they 

 drive the slime forwards to the point where a ciliated peripharyngeal 

 band receives it and carries it up each side of the pharynx just 

 behind the mouth, into the most favorable place for entangling the 

 food, as this enters with the current of fresh water. 



