W. K. BROOKS ON THE GENUS SALPA. 181 



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

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

 pelagic life furnish a much more simple explanation of their advantage, 

 and I believe that the view that they were originally concerned in nutri- 

 tion rather than in respiration will commend itself to all who approach 

 the subject without any philosophical preconception. 



After they were once established they gradually effected a rearrange- 

 ment 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 

 slime-cells were gradually restricted to the anterior part of the pharynx, 

 while the ciliated cells gradually became specialized to carry the entangled 

 food past the openings and to convey it safely into the ossophagus. 



All the parts of the pharynx of appendicularia are beautifully con- 

 structed 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 pharynx, 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. 



On the dorsal middle line the -threads of slime are gathered up and 

 guided along the epipharyngeal band or dorsal lamella, beyond the influ- 

 ence of the current of water which sets backwards, on each side of the 

 ventral velum, to the pharyngeal clefts, and the food is thus safely con- 

 ducted into the oesophagus while the water escapes. 



Up to this point I believe that the ancestral history of the tunicates 

 was identical with that of the vertebrates, for the hepatic ca3cum, the 

 dilated pharynx, the pharyngeal clefts, the hypopharyngeal gland and 

 the peripharyngeal bands have been inherited by all the chordata, and 

 have impressed themselves so firmly in their organization that even the 

 highest vertebrates still retain them, either as vestiges, or as organs 

 which have been fitted to new functions. 



