SALPA IN RELATION TO EVOLUTION OF LIFE. 139 



The Origin of Pelagic Animals. 



All the animals of the ocean are dependent upon the micro- 

 scopic food-supply, and many of them are adapted for preying 

 upon it directly. Among these Salpa is one of the most conspicu- 

 ous examples. It passes its whole life in the open water, and it 

 has no sessile stage in its ontogeny, as many floating animals have. 

 It abounds in all parts of the ocean, and over some great seas it is 

 always present at the surface. As the result of three years' obser- 

 vation, Schminkewitch says that the salpas are perennial pelagic 

 animals, and Chun has shown that they are also found in abun- 

 dance at great depths. 



As long as it is alive and breathing a steady stream of micro- 

 organisms is slipping along its pharynx and down through its 

 oesophagus into its stomach, and sections of the intestine of salpa 

 aiFord most beautiful preparations of radiolarians and diatomes. 



The pelagic food-supply is very ancient, and we have, in salpa, 

 an animal which has been especially evolved to pass its life swim- 

 ming through the living broth of the mid-ocean. 



If we were to select the typical pelagic animal we should probably 

 choose salpa, and it is therefore most surprising to find that salpa 

 itself has not been produced at the surface of the ocean by gradual 

 evolution from a simple pelagic ancestor. 



The structure which fits it so well for its mode of life has come 

 to it by the inheritance of peculiarities which were originally ac- 

 quired by bottom animals in adaptation to the needs of a sessile life. 



This is all the more remarkable since both salpa and its fixed 

 allies show by their embryology that still more remotely they are 

 descended from a pelagic form like appendicularia. 



The place in the pelagic world which Salpa fills so well has been 

 ready for it from primeval times. 



Why then has not the simple pelagic appendicularia given rise, 

 in the open sea, to series of more and more perfected pelagic de- 

 scendants culminating in salpa ? 



Why should the descendants of a pelagic ancestor have passed 

 through a sessile stage before they acquired their improved pelagic 

 structure ? 



