W. K. BROOKS ON THE GENUS SALPA. 149 



and the most distinctive peculiarity of the microscopic food-supply of the 

 ocean is the very small number of the forms which go to make up the 

 enormous mass of individuals. 



The Origin of Pelagic Animals. 



All the animals of the ocean are dependent upon the microscopic 

 food-supply, and many of them are adapted for preying upon it directly. 

 Among these Salpa is one of the most conspicuous examples. It passes 

 its whole life in the open water, and it has no sessile stage in its onto- 

 geny, 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' observation, Schminkewitch says that the 

 Salpas are perennial pelagic animals, and Chun has shown that they 

 are also found in abundance 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 afford most beau- 

 tiful 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 swimming 

 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 acquired 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 descendants 

 culminating in Salpa ? 



Why should the descendants of a pelagic ancestor have passed 

 through a sessile stage before they acquired their improved pelagic 

 structure ? 



