W. K. BROOKS ON THE GENUS SALPA. 185 



It seems probable that after the bottom of the ocean became fit for 

 life, some of the descendants of the primitive pelagic tunicates gradually 

 acquired the habit of sometimes swimming upon or near it in an inclined 

 position with the mouth downwards to suck up the organic sediment, 

 and that they also acquired the habit of resting upon the bottom in this 

 position. 



We may well doubt whether these animals obtained any more food 

 than their pelagic ancestors, but it is well known that it is not the 

 amount of food, but the ratio between the supply and the amount of 

 expended energy which affects size. As this new habit economized 

 energy both during rest and during activity, it permitted an increase in 

 size, and it is interesting in this connection to note that Chun has found 

 at great depths appendicularias which may well be called gigantic as 

 compared with all which are known to exist at the surface. 



With each increase in size, the habit of visiting the bottom must 

 itself have become more and more fixed, until the life upon the bottom, 

 which may have been at first only intermittent and more or less acci- 

 dental, at last became established in the ancestors of the ascidians as a 

 constant characteristic peculiarity. 



As this new mode of life was gradually acquired, some method of 

 aerating the fluids of the body must also have been gradually evolved ; 

 for without it, a minute animal adapted for a free active life in the 

 highly aerated surface-water, could not, at the same time that it grew 

 larger, acquire a less active habit of life in the bottom strata where the 

 water is less perfectly aerated, the products of decomposition of organ- 

 isms more concentrated, and the capacity for passing from exhausted 

 and impure water to a fresh environment, restricted both by the more 

 stationary habit and by the fact that life in space has been exchanged 

 for a home which is limited by a surface. 



Undoubtedly the change of habit was accompanied by the gradual 

 perfecting of the system of blood-spaces around the pharynx, which, at 

 first indefinite and irregular, became constant on the margins of the 

 pharyngeal clefts, which thus gradually acquired a new function and 

 became gill-slits, and also became duplicated as the animals grew larger 

 and the need for more perfect respiration increased with their change of 

 habits. 



I hope that no one will interpret the last sentence as an expression 

 of the belief that the need for respiration caused the gill-slits to multiply. 

 I believe, and shall try to show further on, that the tendency to dupli- 



