W. K. BROOKS ON THE GENUS SALPA. 151 



found at the surface of the open ocean, this is exceptional, and we may 

 state without hesitation that the evolution of the echinoderms has taken 

 place at the bottom. This is equally true of the brachiopods and of most 

 of the animals classed as vermes, the gephyreans, bryozoa, nemertians, 

 and so forth. The pelagic annelids, such as Tomopteris, are secondary 

 modifications of bottom forms, and while some of the more primitive 

 annelids may possibly be originally pelagic, the group as a whole is as 

 characteristic of the bottom as the echinoderms. 



Many groups of Crustacea have pelagic representatives, and the 

 pelagic crustacean fauna is rich and varied, but in most cases the 

 pelagic forms show unmistakable evidence of secondary change of habit, 

 and all the higher Crustacea have been evolved at the bottom in adapta- 

 tion to a bottom life. 



I shall soon give my reasons for believing that there is one important 

 exception to this rule, however, and I shall try to show that there is good 

 ground for holding that the copepods are primitively pelagic, and that 

 while the greater part of the history of the Crustacea is bottom history, 

 the characteristics of the crustacean type were outlined in pelagic 

 animals at a very early period in the history of the metazoa. 



The heavy calcareous shells of the mollusca could not have been 

 acquired at the surface, and that most characteristic molluscan organ, 

 the lingual ribbon, is adapted for attacking more solid bodies than the 

 delicate primitive pelagic animals. The classes and orders of mollusca 

 must have been evolved at the bottom, and there is ample evidence that 

 the swimming shelless gasteropods and cephalopods have, like those 

 great pelagic groups the pteropods and heteropods, been secondarily 

 adapted for a pelagic life. 



Many of the marine fishes are strictly pelagic, and the structure and 

 habits of fishes are in all respects so well fitted for a wandering life in 

 the open water that the pelagic habit of fishes seems at first sight to be 

 their most distinctive peculiarity, although a little examination will show 

 that there is ample evidence that it is secondary, and not primitive. 



The perfection of their adjustment to a free life in the open sea is no 

 evidence that this life is primitive, for the highest marine animals and 

 those whose adaptation to a pelagic life is most complete, the sea-birds 

 and cetacea and marine reptiles, are air-breathing terrestrial animals 

 which have gone back into the ocean. 



The most primitive groups of living fishes are the cyclostomes, elas- 

 mobranchs and ganoids. The cyclostomes are too small a group, and 



