SALPA IN RELATION TO EVOLUTION OF LIFE. 141 



ization among pelagic forms, and the greater part of their history, 

 if not the whole of it, is therefore pelagic. 



The echinoderms are most characteristic inhabitants of the bottom, 

 as they have been from paleeozoic times, and while synapta is some- 

 times 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 brachi- 

 opods 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 adaptation 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 primi- 

 tively 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 molluscs 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 

 molluscs 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 



