150 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



If this were a solitary case it would not deserve notice ; but exam- 

 ination will show that no highly organized animal has ever been evolved 

 at the surface, although all depend on the pelagic food-supply. 



The animals which now find their home in the open waters of the 

 ocean are, almost without exception, the descendants of forms which live 

 upon or near the bottom or along the sea-shore or upon the land, and the 

 exceptions are all simple animals of minute size. The metazoa which 

 are primitively pelagic, that is, those which have been pelagic throughout 

 their whole history and do not owe their structure to competition with 

 improved forms from the bottom or the shore, are astonishingly few, and 

 these few are among the smallest and simplest of the metazoa. 



It is only necessary to review the chief groups of metazoa in order 

 to perceive that most of their pelagic representatives exhibit the clearest 

 evidence of descent from forms which lived upon or near the bottom or 

 the shore. Many indeed have no pelagic members, but are restricted to 

 the bottom. 



The sponges are obviously a bottom group ; most of them are fixed, 

 all are sedentary, and their whole organization is an adaptation for life 

 in the bottom. 



The coral polyps, actinias and alcy on arias, are among the most 

 characteristic bottom forms, and the abundance of the fossil remains of 

 polyp skeletons proves that these animals became established on the 

 bottom very early, and that the whole history of their evolution has 

 taken place at the bottom. The acraspedote medusae are universally and 

 justly regarded as the descendants of fixed polyp-like ancestors, and we 

 may state with confidence that they are not primitively pelagic, but that 

 a fixed period in their history has come between the modern swimming 

 jelly-fish and its remote and unknown primitive pelagic ancestor. 



The veiled medusae are usually held to have had a similar history, 

 but I shall soon give my reasons for holding that some of these at least 

 are primitively pelagic. There can, however, be no doubt that the 

 evolution of hydroid cormi has taken place at the bottom. The siphon- 

 ophores are descended from ancestors like the anthomedusae, and the 

 various families and genera and species of siphonophores have most 

 certainly been produced by divergent specialization 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 palaeozoic times, and while synapta is sometimes 



