W. K. BROOKS ON THE GENUS SALPA. 161 



the ocean from pelagic nauplii, and that the great group Crustacea has 

 been derived from them. 



We have already seen that the eucopepods are the chief intermediary 

 between the micro-organisms of the ocean and the larger and higher 

 marine animals ; that they prey upon the protophytes and protozoa, and 

 in their turn supply either directly or indirectly most of the food for the 

 large inhabitants of the water ; that most pelagic larva? feed upon them ; 

 that they are the food of the great pelagic banks of pteropods and hetero- 

 pods, of many coelenterates, of the young of most fishes, and of some of 

 the most abundant and important adult fishes, like the herring, and that 

 the sea-birds, the cetacea, and in fact almost all of the larger pelagic 

 animals, prey upon animals which in their turn prey upon copepods. 



The animals which are most important at one period in the earth's 

 history are often replaced by others at another period, and it is, of 

 course, possible that the modern copepods now fill a place which was in 

 former times filled by something else ; but as their organization, as com- 

 pared with that of the other Crustacea, exhibits all the characteristics of 

 a primitive pelagic stem-form, and inasmuch as the remains of animals, 

 like the pteropods, which now live almost entirely upon copepods, are 

 found in the oldest fossiliferous rocks, there is every reason to believe 

 that the group formed an important constituent of the primitive pelagic 

 fauna. 



No one who advocates at our time the morphological heresies which 

 are involved in the view that appendicularia is a stem-form which is 

 pelagic in its origin ; that the nauplius is a persistent representative of 

 the primitive Crustacea, and that the whole history of the copepods has 

 been pelagic ; and that the veiled medusa? have been evolved in direct 

 relation to pelagic influences ; no one who makes these statements can 

 hope to escape the charge that his view "ist die unwahrscheinlichste 

 von unwahrscheinlichkeiten." 



The books all tell us that the free active appendicularia is the 

 "degenerated" descendant of an ancestor which crept over the bottom; 

 that the nauplius is a secondary larval form ; that the active free-swim- 

 ming copepods are degenerated phyllopods; and that the locomotor 

 hydro-medusa is, in its origin, a specialized member of a sessile, poly- 

 morphic, hydroid cormus. 



The first of these opinions, that appendicularia is a degenerated 

 form, rests upon a supposed necessity for deriving the body of a verte- 

 brate, which consists of a series of segments homologous with each other, 



