No. 647] PBOGRESSION OF LIFE IN THE SEA 497 



After this digression on the botanical side we must 

 return to the primitive coelenterate and see on what lines 

 evolution proceeded in the animal world. As a purely 

 plankton organism, swimming freely in the water, the 

 progress of the coelenterate was not great, and reached, 

 as far as we know, no further than the modern Cteno- 

 phore. The Ctenophore seems to represent the culmi- 

 nating point of the primary progression of pelagic ani- 

 mals, which derived directly from the autotrophic 

 flagellate. Further evolution was associated with an 

 abandonment by a coelenterate-like animal of the pelagic 

 habit, and the establishment of a connection with the 

 sea bottom, either by fixing to it, by burrowing in it, or 

 by creeping or running over it. At a later stage many 

 of the animals which had become adapted to these modes 

 of life developed new powers of swimming, and thus gave 

 rise to the varied pelagic life which we find in the sea 

 to-day; but this must be regarded as secondary, the pri- 

 mary pelagic life, so far as adult animals were concerned, 

 having ended with the evolution of the Ctenophore.^^ 

 Such is the teaching of embryology, the history of the 

 race being conjectured from the development of the in- 

 dividual. In group after group of the animal kingdom, 

 when the details of its embryology become known, the 

 indications are the same — first the active spermatozoon, 

 reminiscent of the plankton flagellate, then the pelagic 

 larval stage, recalling the coelenterate, and then a bottom- 

 living phase. 



The primitive, free-swimming coelenterate, adopting a 

 fixed habit and becoming attached mouth upwards to 

 solid rock or stone, gave rise to liydroids, anemones and 



Soil/' Jovrnal of Bacteriology, YU, 2. M.xrv\x_ Tlic :nitlior. c'aim 



suggestion. 



