SALPA IN RELATION TO EVOLUTION OF LIFE. 211 



animals which, having been evolved at the bottom, have then become 

 secondarily pelagic and have taken their improved structure back 

 into the open ocean. 



Haeckel has shown that the highest representatives of the craspe- 

 dote stem, the Disconectse (velella, porpita, etc.) and the siphono- 

 phores, are pelagic productions, and that the Disconectse can be 

 traced back to an ancestor similar to the Trachomedusse, while the 

 Siphonanthae or true siphonophores have ai'isen from simple Antho- 

 medusae. (Report on the Siphonophora collected by H. M. S. 

 Challenger; and System der Siphonophoren, Jenaische Zeitschrift, 

 Vol. XXII, p. 1. 1888). 



Both these groups are therefore pelagic in their history, and they 

 go back, not to ancestral hydroid cormi, but to ancestral medusae, 

 but they can hardly be primitively pelagic, and we must regard 

 them as the product of the more modern conditions of pelagic life. 



The craspedota were undoubtedly represented in the primitive 

 pelagic fauna, by floating hydras with stiiF radiating poisoned 

 pseudopodia-like tentacles, and also by small and simple veiled 

 medusae, but the higher forms of the group are probably more 

 modern, although there is palaeontological evidence that they are 

 as old as the lower cambrian. 



