No. 647] PROGRESSION OF LIFE IN THE SEA 495 



found in higher groups of animals make their first ap- 

 pearance in these naked Dinoflagellates in conjunction 

 with this change of nutrition, and we seem to be led directly 

 to the metazoa, especially to the Ccelenterata. In Poly- 

 hrihos there are well-developed stinging cells or nema- 

 tocysts, as elaborately formed as those of Hydra or the 

 anemones. In Pouchetia and Erytliropsis well-developed 

 ocelli are found, consisting of a refractive, hyaline, some- 

 times spherical lens, surrounded by an inner core of red 

 pigment and an outer layer of black ; the whole structure 

 is comparable to the ocelli around the bell of a medusa. 

 In Noctiluca and in the allied genus Pavillardia a mobile 

 tentacle, which is doubtless used for the capture of food, 

 is developed. Division of the nucleus, with the formation 

 of large, distinct chromosomes, has also been described 

 in several of these Dinoflagellates. With the tendency 

 of the cells in certain species to hold together after divi- 

 sion and form definite chains, we seem to approach still 

 nearer to the metazoa, until, finally, in Polykrikos we 

 reach an organism which may well have given rise to a 

 simple pelagic coelenterate. It is difficult to resist the 

 suggestion put forward by Kof oid in his recent mono- 

 graph, that if to Polykrikos, with its continuous longitu- 

 dinal groove which serves it as a mouth, its multicellular 

 and multinucleate body and its nematocysts, we could add 

 the tentacle of Noctiluca, and perhaps also the ocellus of 

 Erythropsis, we should have an organism whose struc- 

 ture would appear prophetic of the Ccelenterata and one 

 whose affinities to that phylum and to the Dinoflagellata 

 would be patent." Or it may be that the older view is 

 the correct one here, and that the first coelenterate came 

 from a spherical colony of simple holozoic flagellates, ar- 

 ranged something on the plan of Volvox, in which the 

 posterior cells of the swimming colony, in whose wake 

 food particles would collect, had become more specialized 

 for nutrition than the rest. 



Before proceeding, however, to consider the further 



15 Kof oid and Swezy, "The Frce-lmng Unarmored Dinoflagellata." 

 Mem. Univ. California, 1921. 



