88 SECTIONAL ADDRESSES. 



partly animal-like. Amongst the naked Dinoflagellates such holozoic 

 nutrition is very much developed, and in many species has entirely 

 superseded the earlier method of carbonic acid assimilation. 



It is really surj^rising how many structural features found in higher 

 groups of animals make their first appearance in these naked Dino- 

 flagellates in conjunction with this change of nutrition, and we seem 

 to be led directly to the metazoa, especially to the Ccelenterata. Fn 

 Polykrikos there are well-developed stinging cells or neniatocysts, as 

 elaborately formed as those of Hydra or the anemones. In Pouchetia 

 and Erythropsis well-developed ocelli are found, consisting of a refrac- 

 tive, hyaline, sometimes spherical lens, surrounded by an inner core of 

 red pigment and an outer layer of black ; the whole structure is com- 

 parable to the ocelli around the bell of a medusa. In Noctiluca and in 

 the allied genus PavilJardia a mobile tentacle, which is doubtless used 

 for the capture of food, is developed. Division of the iiucleus, 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 division and form definite chains 

 we seem to approach still nearer to the metazoa, until, finally, in Poly- 

 li-rikos we reach an organism which may well have given rise to a simple, 

 pelagic cffilenterate. It is difficult to resist the suggestion put forward 

 by Kofoid'* in his recent monograph, that if to Polylcrikufi, with its 

 continuous longitudinal groove which serves it as a mouth, its multi- 

 cellular and multinucleate body and its nematocysts, we could add the 

 tentacle oi Noctiluca. and perhaps also the ocellus of Erythropsis, ' we 

 should have an organism whose structure would appear prophetic of 

 the Ccelenterata and one whose affinities to that phylum and to the 

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

 the correct one here, and that the first ccelenterate came from a spherical 

 colony of simple holozoic flagellates, arranged 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 specialised 

 for nutrition than the rest. 



Before proceeding, however, to consider the further progress of 

 animal life, we must pause for a moment to ask in what direction plant 

 life in the sea developed, from which the increasing animal life derived 

 its nourishment. Here the striking fact is the lack of progress in the 

 free, floating, plankton phase. The plant life of the plankton has 

 never proceeded beyond the unicellular stage, for the jDlankton diatoms, 

 which with the peridinians form the great, fundamental vegetable food 

 suppl}' of the sea, are only autotrophic flagellates which have lost their 

 flagella, having acquired other means of flotation to keep them in the 

 sunlit region of the upper water layers. Deriving their food, as these 

 plants do, directly from molecules in the sea-water, the factor which 

 is for them of supreme importance is the exposure of maximum surface 

 directly to the water. Hence the minute unicellular form has been 

 the only one to survive as phytoplankton. The marine region in which 



'■* Kofoid and Swezy, ' The Free-living Unarmouied Dinofiagellata.' Mem. 

 Univ. California, 1921. 



