September 30, 1922] 



NA TURE 



45i 



the same transition from plant to animal nutrition can 

 be well followed by studying different members of 

 the group. In heavily armoured forms, with a rich 

 supply of chromatophores, nutrition is chiefly plant- 

 like or holophytic. In those with fewer chromato- 

 phores there is, on the other hand, often distinct 

 evidence of the ingestion of other organisms, and 

 nutrition becomes partly animal-like. Among the 

 naked dinoflagellates such holozoic nutrition is very 

 much developed, and in many species has superseded 

 entirely the earlier method of carbonic acid assimila- 

 tion. 



It is surprising how many structural features found 

 in higher groups of animals make their first appearance 

 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 

 Polykrikos there are well-developed stinging cells or 

 nematocysts, as elaborately formed as those of Hydra 

 or the anemones. In Pouchetia and Erythropsis well- 

 developed ocelli are found, consisting of a refractive, 

 hyaline, sometimes 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 division 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 ccelenterate. It is difficult to resist 

 the suggestion put forward by Kofoid 9 in his recent 

 monograph, that if to Polykrikos, with its continuous 

 longitudinal groove which serves it as a mouth, its 

 multicellular and multinucleate body and its nemato- 

 cysts, we could add the tentacle of 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 dinoflagellata 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 the 

 wake of which food particles would collect, had become 

 more specialised for nutrition than the rest. 



As a purely plankton organism, swimming freely 

 in the water, the progress of the ccelenterate was not 

 great, and reached, so far as we know, no further 

 than the modern ctenophore. The ctenophore seems 

 to represent the culminating point of the primary 

 progression of pelagic animals, which derived directly 

 from the autotrophic flagellate. Further evolution 

 was associated with an abandonment of the pelagic 

 habit by a ccelent crate-like animal, and the establish- 

 ment of a connexion with the sea-bottom, either by fix- 

 ing to it, by burrowing in it, or by creeping or running 

 over it. At a later stage many of the animals which 



Unarmoured Dinoflagellata.' 



NO. 2761, VOL. IIO] 



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 primary 

 pelagic life, so far as adult animals were concerned, 

 having ended with the evolution of the ctenophore. 10 

 Such is the teaching of embryology, the history of 

 the race being conjectured from the development of 

 the individual. 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 ccelenterate, 

 and then a bottom-living phase.' 



It is in a ctenophore-like ancestor that we find the 

 line of development to higher animal groups, and this 

 ancestor must have been at one time widely distributed 

 in the seas. Its immediate descendants are familiar 

 to every zoological student in the well-known series 

 of pelagic larval forms. Miiller's larva, taking to the 

 bottom, and in its hunt for food gliding over hard 

 surfaces with its cilia, led to the flatworms ; the 

 Pilidium, developing a thread-like body and creeping 

 into cracks and crevices to transfix its prey, gave 

 rise to the nemertines. A trochophore, burrowing 

 in soft mud and sand, developed a segmented body 

 which gave it later the power of running on these 

 soft surfaces, and became an annelid worm. Another 

 trochophore, developing a broad, muscular foot, crept 

 on the sand, and afterwards buried itself beneath it 

 as a lamellibranchiate mollusc, or migrated on to 

 harder surfaces as the gastropod and its allies. Pluteus, 

 Bipinnaria, Auricularia, first fixing, as the crinoids 

 still do, and developing a radial symmetry, afterwards 

 broke free and wandered on the bottom as sea-urchin, 

 star-fish, and cucumarian. Tornaria developed into 

 Balanoglossus, the structure of which hints to us that 

 the ascidians and vertebrates came from a similar 

 stock. All the phyla thus represented derive directly 

 from the free-swimming ctenophore-like ancestor, and 

 only one considerable group, the arthropods, remains 

 unaccounted for. The evolutionary history of an 

 arthropod is, however, not in doubt. Its marine 

 representatives, the trilobites and Crustacea, came 

 directly from annelids, which, after their desertion 

 of a pelagic life to burrow in the sea-floor and run 

 along its surface, again took to swimming, and not 

 only stocked the whole mass of the water with a rich 

 and varied life of copepods, Cladocera, and schizopods, 

 but gave rise to amphipods, isopods, and decapods, 

 groups equally at home when roaming on the bottom 

 or swimming above it. 



Another important addition to the pelagic fauna we 

 should also notice here. From the molluscs, creeping 

 on solid surfaces, sprang a group of swimmers, the 

 cephalopods, which have grown to sizes almost un- 

 equalled amongst the animals of the sea. 



All these invertebrate phyla had become established 

 and most of them had reached a high degree of de- 

 velopment in the seas of Cambrian times. Among 

 animals then living there are many which have survived 

 with little change of form until to-day. One is almost 

 tempted to suggest that the life which the sea itself 



10 There is perhaps a possibility that further knowledge of the embryology 

 of Sagitta and its allies might make it necessary to modify this suggestion. 



