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On the Origin of the Metazoa By A. C. HARDY {From the Department of Zoology and Comparative Anatomy, Oxford) SUMMARY A new view is expressed regarding the possible evolution of the Metazoa from unspecialized and relatively simple Metaphyta, instead of from colonial or partitioned Protozoa. T HIS is a short paper to present a very simple idea regarding the origin of the Metazoa, one which, as far as I am aware, has not been expressed before. It is in a sense a corollary to the views expressed by my old friend and colleague Dr. J. R. Baker in his article on 'the Status of the Protozoa' (1948); indeed, the idea owes its origin to that article for it occurred to me after reading it. Baker, it will be remembered, defines a cell as 'a mass of protoplasm, largely or completely bounded by a membrane, and containing within it a single nucleus formed by the telophase transformation of a haploid or diploid set of anaphase chromosomes'. He argues that while some Protozoa, such as the Ciliophora and certain Radiolaria, are not cells in the sense of this definition, there are many which can properly be regarded as unicellular organisms. He stresses that there is no fundamental difference between the cleavage of an egg and the multiplication of a unicellular protozoon by division. At the end of his article he discusses the evolution of Metazoa from Protozoa. It is well known that there are two schools of thought regarding this question. On the one hand, there are those who hold that the Metazoa were derived from a group of protozoan cells which had failed to separate after repeated division, i.e. from a colony of cells which have gained a new and corporate individuality; on the other hand, there are those who believe that the origin of the manycelled condition was due to the development of partitions within a multinucleate protozoon. Baker discusses both sides of the controversy and points out how it is more difficult for the zoologist to imagine the evolution of a metazoon from a protozoan colony than it is for the botanist to imagine a similar step in the evolution of multicellular plants. Since my suggestion springs directly from his argument of this point, I will, with his kind permission, quote him at some length. He writes as follows: 'It may be remarked that while such forms as Volvox raise a storm of controversy among zoologists, they are not regarded as unusual by botanists, who are familiar with such colonial forms in groups other than the phytomonads. Botanists regard them as composed of cells, which are comparable on one hand with the individuals of what they call unicellular species, and on the other with the cells of higher but related forms, in which there is somatic differentiation (heterotrichous forms and other algal metaphytes). [Quarterly Journal of Microscopical Science, Vol. 94, part 4, pp. 441-443, Dec. 1953.] 442 Hardy—On the Origin of the Metazoa 'If we seek the reason why zoologists are more divided in this controversy than botanists, we shall surely find it in a fundamental difference between plants and animals. In the lower plants each group tends to present both unicellular and metaphytic representatives, and botanists are repeatedly confronted with the intermediate forms. Metaphytes, in fact, have obviously originated independently many times, so that no one would propose the word "Metaphyta" in any classification that was supposed to be based on evolutionary principles. With animals it is far different. Zoologists have not got intermediates again and again before their eyes. In the great group of Rhizopods, for example, there is no example of a multicellular form (the word "cell" being here used as denned above). It would not appear that anyone has discussed the reason for this striking difference between plants and animals. The following suggestion may be made. The unicellular plant absorbs nutriment from all sides equally, and when, in the course of ontogeny or phylogeny, it becomes a metaphyte, there is no fundamental change in this respect: a cell divides without separation and the two products continue to absorb nutriment over most of their surfaces. The passage from the unicellular form to the metaphyte is therefore easy. In the case of animals, however, there is an important change when a unicellular form becomes a metazoon: a new method of feeding must be adopted. We see this particularly clearly in the ontogeny of Dendrocoelum. The blastomeres feed at first saprozoically, but at last a profound change occurs: a set of digestive organs originates, and a new method of nutrition begins. Most animals overcome this difficulty in ontogeny by placing enough reserve food-stuffs within the single-celled stage to make feeding unnecessary. It is easy to see what evolutionary difficulties must be presented when a new system of nutrition must be acquired before advance can occur. The difficulties would be greatest when a protozoon had a localized mouth. If the products of division of such an animal were to adhere together and each were to acquire its own mouth, no advance would be made towards the evolution of a metazoan alimentary canal. This suggests that the Metazoa may have arisen from primitive Protozoa, unprovided with localized organs of assimilation. These considerations seem to make it clear why intermediates between unicellular animals and Metazoa are rare, while intermediates between unicellular plants and metaphytes are common. (The colonial phytomonads are, of course, only regarded as animals by zoologists on account of their motility: their nutrition, it need scarcely be said, is holophytic or saprophytic.)' We are concerned here with the origin of the true Metazoa, leaving on one side the sponges, sometimes called the Parazoa, which with their more limited integration may well have been quite independently derived from aggregations of choanoflagellate-like protozoa. The simple idea I want to suggest is that the Metazoa have not been derived from the Protozoa at all, but from relatively simple metaphytes which, after they had evolved from protophytes began, perhaps as a result of a shortage of phosphates and nitrates, to capture and feed upon other small organisms as do the higher insectivorous plants. Among the Protista it is clear that animals have evolved from plants more than once; in several different groups of Protophyta, as Fritsch (1935) has so well shown, we can arrange parallel series passing from holophytic forms at one end to holozoic forms at the other. In addition to such changes among the flagellates of the class Euglenineae, Fritsch points out and compares these parallel Hardy—On the Origin of the Metazoa 443 evolutionary lines in four different classes of the simpler algae: Chlorophyceae, Xanthophyceae, Chrysophyceae, and Dinophyceae. If animals have evolved from plants several times at the unicellular level, is it not possible that animals might, at least once, have been so derived from plants at a not too specialized multicellular level ? This suggestion may perhaps be worth bearing in mind because it does get over the very real difficulty which Baker has pointed out, that of how a metazoon could have evolved from a colony of protozoa, each, as individuals, already adapted for the capture of prey. It would be unprofitable to elaborate the idea in much detail. The many different devices evolved by the various carnivorous plants to enable them to secure their food suggest how such a metazoan organism may have been derived. It is not difficult to imagine a spherical volvox-like metaphyte developing a little pocket-like invagination in which small protozoa or protophyta might collect, die, and provide breakdown products to be absorbed by the cells lining the cavity. Such little pockets would be analogous with the bladders of Utricularia which capture water-fleas. How similar in general action, although different in physiological detail, are the 'tentacles' which capture flies on the leaf of the sundew Drosera to those which capture Crustacea on the polyps of a hydroid! The gradual transition from a simple metaphyte to a simple polyp-like metazoon—a bladder-like cavity with tentacles—seems no more difficult to conceive than the evolution of the higher animal-like insectivorous plants; we have only to imagine the process going so far as to cut out all photosynthesis, thus making the organism holozoic—as indeed has occurred repeatedly at the unicellular level of the flagellates and algae. If the Metazoa should have evolved from Metaphyta then the findings of Gohar (1940) are of special interest; he claims to have shown that Alcyonarians of the family Xeniidae feed entirely upon the photosynthetic products of their symbiotic algae and never capture animals for food at all; by a circuitous route they appear now to be metazoa which are on their way back to becoming metaphytes and must, if the whole colony is to grow, be taking in nutritive salts from outside for the growth of their enclosed plant cells. (It is perhaps just possible, but most unlikely, that the zooxanthellae of the Anthozoa, which cannot be cultured outside their 'host', might be plant cells remaining as a legacy from the original metaphytic days and not separate symbiotic algae as in other such associations.) I do not wish here to enter into the controversy between the former two opposing views as to the origin of the Metazoa; the sole purpose of this short communication is to state a third possible view for consideration. I am indebted to Dr. J. R. Baker not only for his article which gave rise to this idea, but for his interest in the suggestion and his discussion of it. REFERENCES BAKER, J. R., 1948. Nature, 161, 548, 587. FRITSCH, F. E., 1935. The structure and reproduction of the algae, Vol. i. Cambridge. GOHAR, H . A. F., 1940. Publications of the Marine Biological Station of Ghardaqa, No. 2, 23. (Cairo.) 2421.4 Gg