THE PHYLUM ARCHAEOCYATHA

Summary 1. The Archaeocyatha are, geologically speaking, a short‐lived group, characteristic of the carbonate‐shelf and reef environments of the Lower Cambrian and early Middle Cambrian. They are found in all continents, excepting South America but including Antarctica. They are the only animal phylum that has become extinct. 2. They were marine, benthonic organisms, probably with planktonic larval stages. Suggestions that larvae had skeletons and that certain small calcareous fossils were planktonic young stages of archaeocyatha are discounted. 3. They existed in depths down to 100 m., could construct bioherms at depths between 20 and 50 m. and flourished best between 20 and 30 m. Surface temperature of the water is assumed to have been that of hot regions. They are not found in sediments with a primary magnesium oxide content of more than 5–8%, and are commonest in sediments with less than 0–2–0.5 % of magnesium oxide. 4. The skeleton or cup is basically a two‐walled cone with the central cavity normally empty. The overwhelming majority of cups attain a diameter of 10–20 or 25 mm. All plates are perforate except the dissepiments. The pores of the walls, but not those of the septa, taeniae and tabulae, may be complex canals or tubes bounded by variously shaped and arranged skeletal structures, and the different types of wall so distinguished are consistent within genera and species. 5. Pectinate tabulae are characteristic of the suborder Nochoroicyathina, but within a single species individuals may occur in which they are scarcely perceptible. 6. Cap‐like structures at the growing end of the cone in some atabulate species raise unsolved problems on the manner of growth of the cup. 7. Archaeocyathan skeletal encrustations on normal cups are interpreted by Zhuravleva as protuberances from the intervallum of the encrusted or some other normal cup. V ologdin , however, interprets them as encrusting species and genera of archaeocyatha in which the inner wall and central cavity are not developed, and Maslov regards them as symbiotic. 8. The skeleton consists of subequigranular, microcrystalline grains of calcium carbonate about 2 μ in diameter, which Russian authors consider to have an appreciable organic content. It is considered to be external in origin by Zhuravleva and internal by V ologdin . Neither spicules nor acicular crystals have been conclusively demonstrated. 9. Ontogeny is consistent within genera and species and is of value in classification. 10. Trends of development, both in reduction and in complication, have been distinguished. 11. Skeletal carbonate sheathing walls, septa and taeniae are regarded by some, including myself, as secondary skeletal thickening, and by others as calcifications in situ of the soft parts that deposited the element sheathed. 12. Canaliculate or tubulose skeletal material found in the central cavity of some individuals is considered by Zhuravleva to be formed by protuberances from the intervallum but by V ologdin to represent calcification in situ of soft assimilative organs. 13. V ologdin , on the basis of his above‐mentioned view that calcified soft parts occur, regards the soft body as cellular, with differentiated assimilative organs located in the central cavity, drawing food currents through these organs and out through the wall pores of the intervallum and depositing the skeletal elements internally. 14. Zhuravleva on the other hand, in an interpretation which I prefer, considers that assimilation and some production of sexual cells took place in the intervallum and that the central cavity was not the site of any vital activity. She thinks of the food currents as drawn into the intervallum through the outer wall pores and expelled into the central cavity through the inner wall pores. The skeleton was external, formed as in foraminifera. Digestion was analogous to that in Protozoa and took place intra‐cellularly in uniform, undifferentiated cells, and not in a distinct layer of epithelial cells as in coelenterates. Thus the phylum was primarily of single‐layered animals, with the possibility that some single‐layered forms with undifferentiated and polar cells, and some two‐layered forms might have been present in the more specialized members. She suggested that the archaeocyatha fulfil the requirements of the hypothetical blastea. 15. Following Zhuravleva's analysis, it seems probable that the Archaeocyatha are a phylum of single, multicellular organisms, with a degree of organization higher than that of the Protozoa but with less differentiation than the Porifera.