FOSSIL INVERTEBRATES/Corals and Other Cnidaria 333 Cupolate Growth form of solitary scleractinian corals in which the corallite has a flat base and a highly domed upper (calicular) surface Dendroid Structural arrangement of corallites in a branching (fasciculate) coral in which each branch is a single corallite and the branches are irregularly divergent Diploblastic Level of organization in primitive invertebrates in which the body wall consists of external and internal cellular layers (ectoderm and endoderm), which may be separated by a layer of noncellular connective tissue Discoidal Growth form of solitary corals in which the corallite has a flat base and a flat to gently concave upper (calicular) surface Dissepiment A vesicular skeletal plate, subhorizontal to inclined zones of which occupy the peripheral areas of many corallites (dissepimentarium) In scleractinian corals, vesicular and tabular dissepiments are distinguished Dysaerobic Environment with 0.1–1 ml O2 per litre of water Dysphotic Environment with low light levels Enteron Cavity in the body of a cnidarian polyp or medusa; the gut Epitheca The external wall of a corallite Flabellate Growth form of scleractinian corals, either a fan-shaped solitary coral with an elliptical crosssection or a meandroid corallum consisting of a single, linear, polycentric corallite Foliose Growth form of colonial corals in which the branches are broad and more or less flat laminae Hermatypic Scleractinian corals that build shallowwater reefs Holotheca The external wall of a colonial coral in which the individual corallites have lost their own walls (epithecae) Homoeomorphy Similarity of form resulting from convergence rather than genetic relationship Increase The process of new corallite formation in a colonial coral Lagerstaă tte Fossil deposit in which organisms not normally preserved, such as those lacking mineralized skeletons, are found Meandroid Structural arrangement of corallites in scleractinian colonial corals in which a number of linear polycentric corallites are intertwined (a brain coral) Medusa The free-floating stage of the cnidarian body plan in which the mouth faces downwards surrounded by tentacles Mesentery A radially disposed fleshy infold of the endoderm of a cnidarian polyp, attached to the inner surfaces of the oral and basal discs The inner end of the lamina may be attached to the stomodaeum, in which case the mesentery is said to be complete Nematocysts Stinging or adhesive structures housed in specialized cells of the ectoderm, characteristic of cnidarians Operculum Skeletal plate or plates covering the calice of a corallite in some corals Periderm Outer stiff organic (scleroprotein) skeleton of some polypoid hydrozoans Phaceloid Structural arrangement of corallites in a branching (fasciculate) coral in which each branch is a single corallite and the branches are relatively closely spaced and subparallel Planula Larval stage of a cnidarian Plocoid Structural arrangement of corallites in colonial scleractinian corals such that corallites are separated by septal costae, dissepiments, or other common colonial tissue Polyp The fixed benthonic stage of the cnidarian body plan in which the mouth faces upwards surrounded by tentacles In the corals and some other cnidarians the polyp secretes a calcium carbonate skeleton at its base Ramose Branching growth form in which each branch consists of many corallites The structural arrangement of the corallites may be cerioid, amural, coenenchymal, plocoid, or meandroid Scyphistoma Benthonic polypoid stage in scyphozoans specialized for budding off medusae Septum In corals, a radial skeletal element, which may be a solid or perforate vertical plate or be reduced to a series of spines In Scyphozoa, one of four radial fleshy partitions of the gut Stomodaeum Cylindrical collar extending down from the mouth into the gut of anthozoan polyps Tabula A horizontally disposed skeletal plate, which may be flat, dished, arched, domed, or vesicular, stacks of which occupy the axial area of most rugosan corallites (tabularium) or the whole width of the corallite in some rugosans and most other Palaeozoic corals Zooxanthellate Scleractinian corals with symbiotic algae (zooxanthellae) present in the endoderm See Also Fossil Invertebrates: Porifera Lagerstaă tten Palaeoclimates Palaeoecology Precambrian: Vendian and Ediacaran Sedimentary Environments: Carbonate Shorelines and Shelves; Reefs (‘Build-Ups’) Further Reading Babcock LE and Feldmann RM (1986) The phylum Con ulariida In: Hoffman A and Nitecki MH (eds.) Problem atic Fossil Taxa, pp 135 147 New York: Oxford University Press