region of gelatinous tissue called the mesogloea, This layer contains numerous small canals and spaces, that link the outer layers with the gastric cavity. The function of these is unknown but they most likely aid in transport of materials throughout the polyp and perhaps act also as a hydrostatic skeleton. The canals may also be related to the transport of products of photosynthesis from the zooxanthellae, much like the zooxanthellal tubules found in tridacnid clams (see volume one), which are also associated with the digestive system. Zooxanthellae are found in these canals (Hyman, 1940) and may also be transported via these canals, allowing for the control of zooxanthellae populations by elimination via the gastric cavity and mouth. The endodermis (=gastrodermis) surrounds the gastric cavity, which contains numerous septa that extend partially or fully towards the actinopharynx, The complete septa have wing-like filaments on their upper ends, as in anemones (Hyman, 1940). The actinopharynx extends upwards towards the oral disc and opens into the small, slit-like mouth, A siphonoglvph is present and is well developed in some genera. The oral disc is surrounded by twro rings of unbranched tentacles (usually) that contain nemato-cysts in varying numbers and types, depending on species. The second ring of tentacles is greatly reduced in some species. The oral disc has a sphincter of muscle around it that acts to open and close the polyp, pulling the tentacles inwards. The extent to which this muscle extends across the mesogloea is used as a diagnostic characteristic for some genera (see chapter eight). Zooxanthellae are found in the endodermis, mesogloea 1 canals and the ectoderm is of some genera (Hyman. 1940).
Zoanthid polyps can be found singly (e.g, Sphenopus spp.), in small unconnected groups (e.g. some Protopalytboa spp.), in small groups interconnected by a thin coenenchyma (Zoanthus spp.), or in colonies connected by a thick coenenchyma with only the oral ends protruding (Palythoa spp.). We will discuss colony anatomy in more detail for each genera in chapter eight.
The most common form of reproduction amongst zoanthids is asexual via their lateral growth over a substratum, be it the reef itself or as in the case of Acrozoantbus, Epizoanthiis and Parazoantbus, over or within other organisms. New polyps can arise from a spreading sheet of coenenchyma, from creeping stolons or via budding from the base of the mother polyp, depending on the
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