The royal natural history . s require a- supporting scaffolding orskeleton, and in nearly all sponges it is the function of certain cells in themesoderm to secrete skeleton-material. A few sponges possess no skeletonwhatever, excepting the gelatinous ground-substance; in some also the skeletonis mainly or entirely composed of foreign particles of sand, spicules of othersponges, skeletons of Radiolaria or Foraminifera. The vast majority form inthe ground - substance a skeleton which is composed of spicules of silica, orcarbonate of lime, or of horny fibres. The sponges whose skeleton is compose
The royal natural history . s require a- supporting scaffolding orskeleton, and in nearly all sponges it is the function of certain cells in themesoderm to secrete skeleton-material. A few sponges possess no skeletonwhatever, excepting the gelatinous ground-substance; in some also the skeletonis mainly or entirely composed of foreign particles of sand, spicules of othersponges, skeletons of Radiolaria or Foraminifera. The vast majority form inthe ground - substance a skeleton which is composed of spicules of silica, orcarbonate of lime, or of horny fibres. The sponges whose skeleton is composedof calcium carbonate form a distinct class—the Calcarea. Of those which secretea siliceous skeleton, the glass-sponges form a second class distinct from the restof the siliceous sponges. All the rest, including by far the largest number., areincluded under a third class, the common sponges (Demospongia). The Calcareous Sponges,—Class this group the skeleton is formed of spicules of carbonate of lime, shaped. a calcareous ascon sponge, Leucosolenia (magnified 4 times). like three-rayed stars, four-rayed stars, or needles. The triradiate occurs mostfrequently in its typical form, the three rays being equal, in one plane, and forming CALCAREOUS GROUP. 535 an angle of 120° with each other. The spicules are transparent and glassy whenviewed separately, but white and opaque in mass. On placing a calcareous spongein acid, the skeleton dissolves away with effervescence. The class is dividedinto two groups, namely, the Homocoela and the Heterocoela, in the former ofwhich the collar-cells line, the whole of the interior of the simple or branchedsac; while in the latter they are confined to the radial tubes, or the endsof branched radial tubes or canals, the gastric cavity with a part of thecanal system being lined with flat cells. The simple ascon sponge (Ascettaprimordialis) forms a minute stalked sac open above, and with its thin wallsperforated by pores and supported
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Keywords: ., bookcentury1800, booksubjectnaturalhistory, booksubjectzoology
