A guide to the fossil invertebrate animals in the Department of geology and palaeontology in the British museum (Natural history) . e their currents (Fig. 10).In this wall the chambers are suspended by rafters (trabe-cule) of soft tissue between an outer dermal membrane andan inner gastral membrane. The spicules are formed by cells POEIFERA (sponges). 33 in the trabeculae; some support the dermal membrane, some Gallery gastral membrane, while others stretch across the body-wall between the two membranes ; in Lyssacina yet otherspicules project from the dermal membrane. Modern Hexactinell
A guide to the fossil invertebrate animals in the Department of geology and palaeontology in the British museum (Natural history) . e their currents (Fig. 10).In this wall the chambers are suspended by rafters (trabe-cule) of soft tissue between an outer dermal membrane andan inner gastral membrane. The spicules are formed by cells POEIFERA (sponges). 33 in the trabeculae; some support the dermal membrane, some Gallery gastral membrane, while others stretch across the body-wall between the two membranes ; in Lyssacina yet otherspicules project from the dermal membrane. Modern Hexactinellids all live in the deep sea, fixed tothe bottom or moored in the ooze by long tufts of rootingspicules. The best known is the beautiful Venuss Flower-basket {Euplectella). Sub-Class I.—LYSSACmA. The spicules of the skeletoneither remain separate or are united at a late period ofgrowth in an irregular manner by siliceous masses or smalltransverse rods (synapticulse). Oeder I.—Hexasterophora. In some of the spiculesin the middle layers of the body-wall the rays branch,forming rosette-like bodies called hexasters. The chief. Fig. 9,—Fossil sponge spicules : Silicispongiae, Hexactinellida. a-f areskeletal spicules ; g, h are flesh spicules. Six-rayed spicules are shownin a,b,e,f; in c one ray is suppressed. Axial canals are seen ina, e and /, and a and / have lantern nodes, e and / illustrate theunion of spicules to form the square mesh of Dictyonina, e heing fromSestrodictyon and / from Coeloptychiuni; g is called a pinule, and h anamphidisc. a is enlarged 66 diam.; b, c, f, 40 diam. ; e, 47 diam.;g, 134 diam.; h, 114 diam. (After Hinde.) families are Euplectellidse, Asconematidse, and representatives of the last two have been found inEocene rocks. Order II. —Amphidiscophora. There are no hexasters,but some spicules in the limiting membranes are in the formof rods with toothed disc-like expansions at their ends ; theyare called amphidiscs (Fig. 9 h). Th
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