. Elements of Comparative Anatomy. 106 COMPARATIVE Fig. 37. 1 Cell with a siliceous spicule of Spongilla. 2 Ves- icle with au amphi- disc of Spongilla (after N. Lieber- kiihn). (Fig. S7, 2). The siliceous spicules are often greatly elongated, and form excessively delicate skeletons (Euplectella), or they form bulky structures which project as tufts of fila- ments far beyond the body (Hyalonema). Lastly, in the Fibrospongise, the skeleton of the body is formed by fibres united into a network, which consist of a substance allied to chitin. In the Acaleph^e also the deposition of in- or
. Elements of Comparative Anatomy. 106 COMPARATIVE Fig. 37. 1 Cell with a siliceous spicule of Spongilla. 2 Ves- icle with au amphi- disc of Spongilla (after N. Lieber- kiihn). (Fig. S7, 2). The siliceous spicules are often greatly elongated, and form excessively delicate skeletons (Euplectella), or they form bulky structures which project as tufts of fila- ments far beyond the body (Hyalonema). Lastly, in the Fibrospongise, the skeleton of the body is formed by fibres united into a network, which consist of a substance allied to chitin. In the Acaleph^e also the deposition of in- organic substances in the mesoderm leads to the formation of various kinds of skeletons. In the Anthozoa they generally have the form of colonies, and the hard substance is almost always formed of calcareous salts. These give rise to deposits (Fig. 45) of definite form (Fig. 38), which are scattered in the soft parts of the body; or to connected masses, which vary in form according to their mode of development. The calcareous bodies (spicula) always lie in the connective-tissue of the parenchyma, and are very varied in form. They have an organic basis, which retains the form of the spicula after the lime is removed. The connected skeletal parts are formed either by the union of sjDicula, which are connected to- gether by a hard organic substance, as in Corallium, or by the direct cal- cification of a secreted horny sub- stance, which lies in the axes of the coenenchyma, and does not possess spicula. "When the organic sub- stances predominate the skeleton is horny, as it is in the Gror- gonidae and Antipathidte. These axial skeletons are sometimes limited to the trunk of the colony, as in the Pennatulid^e, where they lie in the shaft of the stock, or they may be continued into all the branches of the stock. There is another form of skeleton, which resembles the axial; it is formed by the gradual calcification of the parenchyma of the body, in which process spicula sometimes t
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