. Cassell's natural history. Animals; Animal behavior. SPONGE SPICULES. 321 tlie rest by excess or defect of developiiieut, as tlie sluift ((/ to /), ilie other three remaining siniihu- to erxh other being known as the rays or arms. The shaft usually takes a radiate direction in the Sponge, at right angles to the surface, with which the rays, on the contrary, lie more or less parallel; the point of the shaft is directed inwards towards the centre of the Sponge, the head or rayed end out- wards. The rays may grow backwards, recurved, giving the spicule a grapnel-like form (y), or forwards, fork
. Cassell's natural history. Animals; Animal behavior. SPONGE SPICULES. 321 tlie rest by excess or defect of developiiieut, as tlie sluift ((/ to /), ilie other three remaining siniihu- to erxh other being known as the rays or arms. The shaft usually takes a radiate direction in the Sponge, at right angles to the surface, with which the rays, on the contrary, lie more or less parallel; the point of the shaft is directed inwards towards the centre of the Sponge, the head or rayed end out- wards. The rays may grow backwards, recurved, giving the spicule a grapnel-like form (y), or forwards, fork-like, or outwards at right angles to the shaft; they may remain simple, or bifurcate once (//), or more, rarely, twice, or even trifurcate, as in a few recent and souie fossil forms; finally, they may broaden out in the surface of the Sponge into thin lobate expansions (i), and these may become confluent in a circular disc {j), in which, however, the tri-radiate origin can still be traced by the. r\ form of the axial thread. Returning to the quadri-radiate form, in which the rays are all similar (/), another series of changes may result by the ends of the rays becoming branched {I), and closely inter- locking with those of their nearest neighbours. In this way the firm stony network characteristic of tlie Lithistids is produced {k). These branched spicules may be traced through various modifications till their quadri-radiate form remains no longer recognisable (?h). Another group of forms originates in growth in six directions from a common centre along three axes at right angles to each other. The sex-radiate spicule so produced is characteristic of the Hexactinellidse («). One by one the rays of this form may be suppressed (o), so that mingled together in the same Sponge sex-,quinqui-,quadri-, tri-, and bi-radiate spicules may be found, the bi-radiate or a<:erates (;;) often still showing signs of their sex-radiate derivation by the cross-like form of the axial-thread in
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Keywords: ., bookcentury1800, bookdecade1890, booksubjecta, booksubjectanimals
