SPOXGE SPICULES. 



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the rest by excess or defect of development, as the shaft (g to /), the other three remaining similar to 

 each 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, 011 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 (#), or forwards, 

 fork-like, or outwards at right angles to the shaft ; they may remain simple, or bifurcate once (h), or 

 more rarely, twice, or even trifurcate, as in a few recent and some 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 



Fig. 10. VARIOUS FORMS OK SPONGE SPICULES. (After Bowerbank, Ziiiel, and others.) 



form of the axial thread. Returning to the quadri-racliate form, in which the rays are all similar (/), 

 another series of changes may result by the ends of the rays becoming branched (/), and closely inter- 

 locking with those of their nearest neighbours. In this way the firm stony network characteristic of 

 the Lithistids is produced (&). These branched spicules may be traced through various modifications 

 till their quadri-radiate form remains no longer recognisable (?>i). 



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 (n). 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 

 acerates (p) often still showing signs of their sex-radiate derivation by the cross-like form of the 

 axial-thread in the middle of the spicule. 



By suppression of the distal ray of the sex-radiate type, nail-like spicules arise (q), the shaft being 

 stuck in the substance of the Sponge, and the four rays spread out in the skin, forming, with the 

 similar rays of adjacent spicules, a square meshed dermal network. The shaft of such a spicule may 

 become greatly elongated (r), and then it often serves with others for anchoring or supporting the 

 Sponge in the slimy ooze of the sea floor on which it lives. If the four rays of such a spicule become 

 recurved and much reduced in size, we have a grapnel-headed anchoring spicule (s), such as those 

 which compose the twisted wisp-like bundle of the glass-rope Sponge (Hyalonema), and which, 

 measuring eighteen inches in length, are probably the largest spicules known. 



If growth from a centre takes place radiately in a large number of directions, a stellate 

 spicule (t, u) results ; fine examples of this are known in Tethya. By growth along a constantly 

 changing axis various curved forms are produced (v). 

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