SINGLED-RAYED SPONGES 



3517 



nniaxial siliceous needles or rods, which may be isolated and scattered, or united 

 into bundles by the horny cementing substance, spongin; while the bundles may 

 be joined in various ways to form scaffoldings for the support of the soft parts. 

 The spicules are shaped like spindles. In addition to the large spicules forming 



the bulk of the skeleton, 



and on this account called 



skeleton spicules, in some 



groups minute forms 



abound in the soft sub- 

 stance, and are termed 



flesh spicules. The latter 



are frequently shaped like 



buckles or double 



anchors, with prongs at 



each end. A transition 



can apparently be traced 



from this group to the 



horny sponges; the sili- 

 ceous needles becoming 



less numerous and the 



horny substance more 



abundant, till, in the true 



horny sponges, such as 



the toilet sponges, the 



skeleton consists entirely 



of horny fibres. The! 



ocellated Chalina, fre- 

 quently cast ashore round 



the British coasts, ex- 

 hibits an intermediate 



condition between the 



siliceous and horny forms. 



By teasing out and ex- 

 amining a few fibres under 



the microscope, a fine 



core of siliceous spicules 



will be seen in the axis of 



each thread of spongin. 

 The illustration on 



A SINGLE-RAYED p. 3503 represents a group 



SPONGE, Axinella. c 



of sponges growing to- 

 Natural size.) 



gether. Near the base of 



the black seaweed on the stone there arises 



the much-branched Desmacidine sponge, so SIUCEOUS SPICULES OF MONAXONID 



called from its buckle-shaped spicules; while SPONGES (magnified 200-300 diameters). 



